I don't trust the GPS and you prodded me to check my tire height and make a chart of RPM=MPH for my truck.
I have a 29 inch tire and 4.10 gear. The tire height was figured from measuring the loaded radius (center of axle to pavement) and doubled.
Center of axle to pavement =14.5 inches X 2 = 29 inches of tire height.
Once you have that, then you crank in the formula...
Measure your engine’s RPM based on the ring gear and pinion gear ratio, speed, and tire height, with the Spicer Engine RPM Calculator.
spicerparts.com
I don't have overdrive...yet...so the RPM's will be 24% higher than someone with the same tire and rear gear as me...but how it shakes out is this...
40 MPH = 1900 RPM
45MPH = 2150 RPM
50 MPH = 2350 RPM
55 MPH =2600 RPM
60 MPH = 2850 RPM
65 MPH = 3100 RPM
70 MPH = 3300 RPM
75 MPH = 3550 RPM
80 MPH = drastically shortened engine life...
If you have the same size tire and the ZF and you are figuring your RPM in overdrive (5th) then reduce these numbers by 24% (5th gear ratio.76-to-1)
Then...
70 MPH = 2508 RPM
60 MPH = 2166 RPM
50 MPH = 1786 RPM
40 MPH = 1444 RPM
I think that I am going to start looking for a ZF....
On the speedo gear...the last time I made a recommendation...I was wrong...but...I will try it again...
On the
driven gear...
To speed up the speedo in relation to vehicle speed...you...reduce the number of teeth...
To slow down the speedo in relation to vehicle speed...you...increase the number of teeth...
I think that I have that right...we will see if the guys agree or not...
HTH...