I was driving around on the highway today with my new tires, and noticed that I was passing everyone on I-91 going 65 mph. That kinda threw me off, then I found out that my spedo was off 5 at 30 mph, so it said I was doing 30but I was actually going 35...
Is this normal when going from a 255/65 to a 285/75? Bad for the tranny?

 

Normal, definitely. You increased the diameter of the tire by almost 2 inches, although it sounds like your speedometer was off before. Difference should be about 3.75mph. Bad for the trannie, no. It can be corrected. Do a search in the trannie forum. I think yours needs a different VSS gear in the trannie to work the speedometer correctly. Not sure what year they went to programming the PCM for the speedometer. I know that is true for 04.

You can see the difference by going here.

 

RIght, the 97-98's can be corrected with a new speedo gear.

 

summit has the gears for are trucks they got 3 or 4 diffrent ones cost around $18 quick easy correction you can easily do it your self just replacing the gear on the speedo cable in the tranny takes like 10min


not sure what gear u need for 285's though sry but some one will know not sure what one im gonna need ethier sence i regeared and went up in size

 

Thanks alot for the advice guys. I feel alot better knowing my tranny is going to be okay. The speedo doesn't bother me too much, just a convienence thing really.

By the way, does anyone know where I could get skid plates for my engine, tranny, particularly to shield my starter from mud?

Thanks.

 

you can also take your truck to a ford dealership and get your speedometer, rpm gauge, and your odometer recalibrated. if you get bigger or smaller tires your odometer will also be off. i think it would be around $90 to do all of that.

 

But for an extra $10 or so, get a new recalibrator from Ford Motorsport. Got mine from ebay, $100, just haven't actually installed yet. I figure going from 265 to 285 wasn't much of a jump, but if I want to change the gears, or something else like rims and taller tires, no matter what it covers ya. All you have to do is figure out how much you are off, set it's switches to the corresponding charts in the instructions and wammo, you're sailing boy! Of course, get the right one, t99 for 2000 and older, t01 for 2001 and newer.