Mileage Data from Differential swaps?

[crazyfastman]

Does anyone have any mileage figures from before and after changing differential ratios. I'm thinking about swapping in a 2.93 325es differential, but I want to know what kind of mileage gains I will get first. Any help is appreciated! Thanks!

[zboot]

I don't have hard numbers. But if you're going down to 2.93, then your city mileage will go down but your highway mileage will go up. If you went the other way (for example to a 3.73 or 4.10 diff), then the reverse would happen.

Where you do most of your driving will govern your actual experience. If you have a balance of city and highway, you probably won't notice a difference.

[crazyfastman]

I have a 50/50 mix of city and highway. By decreasing the diff ratio, I figured 1st gear would become more usable and lower my engine speed on the highway to yield better mileage.
zboot, could you explain why city mileage decreases with a 2.93 vs. the stock 3.45?
thanks

[zboot]

If you look at the torque/rpm curve for our engine, you'll see they have a sweet spot around 4500rpm.

So, your car is operating at it's most efficient when your engine rpms are at that speed. With a stock diff, you're engine rpms are higher at any given speed than with a 2.93. So, in the city (where you typically drive a lower speeds) you're more likely to be at the sweet spot with the stock diff and get the best mileage.

Now, when you're on the highway, you're typically cruising at higher speeds. With the stock diff, beyond about 65mph, you're rpms start leaving the sweet spot and your car becomes less efficient. If you typically drive between 65-80mph on the highway, you'd be more likely to be within that sweet spot (or close to it) than with the stock diff.

[crazyfastman]

Update: I've done the swap and gone through a few tanks of gas. I get about 31-33mpg now. I got about 28-31mpg driving at 65mph on the highway before the swap. The swap is nice because first gear lasts long enough to be useful and I can cruise between 70-75mph without having to worry about worse mpg. I get the same mileage at 70-72mph as I do at 65mph. Also, the LSD feels nice while accelerating through curves.
If I was going to do it all over again I would go with a slightly higher gear ratio, somewhere around 3.15-3.25:1. I think that should yield a little bit higher rpms (re: power) at speed without sacrificing much mileage, but 325es differentials are ultra cheap and available, so whatever.
Hope this helps some of you guys.

[angel318ti]

good info and great gas mileage.

[thesk8nmidget]

i got about 33-36 with my 2.97 and a max of 38.8 but city driving was not as fun.

not as peppy.