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.

The same diesel engine will use more fuel running 3000 rpm vs 2500 rpm, going the same mph.

All the other stuff stated will also factor in.

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RRGrassi


70's Southern Pacific #5608 Fairmont A-4 MOW car

13 VW JSW 2.0 TDI 193K, Tuned with DPF and EGR Delete.

99 W210 E300 Turbo Diesel, chipped, DPF/Converter Delete. Still needs EGR Delete, 232K

90 Dodge D250 5.9 Cummins/5 speed. 400K

Gone and still missed...1982 w123 300D, 1991 w124 300D

I couldn't imagine only getting 20 mpg.
I think the worst I have ever gotten is around 25 in town
with a lead foot.

I think I would look into the maint. things too
Brakes dragging, injectors, tire pressure, etc.

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81 Mercedes 300SD 289k.......SOLD
82 Mercedes 300CD 252k......slow ride

82 mercedes 300 SD...mi Unknown
83 Mercedes 300D ????ksniff..gone too
84 Mercedes 300D 148k........SOLD
85 Mercedes 300TD 386k and holding some one elses project

My 300D was assembled in 10/81. It is classed as an 82. It has the turbo.

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RRGrassi


70's Southern Pacific #5608 Fairmont A-4 MOW car

13 VW JSW 2.0 TDI 193K, Tuned with DPF and EGR Delete.

99 W210 E300 Turbo Diesel, chipped, DPF/Converter Delete. Still needs EGR Delete, 232K

90 Dodge D250 5.9 Cummins/5 speed. 400K

Gone and still missed...1982 w123 300D, 1991 w124 300D

My '81 TD is most fuel efficient at about 50-55 mph--that's a little unrealistic for highway driving. Nevertheless, speed has a significant impact on mph.
RPM's definitely impact mileage, diesel or gasser--you can sit in your driveway revving to 3500rpm for 10 minutes and burn a lot more fuel than at normal idle.

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1999 E320 4matic really nice Wagon 135k
1981 300TD loud, rattling family car 180k (TGI sold!)
1985 500SEL gas guzzler 220k (sold)
1978 300D my first Turtlediesel (sold)437k

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