Gas Saving Idea - Gearing

[MikeF]

Over the summer, we moved to a new home (on the lake ). I went from a 7 mile commute to an 80 mile commute (each way ). Fortunately it's an 80mph run the whole way. Around this time, as luck would have it, gas prices skyrocketed. I'm now spending about $30/day on gas for my beloved MB. I can't bring myself to part with a nearly perfect MB in favor of a hybrid japanese rollerskate. So, here's my question:

Is there a way to significantly reduce the rpm's at freeway speeds? Either through a lower ratio differential, or regearing the transmission itself. In the case of a lower ratio diff, what's the lowest available and how much effect would it have? Any idea of cost? Obviously low-end torque would go out the window, so can the transmission be modified for 1st gear start?

Car is an 89 300E with 168k on the clock. Oh yes, Donnie Drummond is my mechanic (just lucky I guess). Will give him a ring this week to discuss also.

[Ali Al-Chalabi]

Easiest thing to do would probably be to put a lower ratio differential in. It would probably make at least a couple of MPG difference, I would imagine. Especially since the car is geared with such a high engine speed on the highway. The 722.4 tranny also has a 1:1 4th gear ratio and no torque converter lockup clutch, so that doesn't help a whole lot either.

[raymr]

Drive slower - wind resistance is a big mpg eater. Shouldn't you be getting around 25mpg on the highway as it is? Taller gearing might actually put the engine under a constant load which results in a rich fuel mixture. I'd try to find someone who has already done this before spending the bucks.

[MikeF]

The best I am able to achieve is ~23 mpg. Driving slower (70 ish) doesn't seem to make a difference. Open to suggestions!

[lyonstexas]

Could put a set of monster truck tires on the back.

[t walgamuth]

rear end will help. i have no personal experience with your specific model of car but most of the times that i have changed rear ends it is pretty much a numerical difference. ie if you now have a 3.69 rear end and you put in a 3.31 then divide the difference and find a percentage, in this case you would be around a 10% reduction in engine speed and if you were getting 25 before the change you would get around 27.5. you will also reduce engine noise and wear by the same amount. i have changed the rear ends in at least six vehicles and the only one that the improvment was not clear in was my 81 280e. all others were much as i have described.

tom w

[t walgamuth]

you do you need to change the speedo too. oh,.. on that model i dont know if that is what needs doing or not. on the 123s you change the speedo along with the rear end and all will work out.

tom w

[MikeF]

Not sure why speedo would need to be changed. Rolling diameter of rear tires is unchanged.

Tom,
I would think that low-end torque would really suffer. Was this your experience?

Is there a way to implement 1st gear start? My car starts in 2nd gear unless you stomp on it (or downshift manually).

[t walgamuth]

of course low end torque at the wheel is affected... that is the trade off. once rolling though you just hold the gears a little longer and the effect is not seen again til you are in top gear.

on the starting in second, yes there is a way to make it always start in first. you used to be able to order a kit for it i believe.

if you are fond of jack rabbit starts or want to take your car to the drag strip chang ing gears is not for you. if you want to save fuel it is about the most cost effective thing to do. and unlike some other things it works.

tom w

i have changed the following: 52 pontiac ambulance, 84 suburban, 85 suburban, 83 240d with 300 non turbo d, 81 280e.

[MikeF]

No dragstrip for me. So, is the swap a DIY project, or better left to the pro's?

[t walgamuth]

my 62 fintail about 30 years ago when i was young and full of energy (and other stuff). it was a lot of work. i also did the pontiac swap myself because all you had todo was swap the hogs head. i believe i could still do one of those in about an hour. but all the others i have paid to have done. my indie charged me about 300 i think. the swap of the speedo is about a 20 minute do it yourself thing.

tom w

[Kestas]

Before this spike, when you moved, you were probably looking at paying $23/day for gas. So now you're paying just a little more. I doub't there's anything you can do to your present vehicle to save gas. It's optimized as it is.

You should probably seriously consider getting a "japanese rollerskate" as a commuter car. Not a hybrid, though. Hybrids are designed for city driving, not for steady highway driving.

[t walgamuth]

with the above post. mb has provided a car with what they consider the most reasonable compromise for economy/ performance. you can change the compromise to emphasize it more toward economy. you just have to know going in that you will give up acceleration in the bargain.

as for hybrids, the prius and the insight are more than just an engine package. note the very compact and aerodynamic shape of particularly the insight. that car will use less fuel going in steady state too. 67 mpg aint bad. but in comparing that to a rabbit diesel, at 50 something, the picture blurs. but if you are doing all city stop and go the honda will beat the pants off the rabbit (golf, yeah i know, im showing my age again).

my $.02

tom w

[87tdwagen]

Note, you really need to LOVE the car to do this, it will eat u time and a fair amount of money, but if you are really into keeping the car, you will benefit in the long run.

