Hey guys today I just put on 295/70/18 Nitto Trail grapplers with 18x9 KMC XD Addict. Well here my question. The sticker inside the door says for tire size 275/65/18 for the size or whatever the 18in tire option size is but my truck has 20in rims. So would the spedo be calibrated for the 18's or the 20's? So now i got my 35's on 18in rims. So im wondering is the spedo gonna be saying in running slower or faster than i actually am? and how much will it be off?
Thanks
Ron

 

Either way, it will not be off enough to matter. Maybe 1 mph, 2mph at the most. I run the same tires on mine. I don't think that it is off much at all.

 

A GPS will tell the tale.

 

275/65/18 = 32.08" diameter
295/70/18 = 34.26" diameter

That's a 6.80% difference. Since the new tires are larger, at 70 MPH, that's 65.2 MPH.

But I'm confused. Are you saying you put 18" tires on 20" rims?

 

Well you say that you have 20"s but that you put 18"s on them????
Go to this link and do the calculator:
Car Bibles : The Wheel and Tyre Bible Page 4 of 4
The new size you give VS the stock size from the sticker means that you will be running 65.53 MPH when your speedometer says 70MPH.
Your vehicle should be calibrated for the stock size on the sticker.

 

RIM size makes NO difference on the speedo calibration. Overall tire diameter is the factor here. Your speedo now, with the 295/70/18's should read a few MPH slower than your actual speed. The faster you travel, the larger that gap between actual speed and speedo reading becomes.

 

Nooooo my bad I had 20's. And I got 18's now. so now if im runnin 70 i will be doing like 65. but if im doing 45 i might be doing 48? this is so confusing

 

Actual speed of 70 speedo near 65

Actual speed of 45 speedo near 42

 

so basicly im running 5 slow all the time?

 

No, as stated before you're difference is 6.80%

Your speedo will be roughly 6-7% SLOWER than your actual speed in which you're traveling.

 

Maybe this will help with the confusion.

Which is larger? 10% of $1.00 or 10% of $100?

The faster you go, the more your speedometer AND odometer will be off.

This will also screw up your shift points and put a lot of additional strain on your transmission.

 

So what do I do to fix all this?

 

OK I ran the calculator using the stock size (275/65/18)and the new size (295/70/18)
When the speedometer says 70mph You will be doing 74.7mph.
I understand that the computer can be reset to adjust for the new tire size.
Either you need to visit the dealer or you need to find a programer that can do the adjustment.

 

here's some info for you, you will have to type in the security code and hit submit:
http://www.1010tires.com/tiresizecal...?action=submit

I know that you you can get a programmer to account for the change in tire size that will change your shift points and speedo.

 

I checked mine. The speedo is off by 2mph. When it reads 43-44, I am doing about 45mph. Get a programmer and all your problems will go away as far as shift points go.