RPMs
Instructor
Joined: Dec 2012
Posts: 112
Likes: 43
I use an android app Next Gen GPS for verifying real speed. Works really well for checking speedometer accuracy.
For me with 225/70-15 T/A radials and 411 gears it is just about exactly 3500 RPM at 70 MPH. My speedometer is a couple of MPH slow. I find myself mostly using the tack as a speedometer.
Again with my set up Its very easy. 2500 is 50 MPH ... 3000 is 60 MPH ... 3500 is 70 MPH. On and on.
For me with 225/70-15 T/A radials and 411 gears it is just about exactly 3500 RPM at 70 MPH. My speedometer is a couple of MPH slow. I find myself mostly using the tack as a speedometer.
Again with my set up Its very easy. 2500 is 50 MPH ... 3000 is 60 MPH ... 3500 is 70 MPH. On and on.
Race Director
Joined: Mar 2017
Posts: 11,978
Likes: 4,528
From: Virginia
My 245/60/15 BFG’s have an unloaded outside, diameter of 26.575”.
The loaded diameter of my BFG’s, recommend vehicle weight on each tire, mounted on the approved wheel width, measures 24.98”.
So roughly 1.5” difference in height.
This information can be found on BFG’s website and on most vehicle gearing calculators.
I’ve never seen any information posted on any websites showing the loaded diameter of a tire when mounted on an overloaded unsafe vehicle.
If it existed, can you please show me where?
The loaded diameter of my BFG’s, recommend vehicle weight on each tire, mounted on the approved wheel width, measures 24.98”.
So roughly 1.5” difference in height.
This information can be found on BFG’s website and on most vehicle gearing calculators.
I’ve never seen any information posted on any websites showing the loaded diameter of a tire when mounted on an overloaded unsafe vehicle.
If it existed, can you please show me where?
I looked up your tires at BFG. Using their loose diameter, that calculates out to 758 rev/mile. Using your measured loaded diameter, that calculates to 807 rev/mile. Using the average of these two diameters calculates to 782 rev/mile. The spec on the BFG website is 783. Even using BFG's spec, that's a 3% change.
Fascinating. I'll have to adjust all of my other spreadsheets now. Thanks! And another reason to use a GPS speedometer.
Here's the spec sheet for 255/60R15s, which many of us run, and the link to find your size.
https://www.bfgoodrichtires.com/auto...-a?tyreSize=15
