Highway RPM's

[jmcasaleiz@gmail.com]

What kind of RPM's should I expect at 70 mph on a 1966 140 hp 4speed with 205-55r15 tires? Thank you, Duke

[66vairguy]

What kind of RPM's should I expect at 70 mph on a 1966 140 hp 4speed with 205-55r15 tires? Thank you, Duke

Good question and a complex one. Most 140HP cars came with a 3.55 to one axle ratio, although 3.27 to one was optional. Tire size is also an issues.

LET'S MAKE IT SIMPLE. Ideally the Corvair was designed for 825 tire rotations per mile. Actually GM in that era allowed for tire and speedometer variations by using a tire that rolled more than 825 tire revolutions per mile so the speedometer would never read slower than the car was going (a lawsuit for sure) Road tests of the era said the Corvair speedometer read from 1 to 3 MPH fast at a true 60MPH.

Let's pick 840 tire revolution per mile as rough medium and typical of the stock tire, although 865 would make the speedometer read about 3MPH fast and was acceptable. A 3.55 axle car would be almost 3,500 RPM at 70 MPH. A 3.27 axle car would be about 3,250 RPM at 70 MPH. Keep in mind this seem fast by today's modern engine standards, but IT WAS NORMAL for that era as engines needed higher RPM for good lubrication and cooling under load.

A larger tire at the ideal 825 revolutions per mile would drop the RPM by about 200.

What you have to do is find your tires "revolutions per mile". The online tires sites give ESTIMATES. But it comes down to going out and MEASURING you tires on the car, on the pavement!! Mark the tire, move the car 4 or 5 tire rotations and measure the distance and divide by 5. That is the circuference (forget tire diameter as it is crude and the tire squishes were it touches the pavment). Divide the tire ROLLING circumference into inches in a mile (63360) and you have the your tire/wheel revolutions per mile to calculate RPM based on the axle ratio.

[m37aswell]

I have been driving my 64 Corvair on the 4-lane highway every morning and night going back and forth to work and I typically see around 3500 RPM after I let the oil temp. reach about 180 degrees F. or better. When I glance at the GPS it shows around 68-70 MPH. 25 miles one way. Stock wheel\tire setup with radials. I used to think that RPM a little fast on the highway but now I don't, as that little boxer 6 seems to love it.

[Frank DuVal]

My '64 Spyder with the 3:55 and "stock" tires and bad speedometer was no issue, I just doubled the tach reading.

3000 RPM was 60 MPH. One night I drove to upstate Maryland and thought the tach was wrong, as it was indicating 4000 to 4500 RPM most of the way following some traffic. Up I-95 around the beltway then up I-270. Speed limit was still 55.... Tach was not wrong...