big tires
 

My boy is wanting to put P265 tires on his 99 s/cab 4X4. He's got a superchip that he says will reset the computer to make it run right. I've always heard that if you put too big of a tire on, it will mess up the computer and screw things up not to mention making the speedometer read in error. Am I wrong?

What size tires are on it now? How many issues it will cause depends on how much bigger you go.

computer problems i dont think will be an issue, however his speedometer will be off

dont think it would be out too much.. i went from a 255 to basically a 305 (33") and mine is only off approx 4-5km/h... but it guess it also depends what hes running right now..

i wouldn't see it being much of an issue though.. 265s are larger yes.. but not huge. Theres lots of guys on here that run 265s

I believe 225's are on it now. I'm not sure what size came on it from the factory. It's a 99 4X4 supercab 4.0.

On my 01 4X4 is came stock with 245/75's. I went up to a 256/75 and it changed my speedo to being 2.5-3 MPH's slower. So the speedo was saying I was going 70 and in reality I was going 73 or so. Check out this site to calibrate his tires. It might help, I hope! Here's the site...Tire Calculator It doesn't help in how faster the truck is going with the bigger tires on but it does say how many revolutions per mile the tires spin. I had to use this conversion to update my computer when I put the 265's on. I have an X3 and you can change the RPM setting for the tires to correct your computer/speedo. However I am not acquainted with the superchip one and how it can correct tire sizes.

i have a 99 xlt 4x4 ext. cab 3.0 and stock is 235-75-15 on mine, i have 33 12.50s on it now and mine is only off 4-5 mph but mine has 410 gears stock

the rear axle gear won't change your speedometer, you'd have to change the gear on the trans or speedo cable. the rear axle gearing is more for getting your truck back to where it was performing and near stock rpms at speed.

the tuner should have an option as others have said where you can change the speedo or computer to compensate for larger tires.

you will be fine with the superchip programmer to adjust the speedo. to compensate for oversized tires.

i run a superchip programmer on my 2006 FX4 w/ 265/75R16s and everything is perfectly fine.

changing axle gears will actually change (compensate if matched to your new tire size) your speedo, but that is not a small undertaking.