Gear Ratio Equation?
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Gear Ratio Equation?
I've done a few searches in this forum with no luck. Is there a equation to figure out engine rpm change with different ratios? I'm looking to regear and don't want to make any horrible mistakes. Thanks!!!
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this is the forumla
(RPM X tire diameter ) over ( diff ratio X Trans Ratio X 336 = MPH
so say I have a 3.54 rear ratio and am in 3rd gear trany 1 to 1, 3.54 = 1189.44
27 X say 2000rpm = 54000 devided by 1189.44 = Apx 45mph
now by going to 30 inch tires
30 X 2000rpm = 60000 devided by 1189.44 = Apx 50mph
So as you see we gain 5 mph for the same rpm simply by changing to a larger diam tire.
(RPM X tire diameter ) over ( diff ratio X Trans Ratio X 336 = MPH
so say I have a 3.54 rear ratio and am in 3rd gear trany 1 to 1, 3.54 = 1189.44
27 X say 2000rpm = 54000 devided by 1189.44 = Apx 45mph
now by going to 30 inch tires
30 X 2000rpm = 60000 devided by 1189.44 = Apx 50mph
So as you see we gain 5 mph for the same rpm simply by changing to a larger diam tire.
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Go here for a calculator:
scroll down to the second one.
http://www.novak-adapt.com/knowledge/gearing.htm
scroll down to the second one.
http://www.novak-adapt.com/knowledge/gearing.htm
Last edited by 85e150; Dec 12, 2004 at 02:40 PM.
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From: Calgary, Canada
You need to know the speed of the vehicle that you are interested in knowing the engine rpm for, the engine rpm at that speed, and the tranny gear ratio.
Divide engine rpm by tranny gear ratio, then divide that by the diff ratio. Now multiply the result by the new diff ratio and then multiply by the tranny ratio and you will have the engine rpm at the same vehicle speed.
Divide engine rpm by tranny gear ratio, then divide that by the diff ratio. Now multiply the result by the new diff ratio and then multiply by the tranny ratio and you will have the engine rpm at the same vehicle speed.
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Thanks for everyone's help. My trucks running 3.08s and is totally lugged down in O/D. 1400@60mph. Trying to decide between 3.55 and 3.73 ratios. Truck has 30 inch tires so I think the 3.73's may be a better choice.
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