the question of what happens if you swap the gear ratios from this to that often comes up. i usually jump in and offer my thoughts on the subject, not being a shy individual. yesterday i was thinking and decided to list all the rear end ratio swaps that i have done just for fun.

1. 52 pontiac ambulance. originally came with 4.10 gears. this was a flathead straight eight engine with a two barrel carburator. it weighed something north of 4000 pounds and had a wheelbase that had been stretched about 20" giving it a wheelbase of about 135". one could observe the splices in the frame and the extension pieces. it was built by the national ambulance and hearse company and was based on a sedan delivery. it had a 3 speed with synchro on 2 and 3,. i tested and recorded speedo and odo error and corrected for tire size and drove the car through several tankfuls and several lengthly trips to establish base line figures. the fuel economy was in the low teens.

a. i swapped in a 3.66 rear end from a two door sedan. drove the car on several trips and calculated the improvment in fuel economy. i found that it increased directly proportional to the gear ration with very little deviation.

b. not satisfied, i searched out a 3.08 rear end from an automatic equipped car, bought it and installed it. although now taking off in first was a bit dicy if i got stopped on a steep hill with my trailer attached, once rolling the engine had more than adequate torque in every other situation both driving and pulling my 13' "love bug" travel trailer. i used the car in this configuration for a couple of years, driving and documenting the fuel economy. in this configuration it would get up to 18 mpg. again i found that the mileage was very directly related to the gear ratio.

i once also swapped a very small carburator (taken from my previous 61 mb 220b parts car) onto the engine using an adaptor and adapting the throttle linkage with small cable clamps. i found that the engine started and idled fine and drivabiliity was excellent but that the small carb limited high rpm power. i could open the throttle and the car would accelerate up to a speed commensurate with the amount of air the little carb would flow. it was as if i had the throttle about one third open on the original huge (by comparison) two barrell. the fuel economy on the one trip i took was exactly the same as with the bigger carb.

2. the second vehicle i experimented with was my 51 caddie hearse. i didnt change the gears on this one. by this time i had a larger 19' trailer and wanted to pull that with the hearse, so i actually fitted undersized tires for more mecahnical advantage. i did comprehensive calculations to find the comparative gear ratios generated by the smaller tire size.

3. the next vehicle was my 84 suburban with 6.2. these engines had the reputation for knocking out main bearing webs if equipped with the 410 rear end. so when the rear started showing signs of fatigue (its previous owner was the fleetwood travel trailer factory in crawfordsville indiana and it had over 150k of towing when i bought it), i bought a 3.73 rear with posi and had it installed. i did comprehensive documentation of mileage before and after. this truck with my 24' travel trailer attached would drive with no ill effect for hours on end at wot or very near it. and would top out at about 65 on the level. again i found the economy to pretty much direclty related to the gear ratios. this truck was equipped with a turbo 400 tranny. (a straight 3 speed aiutomatic)

4. the next vehicle was my 86 suburban. this too was equipped with a 6.2 diesel. again i changed the rear end from 410 to 373. again i found the mileage to relate very directly to the gear ratio. (turbo 400 tranny)

5. the next vehicle to receive the gear swap was my 81 280e. this car is a us spec vehicle that originally came with a 354 rear end. it had oodles of torque and an automatic tranny. (the tranny is from a 240d with its relatively low shift points, but ... hey it was paid for and when i put it in i didnt really know if the car was any good 'cause i bought it with a bad tranny). i swapped in a 288 gear from the 85 300dt. this vehicle is the only vehicle that disappointed me in its fuel economy after the swap. it did NOT correspond proportionately to the gear ratios in its economy improvment.

6. and finally when i did the swap of the euro 300d motor into my 83 240 i orignally had the 346 gear installed that came with the 300d. this was a very happy car. it performed very nicely. it had the same friendly driving 240dcharacteristics but felt about twice as lively due to the fact that the 240 has very little extra power to acclelerate after overcoming the power taken to transmit through the power train, run the water pump, power steering and charge the battery, not to mention the ac. i drove the car in this configuration for a month or two then ordered a 307 gear from the "benz store" and a speedo and had them installed. i found that the performance was not affected a lot, but taking off was not an automatic thing, taking a bit of skill to do smoothly. (it is a four speed stick). the engine noise on the highway was greatly reduced. the top speed was still about 100, the same as it was with the 346 gear. and driving at 75 it consistantly got 30 mpg on the highway, actually a bit better than the stock 240d. and with a lot less noise and a lot more usable performance.

