I have a 94 F-250 with a factory ATS turboand 33" tires. I am currently running 4.10 gears with a zf trans and the truck seldomly carries more than a couple of dirt bikes or a ski-doo. Is it really that big a boost to fuel economy to run the 3.55 gears? My father who I bought the truck from swears he will never run 4.10s in another truck again due to fuel economy. He now runs a 94 P.S. 3.55 and loves it. Are the benifits of running lower gears that great. And will it decrease my need for constant shifting.

 

I like my 355 gears, but it does make the take off a little sluggish.

 

With 3.55s you'd probably still be shifting the same amount - just more downshifting on hills vs. upshifting to get to freeway speeds.....

 

I have never had to downshift in this truck. Never had enough of a load. And how much does tire size effect Fuel economy.

 

Go to 3.55 gears and I'd bet that will change....

Tire size affects mileage in much the same way that gear changes do - only in reverse. Taller tires have the same effect as numerically smaller gears. Going from a 30" tall tire to a 34.5" tall tire has about the same effect as going from 4.10 gears to 3.55 gears for example. The only difference is that with the bigger tires you have more weight to lug around, so they'd affect the mileage and power a tiny bit more than changing the gears.

Bigger tires = more distance traveled per revolution of the engine = better actual mileage (though your speedo will be off and it will look like the mileage has gotten worse due to the speedo showing you traveled less miles than you actually did).

Smaller diameter tires = less distance traveled per revolution of the engine = worse actual mileage (though it will look like it improved due to the speedo showing more miles than you actually traveled)

 

What about wind drag. And a stock 31.6" 23585r16 with 3.55 is like 36.5" 4.10 gearing. With 34.5" tires and 4.10 I am running 2000 rpm @ 100km/hr and 5lbs boost. Getting me 15-16 mpg. I want more. What will it take? New pump and injectors already in.