IsItFast

So I got some 22.5" wheels on my truck and the speedo is obviously off. I am taking the truck to the dealer tomorrow for an alignment so I figure I will get the speedo calibrated too while I'm there....hopefully they will do it.

I have 255/70/22.5 tires which, according to Yokohama are 36.8" tall and have 567 Revs per mile. Should I go off of this figure or should I manually calculate it myself? If so, is there a good thread on this? I couldn't seem to find a thread that specifically states how.

Joe N.

The dealer that recalibrated my speedometer didn't want to use the numbers I gave him from BFG. They had a chart, looked at my tire size, and programmed what the chart said. I don't know if the numbers from BFG would have been any closer but the speedometer indicates about 1 to 1.5 MPH fast. Speed has been checked by radar and my GPS unit.

bulabula

I'd go with what Yokohama said - they've made plenty of tires by now, so I figure they know how to calculate revs/mile.

J BODY

I've used the numbers provided by customers from BFG's website and sometimes we were still off. Best bet is to beg, borrow, or steal a GPS unit. That's my prefered method.

IsItFast

How do you calculate revs with GPS? I have a GPS radar that tells my speed...

oldschool

You can calculate the circumference (distance around the outside) of a circle (tire) pretty easily. The formula is C = PI(3.14) * d(diameter)

There are 63,360 inches in a mile...using this we can figure it out.

So what you need to do is measure the height of the tire and plug it into this equation. In my example I'm going to use a 32" tall tire

C = 3.14 * 32
C = 100.48 (distance around the outside of the tire)

To go 1 mile the tire must go around
63,360/100.48 = 630.57 times.

tands1000

I tried this method and it didnt work. I have 35x12.50x17 procomp mud tires. They are actually about 34" though. With that calculation, you get about 593 revs. WAY OFF. I finally got it set at 628 revs and that is about 1-2 mph off using a radar. This method may work for some however. Just didnt on mine.

Travis

drew03

They used the number that i gave them (601) from BFG and mine is dead on.


however i did the math backwards and came up with an overall diameter of 33.55"
When calculating with the 3.14 (PI) *34.5/(5280*12) =584.58 revs/mile
interesting isint it. according to there number it is only 33.5" tall, when they say it is 34.5".

gsbrockman

..........or you could utilize part of this equation and change a small part of it.
Using some children's sidewalk chalk, or a tire/grease crayon, mark the tire in the tread area, drive the truck one tire revolution, and measure the distance center-to-center between the chalk/crayon marks. Divide 63,360 by the distance measured....this will account for the static loaded radius of the tire, and should be a tad more accurate.
Greg

IsItFast

Well I just gave them the number Yoko provided and the tech said it was almost perfect between mile markers...but you never know how acurate those markers are. I haven't had a chance to check it versus GPS yet but it feels much closer compared to traffic.

drew03

or jack it up and use a tape measure.

HeberRam

Just use this handy link and save brain power and getting dirty

http://www.1010tires.com/TireSizeCalculator.asp

SkyHarborCowboy

You cant go by the diameter of the tire. You must take into account the loaded weight on the tire. If you want to use the diameter you MUST measure from the ground to the center of the hub with the vehicles tires on the ground. You then double this to achieve your loaded tire diameter.

2(Loaded Tire Radius) X Pi = True Rolling Diameter

Joe

That is if you want to be truly accurate!