Speedo pinion Calc query • The Dodge Challenger Message Board

by **Goldenblack440** » 22 Sep 2009 16:34

Just another question for Dave, i have used your speedo pinion calculator again ( i love it!). Just wondering about the wheel/tyre diameter. When i measure this the wheel and tyre are stone cold. Did you make any allowances for expansion during normal driving conditions? Especially down here where on a 35-38 degree day i reckon the expansion would be very significant compared to cold. If that makes the pressure go up say another 12 psi, maybe the diameter could change as much as 15mm? I know it won't push it into a whole new pinion size, but just wondering if you had considered it when doing the spread sheet? thanks

by **dave-r** » 22 Sep 2009 20:29

The spread sheet is designed to simply use the tyre size numbers moulded onto the tyre side wall. When inflated to the correct pressure that is as accurate as you are going to get.

There are variations in both actual diameter and width from manufacturer to manufacturer and also between new and old tyres. Not just due to wear either. Tyres shrink with age as well as with wear.

The numbers on the sidewall will get you as close as you will get to matter. Radials do not actually grow very much like drag tyres do. There sidewall and steel wire construction does not allow much. But there is a little as you say.
If you disregard the numbers on the tyre and used an accurate "at speed" diameter I doubt there would be more than a mph or two difference. And then you are left with the question as to at which mph do you want the speedo to be the most accurate?

I really wouldn't worry about it.

by **Goldenblack440** » 23 Sep 2009 11:33

ta, yeah the difference would be negligable, soaked up by speedo error anyway. I didn't realise the spreadsheet was designed just for the tyre numbers. I had also been taking the wheel off and measuring it, both diameter (height from ground) and circumference with a tape to see what the calculator came up with as well.

by **dave-r** » 23 Sep 2009 11:51

Tell me if you do find much difference. But I find it is pretty good and will make your speedo more accurate than factory. :wink:

by **dave-r** » 23 Sep 2009 11:54

Also. Be careful. Only the numbers in black are designed to be altered by the user. All the other numbers are either fixed or calculated (in blue).
If you mess with the blue numbers you will probably mess up any calculations.

by **Goldenblack440** » 25 Sep 2009 15:05

OK! I remember reading that, but then forgot when playing with the spreadsheet. I don't save it with the new figures anyway.

Yes i think it was pretty accurate, ie, the circumference reading matched the diameter input pretty closely to what i measured. But i put in actual measured figures and it did change the pinion result very slightly but only by 10ths, so in reality no real difference. Using it as it is supposed to be used, is very accurate. I changed my rear end from 2.76 to 3.55 and went from a 27 teeth to 35 tooth pinion by the calculator (told me to use 34.9 to 35.2 depending on my inputs). I havn't done a speedo check yet but will in the next few days and see how accurate it was.