Correct formula for RPM & Gear ratio calculation?

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Posted by R Davis on Sunday, January 07, 2001 at 3:30PM :

For years I have been using the formulas :
RPM= mph x gr.ratio div.by tire dia. x 336
Gr.ratio= rpm x tire dia. div.by mph div.by 336
I do the calculations and find sometimes 100s of rpm diff. from what is posted by others.
According to some publications I've read, an engine gets its best fuel economy running at approx. 60-80 % of the rated HP rpms and depending on weight,and driving habits.
I know we're not setting valve clearances for dometop pistons but I'm sure we want the most accurate formulas for caculating this info. So which formula is the most accurate?

Follow Ups:

Re: To Cool ,a new favorate n/t - The Other Ross 7:14PM 01/07/2001

Re: Correct formula for RPM & Gear ratio calculation? - Vaughn 4:58PM 01/07/2001

That calculator is slicker than snot on a doorknob! - Brian in Oregon 5:13PM 01/07/2001

It makes it real easy! n/tx - Vaughn 10:39PM 01/07/2001

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