[joedoh]

a 235/70/15 would not show the correct speed.

the constant here is engine rpm, so a gearing calculator is needed, a tire size comparator will not do. I used the john g burns calc, he has had it online for so many years and it is an invaluable resource I keep going back to.

anyway, plugging in a 245/75/16 tire and a 1:1 4th gear and using just an arbitrary 3.73 rear gear (its not important to the calculation), I get roughly 1700 rpm to display 50mph (this may not be your rpm, but that also is not important to the calculation). keeping everything else constant (any economists in our bunch?) I change the tire size to 235/70/15 @1700 rpm and...


46mph. almost 47. so the tires didnt make enough difference, in fact not even going to an ultra small 205/75/15 makes your speedometer read correctly, still almost 45.

since your speedo is off about 16%, thats a tough one to swallow. my best advice would be to try the speedometer gear swap like the guys above suggest.

but my REAL advice would be to just get a paint marker pen and use the gps speedometer in your phone (or your daughters phone, or even your granddaughters phone, no judgements!) and put some dots on the speedometer lens to represent your REAL speeds... you could even pen the numbers in. if you wanted it to be super clean take it to a vinyl graphics shop, have them scan your dots, design and print out and install a new sticker for the lens. you can even have them use translucent vinyl and it will let light through at night. I am a work smarter not harder guy and would probably throw myself a party for not having to crawl under the truck, even though that may be easier.

good luck to you!