Your statement ignores the point of the question and fact that the efficiency of an internal combustion engine is dependent on RPM. Try to go cross country in 3rd gear .
My attempt is to optimize MPG at a given commuter speed of 75MPH. This speed is probably higher than what the vehicle was optimized for. (Probably 60 or something).
Since a 5th gear to lower RPM is out of question, the only remaining option is the overall gear ratio. After more than 3000 miles I know now that 12% taller tires help about 2-3%. on my MPG.

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Good Patina Comes With Good Miles Driven.
Drive more polish less

220SEB 1963 (excellent driver with no rust but far from pretty)
300 TD Wagon 1981 (Daily Commuter Tractor)
911 MFI 1973

www.vintagemotion.com

Did you correct your calculations for odo and speedo error caused by the taller tires?

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

I have never read up on the various differential gear ratios in the 123s, so reading this string is interesting. I have a simple question....

In my previous 85 300DT (sedan), I could get highway mileage of 30 mpg as long as I kept speed between 60 and 65 mph. In my current 84 300TD (wagon), I can only get 25 mpg. I realize these are two different cars, and various differences may play a role (one car may have better injector spray pattern, better compression, etc.). But if we assume all other factors being equal, is the lower fuel economy in the wagon due to different gear ratios? E.g. is the wagon geared lower because the car is designed to haul heavy loads?

Thanks,

Mark

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1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)

All 1985 models got a 2.88 rear end while 1984 and earlier turbos got 3.07.

Sedan and wagon drivetrains are identical.

Before others flame you, you have a 300D. There is no such thing as a 300DT. All 300D's made after 1981 are automatically turbo except for special imports.

Yes , I recalculated the difference in odometer reading - additionally I compared the MPG commuting the exact same trip I take since 11 years : 35 miles one way and 350Miles a week.

__________________
Good Patina Comes With Good Miles Driven.
Drive more polish less

220SEB 1963 (excellent driver with no rust but far from pretty)
300 TD Wagon 1981 (Daily Commuter Tractor)
911 MFI 1973

www.vintagemotion.com

We need a full write up thread with pics on this! I may be moving in that direction with my sonic wagon.

I put the 2.47 in my CD, along with a 4 speed. I love the more relaxed RPM, much better mileage, and quieter rpm on the highway. I can't comment definitively on my mileage improvement yet, but on the first half of my road trip, I made 35mpg!

I wouldn't do it unless you do a good bit of highway driving. If you do though, the swap is well worth it!

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My Primary Driver - '85 300CD - 4-speed conversion, 2.47 rear, lowered, euro headlights, rebuilding (not restoring so much)

Wife's - '08 Saab Sportcombi Aero

Riding a '03 Yamaha Warrior