i have kept files on all my cars and if i were to dig, i could come up with actual figures on most of these. my conclusion is that, from personal experience, in every case except the 280e, be it gas or diesel, if you change the gear ratio to slow the engine down say 15%, your fuel economy increases by very nearly exactly the same amount.

for years, i have listened to folks who are very knowledgable about cars, but have not done the conversions and done the math, expound eloquently about how a higher gear will result in a deeper throttle position and get into the enrichment circuits of the sysetm etc. etc. (cough cough...bull s###t) they havent done the swaps and done the documentation because they talked themselves out of it before actually doing anything.

but in actual practice, especially in a diesel, i have found that if fitted with a 20% od situation you will realize a 20% reduction in fuel use. my conclusion is that the engines are geared with so much reserve torque and power that they are virtually ALWAYS running at much less than their optimum fuel consumption mode to make them more drivable for the average situation for the average driver in the average weather conditions in the average country with the average..... you get the idea.

this is my experience, if any of you have read all this, i am impressed.

tom w

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Hi Tom,
Very interesting, but I believe you should also mention two other innovations that have also greatly improved mileage. The first is the overdrive transmission - same as reducing the numeric rear end ratio, but with the od transmission you still have a good low gear for starting. The second, and possibly more important with an auto. trans, is the locking torque converter. This invention eliminates the heat generated from a constantly slipping torque converter and thus adds to the efficiency of the drive train. A good modern automatic trans with overdrive and locking torque converter will add up to 30 percent to the mileage on a car with no disadvantages in operation. They also allow lower numeric gearing, as the engine can run at a lower speed without the torque converter becoming "lossy". This is one reason I converted my '82 300D to the 4.3L V6 with the T700R OD transmission with locking torque converter. When in "high" OD and torque converter locked, the auto has very little parasitic loss. My '82 will produce 28MPG at 75mph with 4 adults in the car and the AC on full blast.
Modern American engine design takes advantage of this by designing the engines to be very torquey at lower RPM's where the engine is running most of the time. The newer 4.3L engines actually change the length of the intake runners at different rpms to provide the best charge of air at lower speeds.
Of course, none of this is new info, the late 30's/40's Studebakers and others used an overdrive transmission for fuel economy. Intake tuning has been around for years, mid 50's Buicks used a variable pitch torque converter, etc...

By the way, I just yesterday drove from Vancouver, WA to Seattle, WA and back with my '82 380SL with an '86 560SL engine installed, and it produced an average of 19.2 mpg for the 317 miles, not too shabby, I'd say! Traffic was moving around 80mph for most of the way.

__________________
Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C

Good posts. I see Tom has done gear ratio changes on a variety of automobiles. I hope I'm not misinterpreting if I said Tom was searching, experimentally, to provide a mathematical and experiential foundation in figuring out just what is the optimal gearing ratio is that ultimately can be extended to solve any configuration by extrapolation and/or interpolation; obviously the differences in diesel and gasoline engines being crucial. It seems the primary difficulty in doing this is that for a given vehicle the optimal ratio changes as a function of how the automobile is driven, weight carried, ambient temperaturesetc. The engineers problem is that they have to install a gear ratio(s) that give's satisfactory performance over a very wide range of conditions. A diesel that is used to turn a generator under a constant load presents an easy problem. So, as Tom said, the relationship between diesel engine and gear ratio so as to give the best fuel economy, but not present problems in radically bad acceleration, radically bad drivetrain stresses, radically bad top end speed, etc. Given that, Tom's experience is of great value as it outline's the problem well enough to give some general parameters and how these parameters relate to one another in a general and a limited specific set of cases. Obviously Richard's experience and solutions add much to Tom's original post. My important point, at least to me, is that one has to install gearing that is trading off between a large # of variables, so, in the same car, one driver's optimum gearing is going to differ for another driver in the same vehicle setup.

Basically, faster (either) engine runs, the more fuel it burns.

A situation that gets the throtte (gas engine) open wider is conducive to better fuel economy, too, as it reduces pumping losses

Both are combined when NOT shifting at redline. Perhaps some recall VW's "upshift" light from the 1980s--it suggested REALLY short shifting.

__________________
Kent Christensen
Albuquerque
'07 GL320CDI, '10 CL550. '01 Porsche Boxster
Two BMW motorcycles

Engines are most efficient at low revs and high load because internal engine friction increases with the square of speed. Added to this is spark ignition engine pumping loss. Pumping loss in diesels is low because they are not throttled. Diesels use less fuel at idle and have lower BSFC at part throttle because of their low pumping loss.

