Quote:
Originally Posted by Tom Wesenberg
Measure from the road to the center of the hubcap. This is your rolling radius, which will be less than the radius on the rest of the tire. Double this measurement for the rolling diameter, then multiply by pi for the distance the car travels with one tire revolution. Now divide that number into 5280 to get how many revs the tire needs to make to go one mile. now multiply that number by the rear end ratio 3.78 and you will know how many revs the engine needs to make to go one mile. So, if you are going 60 MPH, that's the RPM the engine would be turning. At 45 MPH the engine would be turning 3/4 of that number.
I'll measure my 28 and do some calculating.
I just measured my 28 and have 14.5" from the floor to the center of the axle, so 29" diameter X 3.1417 = 91.11" per tire roll. Divide that into inches per mile (63,360) = 695.423 tire turns per mile X 3.70 rear end ratio = 2573 engine revs per mile. So at 60 MPH which is a mile a minute, the engine would be turning 2573 RPM, and at 45 MPH it would be 2573 X 3/4 = 1930 RPM.
With a 3.27 rear end the engine would be turning 2270 RPM at 60 MPH, and at 45 MPH the engine would be turning 1702.5 RPM.
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Geezz Tom...you made my head hurt......but you'r figures are correct....lol
I carried this too every track I raced at(engine man) to figure for gear changes since 1973,...it works and it's accurate. Ya!...I still use it to this day!