rear end ratio
6/16/04, 06:20 PM
#1
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Greetings,
Newbies here. Just getting started on a 1968 coupe. How do I find out the rear end gear ratio. I found these #s [WCZ F1 3.00 8FD 931] Do they indicate? or should I be looking somewhere else. The reason is I want to change the rear end to a higher ratio. Based on driving it (302 auto tranny & 9") it sounds and feels like 3.90. Definately not your freeway flyer.
Thanks!!
Deane & Robin
Newbies here. Just getting started on a 1968 coupe. How do I find out the rear end gear ratio. I found these #s [WCZ F1 3.00 8FD 931] Do they indicate? or should I be looking somewhere else. The reason is I want to change the rear end to a higher ratio. Based on driving it (302 auto tranny & 9") it sounds and feels like 3.90. Definately not your freeway flyer.
Thanks!!
Deane & Robin
6/16/04, 07:26 PM
#2
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Welcome! 
I'm not up on my codes on rears, but the door tag should have the axle code on it.
Judging by the numbers you provided, I'm going to take a wild guess and say that you have a 3.00:1 ratiio, and its most likely and open rear. However, someone could have changed that. To find out what gears are in there, do this: jack up the rear and support it with jackstands, get under the car, get some chalk, and put a mark on the inside of the tire, then put a mark on the drive shaft. Spin the tire one complete revolution, counting the number of revolutions. this should give you an approximate idea of what's in there.
Hope this helps.
I'm not up on my codes on rears, but the door tag should have the axle code on it. Judging by the numbers you provided, I'm going to take a wild guess and say that you have a 3.00:1 ratiio, and its most likely and open rear. However, someone could have changed that. To find out what gears are in there, do this: jack up the rear and support it with jackstands, get under the car, get some chalk, and put a mark on the inside of the tire, then put a mark on the drive shaft. Spin the tire one complete revolution, counting the number of revolutions. this should give you an approximate idea of what's in there.
Hope this helps.
6/16/04, 08:57 PM
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mr-mstng is telling you correctly. According to that tag, it's a 3.00 open, but you should verify as he describes.
Do you have a tach in the car? (your ears WILL deceive you). A 3.00 ratio usually turns 2900 rpm or so at 70mph.
Do you have a tach in the car? (your ears WILL deceive you). A 3.00 ratio usually turns 2900 rpm or so at 70mph.
6/17/04, 06:31 AM
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a *general* way to check is to jack up the rear a bit, mark the tires and driveshaft, and rotate by hand. Count the revolutions of each and do the math. Not scientifically accurate, but it would tell you the difference b/w a 3.00 and a 3.90
Definitive test, of course, is to pull the chunk and count teeth.
________
KellyHotTeen
Definitive test, of course, is to pull the chunk and count teeth.
________
KellyHotTeen
Last edited by LMan; 8/20/11 at 07:23 AM.
6/17/04, 06:50 AM
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Ahhh, I completely missed the 9" reference. 
That TAG is definitely for an 8 inch rear. Want to know for sure if you have an 8" or 9" ?. Here you go
That TAG is definitely for an 8 inch rear. Want to know for sure if you have an 8" or 9" ?. Here you go
6/20/04, 04:05 PM
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Good stuff, thanks guys. I'll verify the 8" - 9" thing. My question now is, how do I get the rpms down for freeway flying (and better gas mileage)? If the rear end ratio is 3:1, there isn't much better available, (like 2:90s:1) So, is an overdrive tranny the answer? (any recommendations on an OD tranny?) Again, much thanks
PS: I will be installing a tachometer
PS: I will be installing a tachometer
6/21/04, 06:57 AM
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An OD transmission is the way to go. You can "have your cake & eat it too" - deeper first gear (better acceleration), plus the overdrive for highway cruising.
With a non-OD transmission, you're really limited. Yes, you can go 2.50, 2.79 or other ratios for highway cruising, but 1st gear acceleration will REALLY suffer.
Which OD? Take your pick - there's automatic OD's, 5-speeds, 6-speeds, and even auxilary OD's that attach the the back of your existing tranny. Depending on which parts you use, and the condition/quality of what you swap in, be prepared to spend $1000-$4000...
With a non-OD transmission, you're really limited. Yes, you can go 2.50, 2.79 or other ratios for highway cruising, but 1st gear acceleration will REALLY suffer.
Which OD? Take your pick - there's automatic OD's, 5-speeds, 6-speeds, and even auxilary OD's that attach the the back of your existing tranny. Depending on which parts you use, and the condition/quality of what you swap in, be prepared to spend $1000-$4000...
6/21/04, 08:05 AM
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be prepared to spend $1000-$4000...
5mpg increase from 17-22
$2 per gallon gas
$1000 conversion price
If you manage to get by on the cheap, you'll need to drive that pony 37,400 miles before you break even!
Granted, it's nice to cruise at 2000rpm, but don't think you're saving money by doing the conversion...unless that's what it takes to convince the wife.
Figure out what you want to do with the car. If it's a highway cruiser, install 2.80 and enjoy. If it's an around town car, install 3.50.
6/21/04, 08:21 AM
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Originally posted by Mberglo@June 21, 2004, 9:08 AM
Assumptions
5mpg increase from 17-22
$2 per gallon gas
$1000 conversion price
If you manage to get by on the cheap, you'll need to drive that pony 37,400 miles before you break even!
Granted, it's nice to cruise at 2000rpm, but don't think you're saving money by doing the conversion...unless that's what it takes to convince the wife.
Figure out what you want to do with the car. If it's a highway cruiser, install 2.80 and enjoy. If it's an around town car, install 3.50.
be prepared to spend $1000-$4000...
5mpg increase from 17-22
$2 per gallon gas
$1000 conversion price
If you manage to get by on the cheap, you'll need to drive that pony 37,400 miles before you break even!
Granted, it's nice to cruise at 2000rpm, but don't think you're saving money by doing the conversion...unless that's what it takes to convince the wife.
Figure out what you want to do with the car. If it's a highway cruiser, install 2.80 and enjoy. If it's an around town car, install 3.50.
However, there is reduced wear & tear, the "kewl factor", plus "peace of mind". Can't put a $$$ figure on any of those easily.
Then, of course, we could all sell our classics, and spend $30K on a new Mustang (with factory OD & all the other amenities) to save that $1000! :shock:
6/21/04, 09:53 AM
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No doubt there are worse ways to waste a grand. Say that three times fast.
I like the T-5 conversion a whole lot more than the AOD. I'm not even sure what the "kewl" factor is on an AOD. But that's me. :scratch:
I like the T-5 conversion a whole lot more than the AOD. I'm not even sure what the "kewl" factor is on an AOD. But that's me. :scratch:
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