MPH question
- Thread starter my66coupe
- Start date
jikelly
20+ Year Stangneter 
Do a search for an gear ratio, rpm calculator on the web. I found one and according to the information you provided it says your RPM at 70 MPH will be 3142.49
http://www.superford.org/garage/calculators/index.php#finding_rpm
http://www.superford.org/garage/calculators/index.php#finding_rpm
67coupe351w
New Member
I figure that when your speedo says 70 mph you will be going 55 mph. The old rear end gears are 78.59 % of the new gears so 78.59 % of 70(it thinks you have your old gear still) is 55 mph. Basicly subtract about 20% of whatever the speedo is reading.
Of course this is only close becasue the car didnt come with those tires on it so the whole thing is out of wack by a little bit on top of the rear end gear change.
Of course this is only close becasue the car didnt come with those tires on it so the whole thing is out of wack by a little bit on top of the rear end gear change.
Hack
15 Year Member
My feeling is that with a car that's almost 40 years old, the best method would be to have a friend with a new car drive next to you to clock your speed. The speedo could give you a lot of error.
How's that for a lazy answer?
How's that for a lazy answer?
blown65
Founding Member
Just drive 5 or 10 miles and do the math of the difference from your speedo and the actual mileage.
BeatAChevy289
New Member
I put some some tires on mine that are one size smaller than stock, yet I don't remember what size they are exactly and it threw my speedometer off 10mph. It says 60 when I'm going 50. My Jeep is the opposite. It came with 28" tires from the factory and now has 31's under it. Its off about 5mph. When it reads 50 I am really going about 55. I figured this out on the Jeep by driving through one of those police speed trap things where it flashes your speed up for everyone to see. I figured out the Stang's speedo by having my Dad follow beside me in his car on the way to Church one day. Later <><
