RPM to MPH ratio

[D S.]

I have a 1970 LS-5. When driving the car is there a standard acceptable RPM to MPH ratio? For example when I'm driving 60 MPH where should the tachometer reading typically be? 70 MPH?

Thanks,
Scott Sims
Texas Chapter

[Dan A.]

Re: RPM to MPH ratio

Scott - What is the rear end gear ratio in your car? That will determine the ratio you are looking for.

[D S.]

Re: RPM to MPH ratio

Dan, 3.08. Thanks.

[Jay G.]

Re: RPM to MPH ratio

Dan, 3.08. Thanks.

Also Depends on the height of your rear tires. Wide ratio trans, close ratio, autotrans etc. But guessing apx 2200 RPM @ 60MPH. Good Hwy gear.

[Robert G.]

Re: RPM to MPH ratio

try this:

http://www.richmondgear.com/101032.html

If your tire diameter is 27 inches or slightly more, the rpms should be around 2300rpm or slightly less.

[D S.]

Re: RPM to MPH ratio

Uh-oh. It was running at 1500 RPM at 60 MPH yesterday morning, 1000 RPM in the afternoon, and this morning it would barely start and run. In fact it was almost like it was on life support. When it was running on the road yesterday it ran great. When I stopped at traffic lights or in traffic I had to keep it accelerated to keep it running. Fuel pump? Points?

[Michael W.]

Re: RPM to MPH ratio

Scott, the ratio between rpm and mph is determined solely by the gear ratio of the differential and is not related to health of the engine. It is not possible for the ratio to change from one day to another.

As to why the engine is now running poorly, it would be difficult to comment with the few symptoms you've described. Start with basic troubleshooting common to any engine and it should be simple to narrow it down.

[Duke W.]

Re: RPM to MPH ratio

All vintage Corvette transmissions have a 1:1 top gear, and the revs at 60 MPH are easily computed. From this ratio you can linearly proportion to any other speed.

RPM @ 60 MPH = axle ratio times tire revs per mile.

6.70-15 tires are nomially 760 revs/mile and 7.75-15 and F70-15 are 775. GM did not change speedometer calibration or gearing for these later tires. The speedometers will just read 1.5 percent higher.

Later GR70-15 and 225/70R-15 tires are 760, and the 255/60R-15 is 775.

Calculating revs/mile from inflated OD is not accurate due to the fact that pneumatic tires deflect significantly under load, unlike steel railroad wheels.

Most manfuacturers' technical data list revs/mile for each size, and they are in a narrow range as promulgated by The Tire and Rim Association industry trade group

Duke

[Terry D.]

Re: RPM to MPH ratio

D Scott
Sounds like you have more problems to worry about then rpm's at 60mph. If you had to keep it above idle at stop lights to keep running it sounds like engine is getting too much gas at idle. Could be a stuck float. If tach reads different at same speed then maybe you should check tach out.
Just my old two cents
Terry