RPMs at 60mph

[Jeff B.]

I have verified, using a Garmin GPS, that when my speedometer is showing 60mph, GPS says 58. Close enough. Rear end ratio is verified as a 3.55. With that rear end ratio, what RPM should engine be turning? Tach shows about 2700.

[Patrick B.]

Re: RPMs at 60mph

2700 rpm is about right for a 3.55 rear and tires near stock diameter at an indicated 60 mph.

[Duke W.]

Re: RPMs at 60mph

The basic formula is: revs @ 60 = (tire revs/mile) (axle ratio); 6.70-15s are 760 and 7.75-15s are 775. Given a selection of 18-24 teeth for the driven speedo gear the increment is 4-5 percent. The selection was based on the least error on the high side. I think there were some tooth count changes on some axle ratios with the 7.75-15 tires. You should remove the speedo drive gear, note the color and tooth count and verify that this is the correct gear, and you should also check revs/mile for the installed tires. Your speedo is reading 3.5 percent high, so one less tooth will make it about 1 percent low, but keep in mind that as the tires wear it will get closer to zero error.

[Jeff B.]

Re: RPMs at 60mph

Duke - Thanks for the formula. Did not know what it was. My concern is if tach is showing correct RPM, based on the rear end ratio, at the indicated speed. The speedo driven gear is green plastic, 22 teeth. I'm not concerned that actual speed is slightly slower than indicated. The Vette is probably closer than my 2016 Buick.

[Patrick B.]

[Richard G.]

Re: RPMs at 60mph

Specifications from a 1963 edition of Road and Track, I believe.

SPEEDOMETER ERROR
30 mph actual	28.5
60 mph	58.0

At 60 this is a 3.39% difference with the factory calibration.

Interesting this thread came up as I am dealing with the same issue. My car shipped with 4:11's and the differential was changed to 3:55.
I believe the speedometer is off. Will verify the error soon. I plan on using the speedometer app for my cell phone.
I may be looking for the same gears!

[Tim G.]

Re: RPMs at 60mph

Oddly enough, the Corvette sales brochures have this information in them. It helped buyers plan on the rear end they wanted for their cars.
They list the RPM at various speeds per rear end.

[Jeff B.]

Re: RPMs at 60mph

Thanks to all for their input. I was not so much concerned if the indicated MPH corresponded to actual MPH, but the accuracy of the tach. The instrument cluster has never been out, nor any gauges been rebuilt nor calibrated. However, everything seems to be in good order. The actual road speed is surprisingly close to the indicated speed. Tires are BF Goodrich P205/75R15 radials.