I have been through the search forum and what I understand is that I have 235/70/16 on now and I can switch to 255/70/16 and will not have any problems with the speedo. Is this correct? Thanks. See tag below for detail on p/u.

 

Your current 235/70R16 has a 28.95" overall diameter. The 255/70R16 has a 30.06" overall diameter. That's a 4% change in tire height, which will result in a 4% speedometer miscalibration. That means for every 25mph you'll actually be doing 1 mph less than shown. So at 75 on the speedo, you'll actually be doing 72. That's probably liveable.

You'll also save 4% on mileage. Your truck will only record ~96.3 miles for every 100 you drive.

 

ucfperspicere:

I think you got it backwards. Since the new tire is larger, it rotates less, thereby reducing the actual speed shown on the speedometer. So........... a 75 MPH reading on the speedo is actually 78 MPH vehicle speed, given the following tire sizes:

Old Tires: 235/70R16 has a 28.95" overall diameter
New Tires: 255/70R16 has a 30.06" overall diameter
Difference: 4%

 

My mistake, Rileyrott. You're right - it'll be going 4% faster than seedo. But I had the mileage thing correct - it'll save on mileage.

-Jeff