My speedometer has been off for awhile now , so i started searching around for what is the correct gear for my tire size and gear ratio.

I found this calculator From TCI great little tool
Speedometer Gear Calculator and Charts - TCIŽ Auto
enjoy

 

www.miata.net/garage/tirecalc.html has a nice tire size calculator. It will give you the "revolutions per mile" for any "generic" tire size. Plus you can give it two different tire sizes and it will tell you the difference between the two. Such as if you wanted wider meats in the rear but didn't want to change the O.D. (revolutions per mile) of the tires.

 

thanks JEFFAFA
you can also get tire width /diameter from tire rack

 

You're welcome Mike. You done good Son.

 

If you want to know how many revolutions your tire makes in a mile, post up the diameter. I will tell you.
Tire width doesn't matter.
If your speedometer is off, there is a gear in the transmission to correct. Information is widely available in a search of this site.
Drive parallel to someone with a modern vehicle on a freeway and have them raise their hand at 65. Read your speedometer and report back with what you see on your dial.

 

My 33's add 5 mph over stock size. I drive a lot for work and can tell my speed rather accurately just by feel. I always felt like I was going faster in the 68 so I ran a GPS speed calculator and it gave me the +5 over my speedo.

 

I have 235 85R 16s on my 74 Supercab and it is 5 mph off on higher speeds. Followed by friend in modern car and she said I was doing 70 when my speedo showed 65. I remember these when they were new and recall them having 950 x 16.5s which is a smaller tire than the 235s.

 

Each tooth on the driven gear equals 3-5 MPH I've found in the past. Best way though is to figure the ratio of the difference in the speeds you tested to get that +5 MPH. Then multiply that by the number of teeth your truck currently has. Note. More teeth on the driven gear slows the speedo head down. Les teeth speeds it up.

 

Waited for that one Jeff. If you have a phone app drive with it down the road and compare, then crawl u butt under the truck, pull the speedo gear, count the teeth. You also need to see if they pitch right or left.

This might not be as fun as playing with apps on the phone though.


John