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Discussion Starter #1
Hey what's up, perhaps this is a newb question but i'll ask anyways. Before installing the FC UDP my RPM's at 70 were exactly 3k. Now after the UDP my rpm's at 70 are about 2725. Is that normal?
 
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Should only have to do with gearing dude. Did you change your tires to a different diameter?
 

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Discussion Starter #5
HTPerformance said:
Should only have to do with gearing dude. Did you change your tires to a different diameter?
Well I got some new tires, but they are stock size. Believe me, i'm not bul**iting you. The rpms are lower at those speeds now than before the UDP.
 

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If the old tires were pretty badly worn, it could change it slightly.
 

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while it may change your actual speed, it won't change the readout on your speedometer. your speedometer is reading how many times per second your differential spins, and the computer multiplies that number by the stock tire diameter to get your speed. even if you installed the "same size" tires, you can sometimes see a slight difference in tire revolutions per mile from one brand to the next. it's kinda like how a reebok 10 isn't the same size as a nike 10. like I said, this will affect your actual speed, but it won't change the reading on the odometer.
 

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Discussion Starter #8
crazymechanic said:
while it may change your actual speed, it won't change the readout on your speedometer. your speedometer is reading how many times per second your differential spins, and the computer multiplies that number by the stock tire diameter to get your speed. even if you installed the "same size" tires, you can sometimes see a slight difference in tire revolutions per mile from one brand to the next. it's kinda like how a reebok 10 isn't the same size as a nike 10. like I said, this will affect your actual speed, but it won't change the reading on the odometer.
That's a useful answer, thank you.
 
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crazymechanic said:
like I said, this will affect your actual speed, but it won't change the reading on the odometer.
Sure it will have an effect on the odometer. If for instance I had the stock Conti's which are 215/45/17 and 819 revs/mile and switch to 205/40/17 which are 860 revs per mile there is a 5% increase in the number of revs per mile, so I experience 1/2 mile extra on the car every 10 miles that i drive.

860/819=1.05

So my commute to the lab everyday is 12.8 miles from my home, so... 12.8 miles on the Conti's has now become 13.44 miles.

13.44-12.8 = 0.64 miles extra on the odometer each way on the highway every day. Just cuz I have smaller than stock tire size.
 
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