Join Date: Sep 1999
Location: Southeast Iowa
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Regearing made easy.
[img]/ubbthreads/images/graemlins/smile.gif[/img] I noticed a post on the Main Index from a newbie who needed to know the what and the how that goes along with choosing a new gear ratio to go with the new lift and the new tires. It has been a while since I posted this, so just in case there is someone left on the planet who doesn't know how to do the calculations for regearing, here is a refresher: It is simple (Velocity Squared) divided by 2G. Oops.... that isn't it, that's how you clculate the momentum head for a large axial flow pump.. [img]/ubbthreads/images/graemlins/crazy.gif[/img]... OH WAIT! ... HERE IT IS...!!!! [img]/ubbthreads/images/graemlins/crazy.gif[/img] You find the TURNS PER MILE of each tire, the old ones and the new ones. You can find TURNS PER MILE by looking in the tire dealer's catalog, OR ....you can find it by measuring the circumference of the tire in inches, divinding by 12 to get feet and divide that into 5280, the number of feet per mile. So that would give you the number of turns the tire makes per mile. So you do that for the old tires and for the new tires. THEN you check your existing gearing and whatever ratio you have now, multiply that by the TURNS PER MILE of the existing tires. So it would be something like 715 TURNS PER MILE X 3.54 (your ratio) which would give you a number like 2531. Now that number 2531 JUST HAPPENS TO BE the driveshaft RPM at sixty miles per hour..... WHAT A COINCIDENCE! So....... now stay with me here ..... if you take 2531 and DIVIDE it by the TURNS PER MILE of the bigger tires, the answer you get will be the NEW GEAR RATIO. So .....TURNS PER MILE (little tires) X Gear Ratio (existing) = TURNS PER MILE (big tires) X Gear Ratio (new)... [img]/ubbthreads/images/graemlins/smile.gif[/img] [img]/ubbthreads/images/graemlins/wink.gif[/img] TPM X Ratio(old) = TPM X Ratio(new) [img]/ubbthreads/images/graemlins/cool.gif[/img] And here you thought you'd never use any of that sophisticated high skool math. [img]/ubbthreads/images/graemlins/crazy.gif[/img] Once more... TPM X Ratio = TPM X Ratio
..... The idea is to get the driveline RPM at sixty miles per hour with the new gears and tires to be the same as it was with the original gears and tires at sixty miles per hour. [img]/ubbthreads/images/graemlins/tongue.gif[/img]