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All the calculators i have used on the internet say the stock 275/70R18E tires have a Rev per mile of 607.49.
How did you get 626?
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Ford Truck Source Book
Or you can usually get it directly from the tire manufacturer. BFGoodrich includes tire revs/mile in their specs. But Goodyear doesn't.
Or a back door is to get it from tirerack.com
then click on "specs" for the tire you want. Note the tire revs/mile for LT275/70R18E is:
Wrangler = 630
Michelin = 627
Conti = 628
(Make mine the new Michelin LTX A/T. Expensive, but I'm worth it.) /ubbthreads/images/graemlins/cool.gif
Tire size calculators assume the numbers on the sidewall are exact. But they're not. They're not exactly 275mm wide or 70 percent profile. So if you need accuracy, use the manufacturer's revs/mile instead of a generic calculator.
You can use a generic calculator to get in the ball park, for example to compare the revs/mile of different sized tires. But if you want it accurate enough to, say, calibrate the speedo, then you need the actual.
After you have the new tires on your truck, you can calculate your actual revs/mile with a GPS or by using interstate highway mile markers. Set the odo to zero with your front bumper even with a mile marker. Then drive 10 miles down the highway to another mile marker. Every tenth of a mile off from 10.0 miles is a one percent speedo error. If you want closer than one percent slush, then go 100 miles instead of only 10 miles. My speedo is now 99.07 percent accurate = three tenths of a mile error in 100 miles. /ubbthreads/images/graemlins/warmsmile.gif
Be sure the mile markers you choose are not near an entry, exit, overpass, or underpass. If the highway crew can place the marker where it belongs, they will. But if not, they put it close. And close is not close enough for me. /ubbthreads/images/graemlins/shocked.gif
The generic calculators all said my LT295/75R16 tires were less than 610 revs/mile. But the 100-mile highway test showed they were actually 626. And the manufacturer's specs now show 626. /ubbthreads/images/graemlins/warmsmile.gif