Gear ratio??
Thread Starter
Racer
Joined: Jul 2005
Posts: 317
Likes: 0
From: PA
I did read somewhere that all the low horse L-48's were a 3:36 for that year, that's why I was wondering, I did throw a set of domed pistons in there to up the compression to 9.5:1, I'm changing the cam soon. I'm debating on headers and side pipes or just duals with the stock manifolds, we'll see.
Pro
Joined: Sep 2004
Posts: 690
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From: Madison WI
Originally Posted by Fevre
To be sure jack the back end off the ground rotate the wheels one revolution and count the drive shaft rotation, if 3 then 3.08, if 3 1/3 the 3.36, if 3.75 then 3.70 etc.
Race Director
Joined: Mar 2001
Posts: 11,322
Likes: 1
From: Living in the Hartland
Originally Posted by 72Tornado
Given that it's kind of hard to tell if the driveshaft rotates 3.08 times or 3.36 times, I'd recommend rotating the wheels 10 revolutions instead
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Team Owner
Joined: Oct 2004
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From: Diesel smoke Makes me horny
St. Jude Donor '05-'06-'07-'08
Originally Posted by CGGorman
In other words, trust nothing but the indisputable. Count either the teeth or the revs.
Team Owner
Joined: Apr 2001
Posts: 54,204
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From: S.W. Ohio. . . . . . NRA Life Member
an old rule-of-thumb to determine rear axle ratios was to take the car up to 75mph with the tranny in a gear that's a 1:1 ratio. 4 speeds this was 4th gear, and automatics it was 3 "D".
then divide the RPM by 1000. You must be using the same rear tire heigth as what originally came on the car (or have a corrected speedo if rear tire heigth was not original).
examples:
3080 RPM would mean a 3.08 rear axle ratio.
3700 rpm would equate to a 3.70 rear ratio.
automatics with non-locking TC's tend to mess this test up a bit; the RPM would be a little higher than it should be. (300 to 400 rpm ???)
I haven't tried this on a C3 in many... MANY years. I don't know how valid it is today - across the entire automotive spectrum.
.
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then divide the RPM by 1000. You must be using the same rear tire heigth as what originally came on the car (or have a corrected speedo if rear tire heigth was not original).
examples:
3080 RPM would mean a 3.08 rear axle ratio.
3700 rpm would equate to a 3.70 rear ratio.
automatics with non-locking TC's tend to mess this test up a bit; the RPM would be a little higher than it should be. (300 to 400 rpm ???)
I haven't tried this on a C3 in many... MANY years. I don't know how valid it is today - across the entire automotive spectrum.
.
.
Last edited by Mike Mercury; Jul 28, 2005 at 11:17 PM.
Safety Car
Joined: Oct 2004
Posts: 3,725
Likes: 5
From: Minnesota
I think that may be pretty close Mr. Mercury. 
Pauvil, does your acceleration really suck? Chances are you've got a 2.73 like my 75 had. If you're revving under 3,000 RPM at 75 MPH like Mike says that's would be a good guess. Do you have more than 50K miles? Chances are the side yokes are shot. You can rebuild it with different gears if your case isn't toast ($$$$) or find a different one like I did with better gears (3.36 at least).
Pauvil, does your acceleration really suck? Chances are you've got a 2.73 like my 75 had. If you're revving under 3,000 RPM at 75 MPH like Mike says that's would be a good guess. Do you have more than 50K miles? Chances are the side yokes are shot. You can rebuild it with different gears if your case isn't toast ($$$$) or find a different one like I did with better gears (3.36 at least).
Drifting
Joined: Oct 2004
Posts: 1,259
Likes: 7
From: Ontario
Originally Posted by rihwoods
At 70 I'm reving almost 3500....I know what I got...3.70:1
Last edited by Retro78; Jul 29, 2005 at 12:26 AM.
Thread Starter
Racer
Joined: Jul 2005
Posts: 317
Likes: 0
From: PA
The car has 38,000 original miles, I'm the third owner. I haven't driven it yet, I just like to know what I'm working with so when it does fall on it's face with the joke hp this year makes I can determine the cam I'm going to use. I've already put 9.5:1 domed pistons in. I don't see a cast on the differential either.