Every tire is different, especially between manufacturers. Look at their sales data for the outside diameter. Your speedo reading with the stock tires was probably off low anyway, if the tires are taller it will correct it. Going from a 75 profile down to a 70 may negate the bigger size's effect also. I went from a P235/75/16 on my '02 trac to a LT265/75/16, and my speedo now read right on, where before, it read 3-4 mph faster than what i was actually doing.
What STanner says is exactly right, when i went to the bigger tires i fully
expected my speedo to be off by about 5 miles per hour as i had always
assumed that it was dead on.
after putting the new tires on i grabbed my trusty GPS and took off down
the road, imagine my surprise when i found that from 25 to 40 my speedo
was about 3 mile per hour under but from about 50 up to 80 mph it was now
dead on. i didnt check it past 80 as i have that damn drive shaft vibration
i still need to fix
since most of my driving is in that range my speedo racked up extra miles
on every tankful that i didnt really cover fooling me in to believing i was getting
better MPG than i actually was getting and i wonder how many of you guys
that went to bigger tires and complained of losing fuel mileage actually have
the same issue as me and are now reading accurate miles and
are calculating actual true fuel mileage?
anyone with stock tires and larger tires with access to a GPS if you can please post your
speedo/GPS results at 25, 35,45,55,65 and 75 it would be neat to compare them.