The only real way to extract more economy out of you car is to change the gearing throughout the drivetrane. This means sourcing a MB 5 speed that will mate up to that engine. Luckily these should be readily available since MB installed them in many cars worldwide.

You will need the Transmission, bellhousing, pedal box, or maybe just a clutch pedal and master cyl. slave cyl, and normal clutch related parts. You may also need a new driveshaft, but again OE MB parts can be had. This is probably out of scope of most DIY types, if you feel comfortable around transmissions etc. then plan on a few days and lots of beer to get you through.

The payoffs can be awesome in terms of BOTH performance and economy. I have not done a MB yet, but I did this about 9 years ago with my Jag XJS V12. These cars only cam with GM400 3 speed autos, no manual trannys offered. I was getting 18mpg at best on the highway, and this was with the US spec 2.88 rear end...not good, let alone this 4500lbs pig was not quick off the line. I custom made a bellhousing and adaped a TREMEC TKO 5 speed in the car. The tremec has a .68 OD or (5th gear).

With the better gearing, the performance of the car improved from 7.8 0-60 stock, to a now 6.1 time. I shaved 2.5 seconds off of the 1/4 mile to a respectable 12.92. And to top it all, I saw and increase in economy to 30mpg!!! At 80mph, this car turns 2000 rpm, barely making an effort.

Now mind you, after all that work, I getting burried in that Jag

[Duke2.6]

Originally Posted by MikeF

Over the summer, we moved to a new home (on the lake ). I went from a 7 mile commute to an 80 mile commute (each way ). Fortunately it's an 80mph run the whole way. Around this time, as luck would have it, gas prices skyrocketed. I'm now spending about $30/day on gas for my beloved MB. I can't bring myself to part with a nearly perfect MB in favor of a hybrid japanese rollerskate. So, here's my question:

Is there a way to significantly reduce the rpm's at freeway speeds? Either through a lower ratio differential, or regearing the transmission itself. In the case of a lower ratio diff, what's the lowest available and how much effect would it have? Any idea of cost? Obviously low-end torque would go out the window, so can the transmission be modified for 1st gear start?

Car is an 89 300E with 168k on the clock. Oh yes, Donnie Drummond is my mechanic (just lucky I guess). Will give him a ring this week to discuss also.

Engines operate most efficiently at low revs (minimizes internal friction) and high load (low manifold vacuum), which minimizes pumping loss - as long as the engine is operating in closed loop mode and you don't get into WOT enrichment. Your car has a 3.07 rear axle and the OE tires are speced at 837 revs/mile, which yields (837 x 3.07) about 2570 revs at 60. Later 300Es have a 2.87 axle, and I believe this was implemented with the first gear start to mitigate the loss of initial acceleration with taller gearing.

AT BEST you could expect about a five percent improvement with a ten percent taller gearing, and the car will have slower acceleration performance. If you run the numbers to determime how long with would take to recover the cost with about a 1 MPG improvement you will be surprised. Basically, it's probably not worth it, and you will have to have the speedo recalibrated if gearing or tire revs/mile changes since the speedo keys off driveshaft speed.

One thing you can do to improve mileage is increase tire pressure - up to the maximum placarded on the tire sidewall and use low rolling resistance tires in the OE size. Wider high performance tires will reduce mileage due to both greater aero drag and higher rolling resistance. Also remove excess weight like excess gear in the trunk.

Big V8 engines can be geared to run at 1500 @ 60 to achieve good highway cruise mileage, but there's a limit to how tall you can gear a 3 liter engine before performance becomes unacceptable. A six liter V8 at 1500 produces about the same amount if not more power as a 3 liter six at 3000 because torque is basically proportional to displacement, and V8s typically have better low end torque characteristics than the M104, which is a revver.

Most 103 engines will operate without significant detonation on 87 PON gasoline, especially in cooler weather, which will reduce your fuel cost about five percent - as much as ten percent taller gearing, and there is no upfront cost. (Suggest you search on this subject as there have been MANY discussions.)

Driving slower also reduces fuel consumption since the power requirement increases with the cube of speed, so driving 65-70 even if you can roll at 75-80 will reduce fuel consumption about ten percent.

You can check the EPA website for the EPA fuel economy numbers for your car and later 300Es, but it sounds like, given your driving habits, your mileage is about what to expect, but there are several actions you can take to improve it.

Duke