M40 Mileage

[twright]

I hate to ask questions about fuel mileage, but let's see what happens:

I don't drive my M40 powered Touring much on long trips, but the last one was done as an experiment: I removed the 4.27 differential and replaced it with a 2.93 one. I drove 200 miles at 75mph and got 27 miles/gal.

I have highway driven this car one other time at highway speeds and got over 30 mpg with it, but I wasn't going as fast. This was with the 4.27 gearing.

I was disappointed with the mileage. Is the decrease due to the too-high gearing, or due to the higher speeds?

YMMV.

[bmwman91]

Lower RPM is not always better for fuel economy, actually. There is an optimal RPM (from an efficiency standpoint) inherent to an engine, where a couple of things are balanced against one another. The higher the RPM, the faster the power stroke is, meaning that there is less time for heat to be lost into the water jacket, so more of it gets converted to useful work by increasing cylinder pressure. At the same time, as RPM increases you increase the parasitic losses from pumping the oil & coolant, spinning the alternator, belts, etc. So, the optimal RPM is basically the point at which the sum of those losses is minimal. For a stock M42, I think that the optimal RPM is in the neighborhood of 3500RPM, which corresponds to ~65MPH in 5th gear with the stock 4.10 rear (just a guess based on experience and the fact that the car was designed the way it was).

Now, if you just decided to go faste rot match the previous cruising RPM with the higher gear ratio, then that's your problem. Losses from air drag increase with roughly the square of your velocity (air flow past the body is probably not 100% turbulent), so you could really only run a comparison of diffs if you had been going the same speed on the same highway both times.

[mjweimer]

There is a lot at play when comparing fuel economy numbers. As mentioned above, the conditions need to be very similar in order to make note of the real difference. I think the biggest question is whether the reduced ratio makes the highway driving more reasonable...in other words, is the car more comfortable to drive at highway speed. My experience comparing the 3.46 in an E46 touring to the 3.15 of the sedan (5MT) is that fuel economy is similar but the reduction in revs makes it much less busy on the interstate.

[roguetoaster]

There is a lot at play when comparing fuel economy numbers. As mentioned above, the conditions need to be very similar in order to make note of the real difference. I think the biggest question is whether the reduced ratio makes the highway driving more reasonable...in other words, is the car more comfortable to drive at highway speed. My experience comparing the 3.46 in an E46 touring to the 3.15 of the sedan (5MT) is that fuel economy is similar but the reduction in revs makes it much less busy on the interstate.

I was going to echo the views on peak efficiency and add something about load when making small speed corrections, but that is a very valid point when these are not daily drivers. Ultimately, the M40s I've had have been a bit stressed feeling at 70MPH with a stock rear end, and a reduction in ratio would likely be more comfortable.

[twright]

While the mileage was less, the enjoyment increase was phenomenal. The engine was at 2500 and mostly silent.

If I wasn't afraid of the jail time, I would be comfortable in this car at 100 mph all day long.

[TobyB]

...then maybe stick a 3.46 in it and be happy most of the time? The 3.46 came in the 318ti, and bolts into the E30. It's not a bad match to the M42 and 1:1 5th of the G220

The ti can easily beat 30 mpg as long as i'm not trying to go over 75 mph.

t