I'm fairly confident that the 4 speed auto's and 5 speed's had the same ratios throught the 928's life, and that the final drive was the only difference. Is there a noticeable fuel economy difference between the 2.20, 2.54 and 2.73 final drives in the 87-92 years?

 

Chris,
You may be right about the 4 spd. autos having the same ratios. The 5 spds changed ratios slightly. GT's are different than the S4, and also different than the GTS even though the GT and GTS have the same final drive ratio. There is a noticable difference in fuel economy with the change in final drives. The 2.2 being the best.

 

When I bought my 89GT, we drove it and an 84 5spd back home from Georgia. At every fill up the 84 averaged 3-4 mpg better. (2.20 vs 2.73) IIRC, the GT tacked 1-200 rpms higher at same speeds.

 

I have a computer program called "Fuel Economy Calculator" by Performance Trends Software. The same company that has Engine Analyzer Pro engine simulator. It's a comprehensive fuel economy calculator with inputs for many vehicle parameters starting with detailed info on the engine, then drivetrain, tires, car itself, Cd, etc.

Plugging in all the numbers for an S4/GT, I get these mpg figures for the different final drive ratios for the 5 spd trans. at 70 mph.
2.73, 21.23 mg
2.54, 22.06 mpg
2.20, 23.52 mpg
Just for fun, I checked the 3.09 and it would give 19.6 mpg.
The numbers seem very close for my '87 S4( 2.20), but I could never get to 20 mpg with my GT, may be the different cam, or because I seldom drove at 70.

The auto trans S4 should get, @ 70mph, about:
2.73, 19.16 mg
2.54, 19.55 mpg
2.20, 21 mpg

 

I do almost all city driving, and hard city driving at that....and to be honest, i doubt i get 10mpg with my 2.2 rear.

 

Figures don't seem to far from factory claims:

 

Wow, I always thought everything else being constant that the 5 sp would give better fuel mileage.

 

Looks like they got the data for the different trannys reversed. It wouldn't be the first mistake in factory published 928 data.

 

Typically the 4 speed autos are rated better in town mileage than the 5 speeds because they start in 2nd gear when not driving hard. Look at the EPA mileage estimates on window stickers....autos are better in town vs the 5 speed (but worse on the freeway) my 88 is rated 16-21 vs 15-23 for the 5 speed? The best mileage I have seen is about 21mpg (but I drive fast)

 

Maybe I misunderstand final drive ratio - shouldn't the auto and manual have the same gearing in top gear? Or does this include an efficiency loss for the torque converter?

 

wow, 30mpg at 70mph. ill have to try that out some time. maybe someone can drug me with a seditive and ill do a road trip. singing "i cant drive, seventy-five" on hyway 5!

Mk