Hey guys

So i know if you run bigger tires you go faster and cover more miles than with stock tires but what is the actual formula to calculate how fast you are going based on your tire diameter, rpms and gear ratio

thanks

 

If all you did way change the tire size then just go to the tire mfr's web site for both the old tire and the new tire to get the diameter in inches. Then, whatever the percentage difference is between the two will be the percentage that your speedo is off. For example going from a 30" to a 33" would be a 10" difference and when your speedo says 50 you'll be going 55.

John

 

Don't know formula...I just use this site.
http://www.sjdsm.com/tech/tire_size.htm

RJ

 

I was hoping to get the actual calculation for how rpm is connected to tire diameter and speed, i guess what i was hoping to find is the formulas that the website uses

 

The RPM will change by the same amount. 10% taller tire = 10% lower RPM at the same speed.

 

try clicking on tech Info, HERE
May take you close to where your trying to get?

RJ

 

just found what i was looking for on another thread

https://www.dieseltruckresource.com/...ght=gear+ratio

there is a link to a spreadsheet that has the formulas in it


http://www.klenger.net/dodge/general...calculator.xls

thanks for your help

 

The MOST accurate way to do tire/speed conversions is to use the manufacturer's published number of tire revs per mile.

This can vary even with tires that are nominally the same size. Some 285s for example can be as low as 627 revs/mile, some are up to 635 if they have shallower tread.

Almost every manufacturer published the revs per mile as part of their tire specs. Then you just do some basic math using the figures for your current tires.

Example: my OEM michelins are 655 revs/mile. What if I changed to 315s that were 599 rev/mile?

Math: Old revs/New revs= new speed/old speed.
= 655/599= 1.093
= 1.093= 70mph/X mph
= X mpg= 70mph/1.093
= X mph = 64.04mph

So, going from a 655 rev tire (my 265 michelins) to a 599 rev tire (like a 315 mudder) will give me almost 10% speedometer error, indicating 64.04mph when my actual speed is 70mph.

Hope this is what you're looking for.

Justin