what kind of a difference in rpm change at 75 mph i would see with a 265x75x16 tire

Well if I remember correctly the difference is 3% between the 245 and the 265. So just calc the diff and you should be good to go.

That will be about right. Remember that if you don't recal your speedo, that it will still show the same RPM for the speed that you are registering. So, you just have to know that you are going a little faster than what is posted on the speedo. The RPM will be correct, but speed will be off my 1.5 mph.

70 mph @ 2200 rpm......for me-

tire circumference for 265/75r16: 99.4301706 inches
for 245/75r16: 95.7196281

ratio is 1.03876, or 3.9% approximately
http://www.car-videos.net/info/tirespecs.asp

The speedo reads about 2 mph fast with the stock 245s so don't worry about re-calibrating; the 265s will put your speedo almost exactly correct.

I never bothered to recalibrate mine cuz it is that close

That 'interesting'. With 265s, my speedometer is 2 mph fast per GPS calibration at 65mph.

I have 265's went up north and hit a lot of them radar speed sign's and them all showed the speed speed that my speedo showed with no correction. Go figure? :cool: JD