View Full Version : MPH vs. RPM ????

George Zeck
09-09-2006, 09:04 PM
Hi All --

Though I'd ask the question since you are all the speed experts. Have heard many describe highway usage at approx 70MPH @ 3500 RPM (or so).

On my Sprite -- run 60MPH @ 4500RPM. I swapped the diff 2 years ago from stock (3.9) to "upgraded" @ 4.2 (could have those flip floped). When trying to pass someone (ie 65 + ) I get up to 5500 - 6000 and afraid I'm runningthe engine too hard.

My question -- how can i verify diff gearing (stamping plate ?) is truly what I thought I was getting. And also -- could my VDO (aftermkt) tach be off ? any other thoughts ?


George Zeck

Geo Hahn
09-09-2006, 09:28 PM
My question -- how can i verify diff gearing (stamping plate ?) is truly what I thought I was getting...

[/ QUOTE ]

The most straight forward way is to raise one rear corner, put the gearbox in neutral and count the number of times the driveshaft turns in two revolutions of a rear wheel.

Put a bit of masking tape or such on the driveshaft to make it easy to count the turns.

You do 2 revolutions of the rear wheel since the other wheel is on the ground and not turning.

Not real precise but should be close enough to tell the difference between 3.9 & 4.2.

If I've got the math all wrong or backwards someone will speak up I hope (I try to visualize this process but no easy w/o a car in front of me).

BTW -- 4500 RPM at 60 mph sounds brutal. Hopefully some Sprite owners can help you sort out if that is indeed true.

09-09-2006, 11:05 PM
This was discussed ad nausea in the spridget forum. Running at 4000rpm with a 4.22 rear I am doing 64 mph. You can use this calculator to figure out what your speed at what rpm is also factoring in your tire size.
Sounds like either one or both your gauges are off. You will have to do the above to find your diff ratio in order to use the calculator however. I cruise the interstates at 4500- 4700 and my motor loves that rpm and purrs. If yours sounds like it's running to hard something with the motor is wrong. The A-series engines thrive on rpm.