Question about non-stock tire sizes...

[HillHound]

If my stock tires are 265/75R16s and I change to 285/75R16s how will this affect the following:

 

A.  How much will my speedometer be off and

 

B.  How much and in what way will it affect my odometer??

 

 

My stock tires are 235/85s which are the same height as 265/75s.  I'm trying to decide whether to go to BFG ATKO 285s or stay with a stock height of the same type tire in 265.  I'd like the extra width and 1" of height of the 285 but I just don't want to mess up my instruments too bad or have to buy a chip/programmer for $300+.

[MountaineerTom]

Here you go Ron.  Axle Ratio/Tire Size Calculator

 

You can figure out lots of different things with this site.  

 

Looks like around 2 - 2 1/2 MPH difference at usual highway speeds.

[Torque Junkie]

265-75-16=10.4" wide = 31.6" dia.

285-75-16=11.2" wide = 32.8" dia

32.8/31.6=1.0389 = 4% difference roughly

 

looks like a 4% difference across the board.... not too significant I would think... 100 mph would show as 96 on the speedo, 50 would show as 48, 25 as 24.   Like Tom said, roughly 2-2.5mph at highway speeds.    I just took the long way home

 

-Mike

[GreenRado]

Ron, for what it's worth, my '99, perhaps yours also, shows about 2 mph faster than it's actually going, but that's true whatever my speed (I have 265/75r16), so going slightly larger tires, may actually make it closer to your actual speed.

 

Of course, I don't know whether your F-250 will be similar, or not!  

[MarkD]

I haven't checked my truck, but one way to check a vehicle speed is with a GPS.  That will tell you exactly what your speed is now so you could calculate you speed with the new tires.