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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