Any difference in MPG between 2.88 and 2.47 gears?
 

I am reading that my 85 300D has 2.88 gears. This still seems too high (ratio number) to me. Has anyone swapped in a diff out of another car with 2.47 gears. Is this worth the difference?

I have 2.47 with a manual. it makes a difference taking off. It is basically the same to me as taking off in second. I think with an automatic trans it would not be an issue. on the highway I turn around 2000 RPM at around 60. my speedo is off so this is my best guess. my milage jumped about 3-5 MPG depending on the mix of driving highway and in town

lutz, you went from 3.07 to 3.47 though right?

I have the same setup, and that is a very accurate description of the feel.

I have a GPS speedometer, and it's about about a 12% difference in the speedo, meaning 63 on the speedo is about 70. On your odometer, it is about 17% off, for every 100 miles on the odo, it's really 117 mi. Doing the math, I was averaging about 28 mpg/hwy before, and I averaged 35 with the tank I have burned through since...all highway. Since that is only 1 tank, it's not an accurate mpg rating...I'll have to get back to you after a few more to get you a better idea.

All in all, if you do a lot of highway driving, I would go for it...otherwise, it's not going to help, might even hurt in the city.

yes, but I was just explaining the 2.47. I dont know what a 2.88 would be like so I did not compare the before and after just the feel of the 2.47 which would give some indication of the final result. but you are right, the milage comparison is indeed the difference from 3.07 to 2.47.

I am setting up this car to make a 140 mile highway commute everyday. I know I already have the 2.88. Just don't know if 2.47 is "that much" different. My wife will drive this car and she will drive approximately 65 MPH on the highway. It would be nice to get the RPM's down to 2500ish at 65 MPH. I feel that 2500 RPM probably will deliver the best fuel economy vs. speed achieved. It would be nice to get 30+ MPG. I have a feeling that diesel will be $$$$$$$ in the near future.

With a commute like that the taller gears will help. You may also want to look into taller tires, they're a bit easier to install than a new differential.

go for a 2.24 diff if you want better mpg. I'm not sure but I dont think the 2.47 was offered in a housing that will fit your car.

2.88 offers better highway mpg's compared to the other years or so im told

Keep the 2.88. At 65 mph, it'll be turning about 2650 rpm with 205-70-14's or 205-65-15's.

The change to a 2.47 won't yield any better economy (as proven by others) and the driveability suffers significantly on upgrades.

If you want 30+, tell her to slow to 60 mph and advance the IP timing to 26 BTDC by the drip method.

Agreed. The 2.47 is to lower the RPM of those big V8 gas engines.

Thanks!! This all good information. What is the factory IP timing set to? Does advancing to 26 BTDC change the performance of the engine at all? My engine is very peppy now (thanks to ALDA adjustment and getting turbo boost from 7psi to 11 psi). I still would like to have decent "off line" take off, but on the highway achieve optimum MPG.
I think getting her to drive 60-65 MPH will not be hard, especially once I get the cruise control working (another project i am figuring out!)

Unfortunately I just bought new rear tires. I think they are 195/70/14.
I should have looked to see if they had a 195/75/14 to lower (numerically) my overall gear ratio.

Thanks for all the info!!

Top speed people! When I get the 5 spd together (which is put off by the Motoroam) I'll set some W123 OM617 land speed records :)

maybe not without some motor mods...... my final drive ratio is about the same as 5th in the 5 spd when I am in 4th with the 2.47 and it starts losing its pep around 95

Its all in the pump.