Gear ratio??
07-28-2005, 11:04 AM
#2
Le Mans Master
I need the 2 letter code stamped on the bottom of the differential housing, adjacent to the strut rod bracket. AY=2:73, AW=3:08, AX or LR= 3:36, AA=3:35, AB=3:70, AC=4:11......
07-28-2005, 12:48 PM
#4
Tech Contributor
Unless it was changed I'll bet it's a 308 with that setup.
Gary
Gary
07-28-2005, 01:26 PM
#5
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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.
07-28-2005, 02:22 PM
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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.
07-28-2005, 02:34 PM
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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.
07-28-2005, 02:47 PM
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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
07-28-2005, 04:17 PM
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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.
07-28-2005, 11:06 PM
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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; 07-28-2005 at 11:17 PM.
07-28-2005, 11:54 PM
#14
Safety Car
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).
07-29-2005, 12:24 AM
#16
Originally Posted by rihwoods
At 70 I'm reving almost 3500....I know what I got...3.70:1
Last edited by Retro78; 07-29-2005 at 12:26 AM.
07-29-2005, 03:51 PM
#17
Le Mans Master
My owners manual says an l48 auto had a choice of 2.73(Econ.) or 3.08 (Std). No other choices were listed for this combo. Page 77 of the 1975 owners manual.
07-31-2005, 01:52 PM
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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.