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Join Date: Oct 2003
Location: Scottsdale,Az
Posts: 3,738
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7:31 ring & pinion question?
My 1980 SC had the tranny rebuilt 2 years ago and they added the 7:31 ring & pinion.
Ok I know its for quicker starts but whats the difference between 7(205/50/15) & 8 (225/50/15) x15 Fuchs Vs 7(225/45/16) & 9 (245/45/16) x 16 Fuchs Which wheel combo would be a better choice for the above gearing? Street driving only
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Frank 1980 SC Cab Conversion (sold) 1974 914 2.0 RIP rear ended Looking for a 996 Silver Cab 2002-2004 |
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09-10-2006, 01:23 PM
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Well, the 225/50-15 is approximately 23.85 inches in diameter and the 245/45-16 is 24.68 inches, so the 225/50 is about 3% shorter than the 245/45-16. This means that you're seeing about a 3% difference in gearing as a result.
Another way to look at it is that a 7:31 is a 4.429:1 ratio. Going from 245/45-16s to 225/50-15s is about the same as changing the ring and pinion to a 4.56:1. Or, if you go the other way (from 225 to 245) it would be like a 4.29:1 ratio. It's all "gearing", in other words. IMHO, a 225/50-15 with a 7:31 in a later gearbox with a relatively short 5th gear is going to be too "short" of gearing for typical street use. I wouldn't want to drive that car on the interstate for long distances... Definitely go with the 245/45-16s. Mike
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Mike 1976 Euro 911 3.2 w/10.3 compression & SSIs 22/29 torsions, 22/22 adjustable sways, Carrera brakes |
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09-10-2006, 01:35 PM
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Location: Atlanta
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Hey Mike what is the equivalent 8:31 ratio. Just curious.
Fender was there a profound difference when they rebuilt your box with the new R&P?
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erik.lombard@gmail.com 1994 Lotus Esprit S4 - interesting! 84 lime green back date (LWB 911R) SOLD  RSR look hot rod, based on 75' SOLD 73 911t 3.0SC Hot rod Gulf Blue - Sold. |
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09-10-2006, 02:07 PM
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Location: Long Beach CA, the sewer by the sea.
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Yeah, it's at least 3%. Closer to 3.5, me thinks (and according to my calculator
 ). Now, someone multiply 3+% by the max torque and tell us what really happens! And, that's just with the tire difference. I have no idea how to factor the 7:31 vs. 8:31 difference, although it should be the same forumla.
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09-10-2006, 02:20 PM
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Join Date: Oct 2003
Location: Scottsdale,Az
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Quote:
I have no clue, this will be my first 911 (still on its way to me) , so I have no clue what the difference would be? I just know the car has 6 & 7 x 16 Fuchs and as soon as Al Reed does my 7 & 9 x16 Fuchs, they are going on. Wanted to see if there was a difference between 15's and 16's....
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Frank 1980 SC Cab Conversion (sold) 1974 914 2.0 RIP rear ended Looking for a 996 Silver Cab 2002-2004 |
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09-10-2006, 02:23 PM
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An 8:31 is essentially a 3.875:1 ratio. Much taller than a 7:31. Simply divide the number of ring gear teeth by the number of pinion gear teeth to get the ratio...
Mike
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Mike 1976 Euro 911 3.2 w/10.3 compression & SSIs 22/29 torsions, 22/22 adjustable sways, Carrera brakes |
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09-10-2006, 04:11 PM
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1976 Porsche 911
