RPMs vs MPHs
Le Mans Master
Joined: Oct 2006
Posts: 7,123
Likes: 433
From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
Things from tire diameter, rear axle ratio, and transmission all play a part in RPM vs. MPH..
Trans is pretty easy- unless it's a known overdrive, the final there is 1:1. Rear axle and tire diameter are the 2 real unknowns, but there are all kinds of calculators out there that you just plug in the numbers and they spit out the number.
If you have a GPS, (or a smartphone with that app), you can see the numbers on that. Have a passenger call out the speed on the GPS and make you a chart.
Trans is pretty easy- unless it's a known overdrive, the final there is 1:1. Rear axle and tire diameter are the 2 real unknowns, but there are all kinds of calculators out there that you just plug in the numbers and they spit out the number.
If you have a GPS, (or a smartphone with that app), you can see the numbers on that. Have a passenger call out the speed on the GPS and make you a chart.
Instructor
Joined: Aug 2013
Posts: 108
Likes: 0
From: Kenosha Wi
http://www.prosystemsracing.com/calculate.html
My 55 belair has no working speedometer and I have a chart I made up from this calculator link posted above. Works well so far,
I have a tremec overdrive but this is a 1:1 calculator which I believe is needed for the turbo 400.
My 55 belair has no working speedometer and I have a chart I made up from this calculator link posted above. Works well so far,
I have a tremec overdrive but this is a 1:1 calculator which I believe is needed for the turbo 400.
Thread Starter
Intermediate
Joined: Nov 2012
Posts: 39
Likes: 0
From: Dallas Texas
The speedometer was rebuilt. I am hoping it is the gear in the transmission. I am taking it to the tranny shop next week for other stuff. I will have them check the speedo connection while it is on the rack. If that's not it, I guess it would be a bad cable
Thread Starter
Intermediate
Joined: Nov 2012
Posts: 39
Likes: 0
From: Dallas Texas
According to the pro systems chat as posted above with the 3.08 ratio and 245/60/15s.
2000 rpm. 51 mph
2500 rpm. 64 mph
3000 rpm. 77 mph
That's seems about right based on driving with traffic at highway speeds
Thanks to everyone. As always, the forum knowledge comes through.
2000 rpm. 51 mph
2500 rpm. 64 mph
3000 rpm. 77 mph
That's seems about right based on driving with traffic at highway speeds
Thanks to everyone. As always, the forum knowledge comes through.
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Race Director
Joined: Jul 2001
Posts: 15,892
Likes: 42
"This calculation is for a final drive ratio of 1:1"
I think every GPS unit these days will display vehicle speed.