How much will tire size change affect speeedometer

[Mr4spd@work]

I am replacing my 245/75/16's w/285/75/16's. How can I make sure the speedo reading is correct after this change? I have a 99 2500 4x4 w/360 5-spd that I am doing this to. Anybody have any answers? Thanx in advance.

[gthomas]

Believe it or not each tire brand can be a little different (not always). The best way is to figure out the difference in circumference and divide that difference by the larger tire circumference. It will give you the percent difference. I changed to 265/75's tires on my 99 2500 4x4 and it changed about 3.846%. I checked it with my GPS and I was doing 65 when the speedo read 62 1/2 mph. You'll need to correct your speedo a lot when you go up 4 sizes.

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1972 340 Duster
-Dyno @ 420hp/440ftlbstq
-Built 727 w/3000 stall
-3000lbs
1999 2500 4x4
-V10 w/5sp
-Manual xfer case