FACTORY GEARS?????????

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04-16-2008, 08:31 AM

#21

ensign_lee

3rd Gear Member

Join Date: Nov 2007

Location: TX

Posts: 820

RE: FACTORY GEARS?????????

That story about getting to max speed is priceless. Get pulled over "son, what do you think you were doing?"

"I was trying to see what rear differential I had by testing my RPMs versus my speed."

....

and if you're Mickey and have Barricade., "plus, Megatron is coming" [8D]

04-16-2008, 09:11 AM

#22

Norm Peterson

6th Gear Member

Join Date: Feb 2007

Location: state of confusion

Posts: 7,635

RE: FACTORY GEARS?????????

Quote:

ORIGINAL: grampa_stang

Lets go with a manual and a stock tire size ofP235/55-17 (27.18 inches tall) and 3:31's.
Give or take up to a few tenths as allP235/55-17 are not created equal.

1st gear at 6000 rpm = 43.36
1st gear at 6500 rpm = 46.97

2nd gear at 6000 rpm = 73.28
2nd gear at 6500 rpm = 79.39

and asposted earlier,
5th gear at 5000 rpm = 180.94

Oh, and if you went with a shorter tire, you're mph will be lower in the same scenerio as above
If your tire is taller, you'll have a higher mph in the same scenerio as above.

Tire size can change your gear ratio.

Close, but not quite there. There's about a 3.5% correction that you have to make that's due to tread compression where it's in contact with the ground. It's not the same as using 'loaded radius' to determine tire circumference either.

With the correction factor included, you cancalculate revs per mile that are very close to the figures tabulated on the 'Specs' pages at Tirerack.com . Close enough to use for speed correction in a tune. With a 27.18" OD tire, you get 769 revs per mile and the following (still with 3.31's and a manual tranny:

1st gear at 6000 rpm = 41.8
1st gear at 6500 rpm = 45.3

2nd gear at 6000 rpm = 70.7
2nd gear at 6500 rpm = 76.6

and asposted earlier,
5th gear at 5000 rpm = 173

Norm