Tire size vs speedometer

[MuddFlapp]

Let me see if this is correct. If the stock tire is 275/70/17, and the truck tires are 315/70/17. The tires on the truck will be approx. 27 mm taller which will make the speedometer run slow. Does that sound right?

 

[RRB]

I did some quick math. Your total tire diameter will be 56mm larger (2. 2" taller). Your speedometer will read 6. 8% slower than actual speed with the 315's.

 

[SWithrow]

Tire Dimensions Made Simple

Try the link below, going from 275 to 315 your speedo will be reading 65 when your really doing 69. 46

Tire Dimensions Made Simple - Discount Tire

 

[Shadrach]

If you ever decide to "upgrade" the power a Smarty will cure the tire size problem. Shadrach

 

[MuddFlapp]

If you ever decide to upgrade the power a "Smarty" will cure the tire size problem. Shadrach

Would that be a Smart Jr. ?

 

[seafish]

I ahev heard that the Smarty Jr can reprogram the ECM for tire size also, but the change to the ECM does NOT stay with the truck if you return the ECM to stock programming.

 

[Bill Weber]

Have 315's on my truck. Checking the speedo against a GPS and the odometer against the mile markers on the interstates (at least 50 miles) both agreed I needed to multiply my speed and milage by 1. 09.

 

[MuddFlapp]

Thanks to all. I checked my speed against a GPS today and 1. 09 is what I get also. 315s work for me.

 

[makelemen]

You could also do a test using the highway mileage markers. Go 10 miles using the markers and compare that with the odometer. Assuming the speedometer and odometer are in sync the % can be carried over for your speed.

 

[Regcabguy]

I ahev heard that the Smarty Jr can reprogram the ECM for tire size also, but the change to the ECM does NOT stay with the truck if you return the ECM to stock programming.

The '06'-'07's will stick when returned to stock. You have to physically return the tire size to stock which on mine was 31. 40". '05's on down will revert to stock automatically.