Tire size help

pickemuptruck13 · #1 Tue, March 31st, 2009, 02:11 PM

Hi guys i was wondering if someone could help me with tire size calculations? Before anyone say it i used the formula that came with the book but my speed still seems to be off about 5 miles and it seems it gets worse the faster i go i am using a gps for the speed comparison. My tire size is 365/60/r20 i also have 456 gears but i put that in so it shouldn't affect what size i put the tires. Thanks for all your help in advance

cj3511 · #2 Tue, March 31st, 2009, 02:45 PM

Try the calculator that is downloaded with the Pegasus software, used 7% instead of 5.

Jim

***Jackpine*** · #3 Tue, March 31st, 2009, 07:11 PM

Very good Jim! Exactly what I would have recommended. Good on you!

Pickemup, your speed is probably off because your tires "squish" under the weight of the vehicle. So, the actual distance traveled per revolution is somewhat less than the formula in the book shows. A rubber squish factor of 7% is what I found to give me the best results.

There is some "play" in this though: Your wheels do not "squish", this only happens to the rubber tires. If you compare the squish between very low profile tires and very high profile tires, I bet you will come out with some significant differences.

Try the calculator value at 7%. If it agrees with the GPS, cool! If you are now indicating SLOWER than the GPS, your squish factor is too big - reduce it. If you are indicating FASTER than the GPS, increase the squish.

Don't be a "slave" to the calculator. Play with the circumference values by "feel" until you get the speed on the Gryphon right. At this point, the dashboard speedo will read about 2 mph faster, because that't the way Ford designed it. The odometer will be correct though.

(Jim - I was an instructor. Someone asks me the time, I tell them how to build a clock! Sorry, I can't let go of it!)

- Jack