Question about oversized tires

[T-Town-Z]

I have a "math" question of sorts about installing oversized tires. I switched out the factory Goodyear SR-A tires on my truck for Bridgestone Dueler A/T. The SR-A's were P275-55-20 measuring 31.9" tall. The Duelers are LT275-65-20 that measure 34.1" tall. That makes the new tires 2.2" taller.

I'm no math wiz, so can someone tell me how far off my speedometer is at say 65mph ? I'm guessing I'm actually going faster than what the speedometer is showing ?

Now, along that line of thinking, I'm guessing that the mpg reading on the dash is off by the similar percentage ? Seeing that it is now less than what I was getting before the tire swap, I'm thinking that my mpg may actually be better than what it is now showing ?

And finally, what is the easiest way to get the speedometer re-calibrated, and will it show as a modification to the computer which may cause warranty problems ??? Thanks

[8.1HD/ALLISON]

At 60 mph you are actually doing 64 mph.

[Horhay7]

Hey Andrew, just wondering, did you have to do a level to get those to fit? If not, no rubbing at all? I'm thinking of going to that size or possibly 275/60R20 up from the original 275/55R20 but would like to avoid a level if possible.

[T-Town-Z]

Thanks for the mph Matt.

As for the level, I did put a 2" lift spacer under the front struts. It's just enough to clear everything with a stock 20" rim, with just minor rubbing of the "felt" covered wheelwell liner. I fixed the rubbing by drilling a hole in that area, through both the liner and the frame bracket behind it, and then used a zip-tie to pull the wheelwell liner back about 1". (see photos) It was enough to fit a 34" tire !

[T-Town-Z]

The zip-tie was a quickie job where I wanted to get it pulled out of the way to keep it from rubbing. When I get a chance, I'll get a decent looking bolt and large washer, and upgrade it from my zip-tie.

Been wanting to wash the truck to get a nice pix to post but been busy. Here's the one I took right after getting the new tires the other day. They are Bridgestone Dueler A/T's in LT275-65-20 and with the 2" lift up front.