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Old 09-18-2010, 09:59 PM   #1
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Join Date: Jul 2010
Location: BC Canada
Posts: 427
Basic car math everybody should know

Want to figure out why your engine seems like such a dog after you bolted on those 35's? Wonder why you cant' pull O/D all of a sudden? Get these numbers and you will have the answer.

Find your:
1) Tire Revolutions per Mile (20,800 / tire diameter = Tire revs per mile.)

2) Transmission final drive (usually 1.0 in direct or .71 in O/D)

3) Final drive (3.7, 4.1, 4.56, etc.)

Now you can find the answer:

Transmission final drive x Tire RPM x Final drive = Engine RPM @ 60 MPH

Engine RPM / 60 x your desired speed = Engine RPm @ your speed

"there is neither science nor fact prevailing here" - vlamgat
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Old 10-06-2010, 11:28 AM   #2
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Location: Las Vegas, NV
Posts: 27
Re: Basic car math everybody should know

Good Info, Thanks

'08 E350 SD EB Cargo 6.0
Bought New 5/2010
Quad Van 4x4
Rudimentary DIY Camper
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Old 10-07-2010, 09:49 AM   #3
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Location: Arizona
Posts: 225
Re: Basic car math everybody should know

People often want to lift a rig and put on big tires and then get back to stock RPM with a ring and pinion change. Whats often forgoten is the extra weight and the extra wind resistance due to the additional height. These add up to the need to run a higher RMP than stock to be in the powerband and to keep it from hunting in and out of high gear. I have never heard anybody say they wish they had gone with higher gears but often hear "I should have gone lower."
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