Mileage change with tire change?
 

I have 295/70/17D tires and am considering 255/80/17E tires. Both are aggressive "mud terrain" type tires. Do you think the difference in width will make much or any difference in mileage? They are both basically 33ish in height.

yes, it will probably make some difference. probably a little bit better since you have less tire touching the pavement decreasing your overall rolling resistance.

Should not matter with width. What matters is how tall the tire is. Just like with gear ratios, the mileage (and speedo) count based on revolutions that the VSS counts, wich is based on tranny speed, which reads after the gears, which controls the tires. After it is all said and done, the speedometer and the odometer both read off of how many revolutions the tire turns, then calculates the speed from there (either VIA the ECU or a mechanical gear). This means that if you are running a 29" tire factory and it turns 1000 times per mile, the speedo will be calculated to read 1 mile for every 1000 turns the tire takes. If you put 33's on it which only turns 800 revolutions per mile, the truck will think that it has only gone 8/10ths of a mile. This will lower your odometer reading as well as bring up your speedometer reading. If you down size tire profile, it will have an opposite affect.
Tire width has no affect on how many revolutions that the tire turns per mile, and therefore would not affect the speedometer and odometer as long as the height of the tire remains the same.

Are you talking about fuel mileage or how the odemeter/speedometer reads mileage?

Oh, whoops... I might have misread that. If talking about fuel mileage, then yes, the less weight and tire contact the better for mileage.

Hmmm.... I'll place my bet on 0.5 to 1 mpg.

The lower weight will help in town mileage (less mass to accelerate). The reduced windage and rolling resistance will help freeway.
I'm also assuming you will be running a bit more air pressure in the narrower tire, and that the two tires are of similar construction, tread pattern, and rubber compounds. That can make more difference than size.

If you're worried about mileage, go with an AT or highway terrain tire instead of a MT.

Sorry for hijacking the thread but it made me think....How DO you reprogram the computer to calculate based on the new tire diameter? I went from stock (265's) to 35's and my speedo is a craps shoot..

Sorry for hijacking the thread but it made me think....How DO you reprogram the computer to calculate based on the new tire diameter? I went from stock (265's) to 35's and my speedo is nuts..

Depends on the truck. Mustangs have a mechanical gear that can be swapped out to correct for gear ratios up until 1999, past that you have to use a speed cal box and wire it to the VSS because the 1999+ has electronic driven speedometers. If this is the case, you can also correct this through a tuner.

I checked with the dealership the other day and they wanted an hour's worth of labor, $60.

Tire diameter is programmed in the ABS module, which you can communicate with through the OBDII port. AutoEnginuity is the only non-Ford tool I know of that will calibrate it.

Very cool. I don't wanna buy something to use it once, unless it's less than an hour of Ford Tech time of course. Thank you

~ Jim

They are a very useful diagnostic tool. Mine paid for itself when I had to diagnose and replace two injectors a few weeks ago.

How much does it run BTW?