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

[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

[t walgamuth]

i like the five speed idea. it isnt that hard if you get all the parts. it was offered in the 300e but prob not all years. they are rare as hens teeth though. prob hard to find in salvage yard as folks put them in when their automatics eat it.

tom w

[MikeF]

Thanks guys. I love my Benz, but although it's in amazing condition and runs like new, it's still a $2500 car at best. No tranny swaps for me.

Duke...great advice. I'm running my tires at 35 cold now. Maximum is 44. I won't go that high, but may add a few psi's to see how it helps (38 ish).

I alternate between mid-grade and premium at fill ups (89/93). Car seems to run just fine on either with no noticable change in mpg. When the weather cools off, I may try running some 87 as well.

Plan B will be to pick up a 3 year old 5-speed Corolla for the commute. I suspect I'll hate it, but they get close to 40 mpg highway pretty consistently. It'll also help preserve the MB.

Better yet, why don't we have a national boycott on gasoline? If the entire country goes 3 days without buying a drop of gas, I'm betting prices will plummet. As of now, we're not in control....they are!

[manny]

Just to add a little " Ray of Hope " to your situation........I don't believe this pricespike will last very long.

[Kestas]

Originally Posted by MikeF

Better yet, why don't we have a national boycott on gasoline? If the entire country goes 3 days without buying a drop of gas, I'm betting prices will plummet. As of now, we're not in control....they are!

All that'll do is increase the buying to the days before and after the boycott with no net effect.

Plus, this'll mean that women have to give up their shopping for a few days. That won't fly too well with them. I've always considered shopping a significant waste in gasoline.

[manny]

Originally Posted by Kestas

I've always considered shopping a significant waste in gasoline.

Kestas

Don't paint all women with the same brush.
It depends what they're shopping for.
I don't mind my wife using the car to buy me presents or beer.