Finding the right wheel circumference
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5th Wheeling
Joined: Jul 2018
Posts: 34
Likes: 4
From: Pinson Alabama
Finding the right wheel circumference
Iv replaced my stock 20 inch rims with 2020 raptor wheels 315/75/17. An have tried
a few different circumferences on forscan none seam to get me close to right.
my speedometer is still 10 miles off. Any have an different numbers for me to try.
please help me out
thank you for any an all help
a few different circumferences on forscan none seam to get me close to right.
my speedometer is still 10 miles off. Any have an different numbers for me to try.
please help me out
thank you for any an all help
Fleet Mechanic
Joined: Jul 2005
Posts: 1,464
Likes: 103
From: Staten Island, NY
You could use a tape measure to measure the circumference or you could put a chalk mark on the sidewall at the 6 o'clock position and on the ground, roll the truck forward until the mark is again at the 6 o'clock position and measure the distance.
If you can find the exact tire you have listed on Tire Rack it should list all the specific dimensions. If it gives "revolutions per mile" get out your calculator and divide that number into 63,360 and that will give you the answer.
If you can find the exact tire you have listed on Tire Rack it should list all the specific dimensions. If it gives "revolutions per mile" get out your calculator and divide that number into 63,360 and that will give you the answer.
Moderator
Joined: May 2005
Posts: 8,224
Likes: 1,236
From: San Antonio, TX
You don't need circumference, you need diameter. 33.25 got my speedometer perfect. Although the tires are spec'd at 34.4", that's a fully inflated tire with no load. Once it's mounted on the truck and the weight is on the tires, they squat somewhat. Now, when rolling at highway speeds they will increase slightly due to centrifugal force, but they still won't be 34.4" tall. Some call them 35s, but they aren't. Even then most 35s spec at 34.5"
Check it with an app like Speedbox to confirm.
If you really need circumference, 33.25/3.14 will get you there.
Check it with an app like Speedbox to confirm.
If you really need circumference, 33.25/3.14 will get you there.
Lead Driver
Joined: Mar 2010
Posts: 5,505
Likes: 256
From: Woodbridge, Ontario, CA
Assuming your truck was originally equipped with the factory P275/55R20 tires, that puts the original specs at 31.9" diameter, 100.2" circumference and 632 revolutions per mile. A 315/75R17 size tire alters the specs to 35.6" diameter, 111.8" circumference and 567 revolutions per mile. Hopefully one of those numbers will help you enter what you need into Forscan to resolve your programming dilemma.
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