Drivetrain Tech General discussion of drivetrain for the Ford Ranger.

transmisson ratios and figuring rpm at certian speed

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Old 02-06-2006
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transmisson ratios and figuring rpm at certian speed

i got this from another post from bob

Originally Posted by rwenzing
1st 2.47
2nd 1.85
3rd 1.47
4th 1.00
5th 0.75 (OD)

Rev 2.10
with that information how can i figure my rpm at 60mph. there has to be a formula to figure what it should or close to it.

with 33s and 4.10s its like 2100 rpms right now

so what should it be with 35s and 4.88s ?
 
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Old 02-06-2006
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I can't think of a concise formula for it, but it seems to me that you would have enough information to calculate it...

engine rpms * transmission ratio * axle ratio * circumference of the tire = distance travelled per minute, right? 1 mile/min = 60 mph
 
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Old 02-06-2006
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idk that is alot of multiplying and it comes out to a big number.
 
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Old 02-06-2006
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You could easily do it with multiplication but this calculator makes it even easier:

Go to -> http://www.bgsoflex.com/auto.html
Select "RPM amd MPH Correlation" from the LH panel.
 
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Old 02-07-2006
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thanks bob. that is exactly what i wanted.
 
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Old 02-07-2006
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Don't forget that most '33' and '35' tires are 0.3~0.5" smaller than their sidewall numbers would suggest, even when brand new. Using a number that is 1/2 inch smaller in the calculations is a good rule of thumb for improved accuracy. For example, use 32.5" for the diameter of a '33' and 34.5" for a '35'.
 
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Old 02-07-2006
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ya thanks for the reminder. i didnt forget.

i figure the match wont be perfect as there is wind resistance, and alot of other factors. but it should be close.
 
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