Re: Differentials

Posted by JeromeSolberg on 2021/9/14 21:16:40
Here's one way to look at it:

Let's say your car originally came with 8.00-15 tires. Use the Vintage Tire Size Conversion Chart: ... ire-size-conversion-chart

So let's say you have 225/75 R15 tires on there now. Use the tire size calculator:

Those tires are therefore 28.3" diameter, 713 Revs/Mile. Which means at 60 mph, the rear wheel will be spinning at 713 rpm.

Now you say you have an effective ratio of 2.55:1 including the diff ratio and the overdrive. So you get at 60 mph the engine is spinning at 2.55*713 = 1818 RPM

I don't know what the "best" engine speed at 60 mph would be for your engine, but that's only about 3 times idle speed - probably about right.

The Packard 300/400 in 1953 with the Ultramatic had 3.54 gears on there, so they would be spinning 2500 RPM, which is a bit high.

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