Will tires that are over sized cause a cars speed to read high?

 

No it will read too low

 

See if this chart helps. http://www.4lo.com/4LoCalc.htm

 

Try this!

Randy's Ring & Pinion

 

Anytime you change the tire size (diameter) you will change the speedometer reading. Say you install 23.5" diameter tires. Multiply 23.5 times 3.14 to get the circumference in inches (73.79) divide by 12 to get it to feet (6.149) and divide this into 5280 (feet in a mile) and you have the rpms of the wheel at 60 mph (858). To figure out the gears you need on your speedo cable to get the speedometer reading correctly use this formula:

Driven gear teeth (on cable) equals Drive gear teeth on tranny output shaft x rear end ratio x revolutions per mile
-------------------------------------------------------------------------------------------
1000




For example : A tranny with a 6 tooth gear x 3.25 rear end x 858 equals 16,731

Divided by 1000 equals 16.73 You round this to the nearest whole number and you would need a 17 tooth gear on the cable to have a speedometer reading accurately.

 

Man that is some great information! I always used the tried and true method of yanking the gear out and the trying one with more or less teeth until I got it as close as I could.

 

Glad all that formula stuff was helpful to someone. Could have saved me some problems if I knew it when I replaced the speedo gear on my tranny output shaft. Apprently I put an eight tooth on my Toplaoder and now with my 225x50's and a 3.25 rear, I need aabout a 22 tooth speedometer cable gear (they go to 21!) With a six tooth gear it would be a 17. Darn. Oh well, I will live with the offspeed until I get a T-5 and then get it right.