BIG WHEEL: you cannot rely on charts for the best accuracy BECAUSE TREADTHICKNESS VARIES A GREAT DEAL FROM TIRE MODEL TO TIRE MODEL, EVEN WITHIN THE SAME BRANDS FOR THE EXACT SAME TIRE SIZE!!! Yeah, the charts will get you CLOSE in MOST CASES, that the difference probably might be immaterial, but be forewarn that you won't likely be nearly as accurate as using 2 pi r , or doing a dummy style measurement from marks on your living room's hardwood floor.
Why not try going with 2 pi r ???? IT IS SUPER SIMPLE AND IT IS ACCURATE.
r is RADIUS , which is just your MEASUREMENT FROM THE CENTER OF THE WHEEL TO THE OUTER TREAD OF YOUR BIKE'S TIRE....simple right, yep!
HERE IS A SIMPLE EXAMPLE:
Lets say Bubba measures exactly 14 1/4 inches or 14.25 inches from the CENTER to THE OUTER TREAD....... converting to millimeters, oh well according to GOOGLE's calculation, 14.25 inches = 361.95mm ................SO YOUR RADIUS is 361.95mm or 14.25 inches......you measured with a tape measure to get the RADIUS. Okay simple enough.....you've got that!!
You remember Pi from your school math class forty five years ago right.................Pi = 3.14 but the decimal placement is said to go on to infinity but 3.14 is Pi with just two decimal places......... but you could express Pi as 3.141592653589793 if you wanna go out fifteen decimal places
2 x pi
2 x 3.141592653589793 = 6.28318530718
So you see that 2pi = 6.28318530718
YOU REMEMBER THAT THE FORMULA IS 2 pi r which translates to 2pi TIMES radius , or 2pi X r
Remember that we know that our measured radius is 361.95mm
r = 361.95
So doing the formula calculation 2 pi r which is 2pi TIMES radius or 6.28318530718 X 361.95
okay 6.28318530718 X 361.95 = 2274.19892193
so then 2274mm IS OUR CALCULATED TIRE CIRCUMFERENCE FOR THE SPEEDOMETER.
No joke, it is much simpler than my lengthy example as if I was trying to explain it to 4th Grade ten year old kids.
MEASURE the RADIUS.
(6.28318530718 ) is the figure you can use for 2pi
Multiply your measured RADIUS figure X 6.28318530718
Obviously, you want your RADIUS figure to be expressed in millimeters, so depending on what unit of measurement that you did choose to get a reading with the tape measure, you'll want that figure converted to exact amount in millimeters for your calculation
Again it is simply this:
Your measured RADIUS figure X 6.28318530718 = Speedo's needed wheel circumference in mm
this is " TWO PIE ARE " and it is extremely accurate assuming your wheel is round and your Radius measurement is good.
As you know, it shouldn't matter one damn bit as to where you measure the RADIUS as in a true circle or perfect wheel, every location would be exactly the same.
Your wheel should be round enough that it shouldn't have any material differences as tiny variations should be negligible and not significant at all if your wheel is anywhere close to true.