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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 ?

Tx-

George Zeck
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Geo Hahn
09-09-2006, 09:28 PM
[ QUOTE ]
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.

Biff
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.
https://www.fisc-racing.com/misc/ratios.html
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.