I HAVE A 77 F100 THAT HAS HAD A NEW SUSPENSION INSTALLED AND THE TIRES THAT ARE ON IT ARE 215/70/15 I WAS TOLD THAT I SHOULD HAVE 235/75 15 ON IT WHAT IS THE RIGHT TIRE SIZE WITHOUT CHANGING THE SPEEDOMETER RATINGS

 

The 235/75 15 tire size is the factory size. As far as speedo variances you can change to 235/50 15 ot 235/80 15 without changing speedometer
gears. Those didgits are the width If you change the 235 to 175 or something simler is the hight
and you will have to change the speedo gear.

Good luck