RE: 1969 Corvette 350/350hp

[Jerry B.]

I am trying to caculate the correct rpm @ 60 mph. with a 3:55 and a M21 trany. Second question is what color speedo gear should be used. thanks for any help.

[Joe L.]

Re: 1969 Corvette 350/350hp

I am trying to caculate the correct rpm @ 60 mph. with a 3:55 and a M21 trany. Second question is what color speedo gear should be used. thanks for any help.

Jerry------

As far as RPM goes, a lot depends upon your tire size.

As far as speedo DRIVEN gear goes, I believe that you require a 22 tooth GREEN gear. What color gear did you remove?

[Duke W.]

Re: 1969 Corvette 350/350hp

For a direct drive top gear manual transmission true revs at 60 are ALWAYS installed axle ratio times installed tire revs per mile, and Corvette speedos are set up for the 760 revs per mile of the old 6.70-15 even though '65 through '72 tire sizes were nomially 775 revs per mile.

760 x 3.55 = 2698 - call it 2700.

So if you have the correct speedo gears installed for the installed axle and the revs/mile of the installed tires are in the OE range, the speedo should read within about 3 percent of dead on.

Duke

[Jerry B.]

Re: 1969 Corvette 350/350hp

Thanks Joe and Duke for you rquick response. To answer the tire size question, I am running 215/70X15. As far as gear color, I will determine that this weekend. I will use Dukes formula and see how close I am.

[Joe L.]

Re: 1969 Corvette 350/350hp

Thanks Joe and Duke for you rquick response. To answer the tire size question, I am running 215/70X15. As far as gear color, I will determine that this weekend. I will use Dukes formula and see how close I am.

Jerry------


The 215-70-15 size will have a revs per mile of somewhere in the range of 765-785 depending upon the particular tire brand and model.

[Duke W.]

Re: 1969 Corvette 350/350hp

The nominal revs/mile figure for 215/70R-15 promulgated by the Tire and Rim Association is about 775. Most will be speced very close to this, which is also the nominal revs/mile for 7.75-15 and F70-15.

New tire revs/mile increases about two percent when the tires are worn to the wear bars.

Duke