correcting speedo

[Scrappy]

I put 35/12. 5R17's on my truck and almost got a ticket already so need to get my speedo corrected. The dealer can do it right? I have heard that they can but they need to know revolutions per mile, I think that I have the correct formula(34. 6X3. 14/12= circumference in feet) and 5280/ circumference= revs/mi. I came up with 586 revs. ? is this correct?

 

[XJSuperman]

In your signature it says you're running the BFG MT's??



If so then the revs per mile stated by BFG for this tire is 598.

 

[Scrappy]

thank you!

that is the info that I needed. I guess my math is bad .

 

[StephenThomas]

I thought the rpm for BFG 315's = 601? As for 35x12. 5's ... . I don't know.

 

[RAMINSPEED]

The actual diameter in 315's / 35's are all a Little different. The MT's are slightly larger than the AT's.

 

[GColeman]

Just take it to your dealer. They will do the math and reflash the unit. Most will charge you about 1 hour labor. They should do it while you wait.

 

[Moterhead]

I wouldn't bet the dealer would (or could) do the math. This is a great website for figuring out speedo error caused by changing tire size.



http://insightnl.extra.daimlerchrysler.com/



it shows your 275s as having 627. 17 RPMs and

315s and having 586. 93. This would give you about a 6. 5% error. IOW, at an indicated 60, you will be actully doing 64. 70 would be 74. 5. Not really a big error margin. If, however, you, like lots of folks, like run the limit plus 5, at an indicated 75, you would be doing almost 80. That would likely get you pulled over.