At high load spark ignition engines can be competitive with diesels, but most automotive engines are sized and geared to have considerable "reserve power" under most conditions, so they end up spending 99.99 percent of their time throttled with the corresponding pumping loss.

Stationary natural gas engines are competitive with diesels in terms of BSFC. Such engines have high compression ratios (natural gas has a RON of about 104) and operate at low speed, high load running pumps and generators.

In recent years cars have acquired more gears and lower top gear revs at cruise speed. This both reduces internal friction loss and reduces manifold vacuum pumping loss to maintain cruise power. Consider that a new 7-liter Z06 Corvette is rated at 26 MPG highway - the same as my Merc - and this has much to do with its very tall overall gearing in sixth that yields about 1400 revs at 60, versus 2250 for my Merc, and the auto trans version of my car was rated three MPG lower in highway fuel economy due to both the lack of the 20 percent overdrive and torque converter loss, but most is due to the gearing.

If you have a vehicle that is "overgeared" my rule of thumb is you increase fuel economy about half the percent ratio change, so if you lower the numerical ratio by ten percent you will achieve about a five percent reduction in steady speed highway fuel economy.

However, just lowering the numerical axle ratio without changing the number of gears will result in more sluggish acceleration, and will require downshifting more often to accelerate or climb hills, and if you carry it to extreme there may be no benefit.

Years ago my late father had a '76 Vega station wagon. It had a five speed (0.80:1 fifth) and a 2.92 axle, and the engine was rated at about 84 HP@4400. Revs at 60 in top gear were about 2100. I recall driving it once heading north on I-5 out of Seattle in a stiff headwind, and the engine didn't have enough grunt to maintain cruise speed in fifth. I had to use fourth even though that stretch of I-5 is virtually level.

In an attempt to squeeze as much "fuel economy" out of cars of that era as possible many were so "undergeared" that performance was abysmal. GM recognized the mistake and bumped the standard axle with the five-speed to 3.42 in '77. My Cosworth Vega has the same five speed, but being as how it produces peak torque at higher revs than the 140 lump produced peak power, GM wisely installed a 4.10 axle, which yields about 3000 at 60 in fifth.

Duke

the newer cars with many many gears combine the bestof both worlds. good grunt at takeoff and very high gearing at cruise for economy.

tom w

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

You will never get a 20% reduction in fuel use from a 20% OD situation on a diesel engine. This has been proven by every member of this forum who changed from a 3.07 to a 2.47 on the SD. The best that any of them can document from this 24% change is about 10% or so.

In my specific case, dropping from a 3.07 to a 2.88, a 7% reduction, resulted in about a 3% fuel economy gain.

The problem with your theory is that you forget about the work that engine must perform, no matter what axle ratio is in the vehicle. A diesel engine must burn a specified amount of fuel to do a specified amout of work........and all your arguments in the world can't get around this little bit of physics.

The only benefit that you receive from a taller axle ratio is reduced engine friction based upon the slower engine speed. It takes less fuel to turn the engine at 2600 rpm than it does to turn it at 3000 rpm. However, this portion of the fuel consumption is probably 30% of the total fuel consumption on the highway. Most of the fuel is consumed to push the vehicle through the air..........and you can't produce any magical energy by changing gear ratios.

Change differentials until you are 85 years old.........but.......don't challenge the engineering of a diesel engine.........the physics speak for themselves.

brian, brian, brian. you have experience with one car and one engine type. it is a turbo motor. as you will note none of the vehicles i have done have a turbo diesel motor.

so therefore your experience will naturally be different than mine.

you are entitled to conclude whatever you wish from your observations, but personally i will not begin to think that my mileage calculations will be as precise as 3% vs 7%. round numbers make a lot more sense. 20 % engine speed reduction won't equal exactly 20%. but i have found it is very close to that in my practical experience.

your personal experience doesn't equal all the physics in the world.

tom w

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

My personal experience includes several years of working for Chrysler as an emissions control engineer. We routinely swapped axles and noted the results of same. Even with SI engines, we never achieved a reduction in fuel economy equivalent to the axle ratio change.

You've got a patronizing attitude with some poor anecdotal evidence and no data. You cannot even get your own data accurate within a couple of percentage points. By your own admission, you've rounded your data to a convenient 20%. This is simply BS.

I've got an engineering degree, several years of hard data from an automobile manufacturer, personal experience, and the knowledge of diesel engine theory.

We'll let the forum decide who to believe.

further thoughts on the 240 with 300 motor and a 307 rear end vs the stock 240 with 369 rear end.

an excellent 240 with a stock 369 rear end will be lucky to break 30 mpg running 75.

my 240 with a fresh 300 engine and the 307 rear end got 30 at 75 mph consistantly.

the 300 motor is about 20% larger and with a rear end that is 20% higher the mileage is equal to the stock 240, perhaps even a little better.

now you can put a sharper pencil to that if you wish but those figures are pretty accurate for the purposes of discussion.

i dont know what the results would be with a turbocharged engine. you have results on that, which i don't challange. i simply have made my own observations about what i have personally done.

tom w

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Just for the record, you've got two different engines with two different axle ratios.......with "similar" fuel economy. But, we really don't know the exact fuel economy because you are just estimating the economy.............as usual.

The axle ratio certainly didn't give you a 20% increase in fuel economy for that swap........did it?

By your own calculations, if the 3.69 was "lucky to break 30 mpg", then the 3.07 should be giving you over 35 mpg.......but it never even came close to that figure.

Naturally, you will probably blame the fact that you didn't get the 20% fuel economy benefit due to the larger engine. But, it's highly unlikely that the 300 gives up 20% fuel economy to the 240 in the same chassis.

brian, you take my words and twist them around to make it sound as if i said the opposite of what i actually said. is this your intention?

and while i am admittedly a generalist, in college i had to do structural calculations, using lots of algebra, etc. we learned to round to three significant figures on all calculations. this type of process is engineering. i did very well with it, thank you.

as for estimating, what does it matter to this discussion if a 21% gear ratio change resulted in 17% better economy or 18% better economy. i said that the benefit is very close to the gear change. not exactly the same.

i know exactly how to measure fuel economy. i keep track of each fill and i keep it to the nearest tenth of a gallon. i use full tanks and i fill usually in the same station at the same pump. i always let the pump shut itself off. i don't estimate it. i calculate the mileage with the calculator on my cell phone.

the 240 with the 20% larger 300 engine and the 307 axle ratio consistantly got 30 mpg running at my usual 75 mph.

the 240 with the 240 engine and the 369 gears (20% lower gearing) is lucky to hit 30 under similar conditions ( i actually run only 72 in a stock 240).

the bottom line is that the higher gear equalized the increased displacement and made the fuel economy equal. in other words both cars pump about the same amount of air through the engine per mile traveled.

now this wouldnt work if both cars began with their gearing optomized to their engines for maximum fuel economy. they are heavily compromised from the factory to give excellent all around service. in my example the gear change worked as it worked. in another situation it may not work. on newer cars which are much more optomized for economy i am sure these results would not be possible.

i checked the odometers and speedos on both cars against the highway markers. i know how to do this.

if you didnt get as good a results with your rear end swap, i have no explanation for that.

and while you may have kept strict scientific records at chrysler, i bet you didnt have those controlled conditions when you checked you own 247 gear swap, did you.

tom w

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

........yep, I twist your words alright..........

none of the things you highlighted apply to my statement.

tom w

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Gentlemen:
Sorry to step in the middle of this dart throwing contest, , but I think there's yet another variable that needs to be considered. That is the driver - IF the driver drives a numerically low differential ratio'ed car gently and not too fast, the engine won't be loaded to the point that it's being fed more fuel than it can use. (IE, the diesel car won't smoke, the gasoline car won't be running on full throttle enrichment). If the driver is mostly in a hurry and has a heavier footed style of driving, the numerically low differential ratio'ed car won't give nearly the mileage that it could be capable of, and the numerically high differential ratio will be better suited to this driving style, as the engine will be operating within it's design parameters.
A good example is a pickup truck with a small engine towing a heavy load, compared to the same truck with a larger displacement engine towing the same load. The larger engine will usually give better mileage because the engine can handle the load. I'm sure most of us have experienced that situation. My brother has a GMC pickup with the 4.3L V6, I have the same truck with the 2.8L V6. Same transmission & rear end ratio. He consistently gets better mileage. Why? Because my truck just doesn't have the poop to pull the truck in OD at faster freeway speeds, and his does.
So, there are many factors to consider... maybe Tom is a gentle footed driver.
Regards,

__________________
Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C