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Discussion Starter · #1 ·
Due to their reputation and being a fellow ISU grad only 75 miles away, I'm going to have Quick Performance build me a 9" for my Chevelle with LSA swap. With the upgrade cam, pulley, and HX, I can only guess the car is going to make around 625 hp/tq at the wheels. It'll be mated up to a 4L80e trans with a Circle-D 2800 triple disc stall. This car is 100% street. It's for a lot of show and a lot of go! Question is what should I choose for a rear gear w/ that setup. Old gear setup w/ my 4L60e and factory Chevelle 12 bolt was a 3.73. I really didn't have too much complaint about that ratio. The 4L80e does have quite a taller first gear tho. Just not sure which the optimal ratio would be with my current setup. Thoughts?

Also, for axles. I'm quite sure a 31 spline will be plenty, however moving up to a 35 spline is the same price. Is there a noticeable power loss at the wheels w/ the beefier and stronger axle? Or just do it once and go with the 35 spline? Limited on those 2 choices as I'm going to go with the Wavetrac diff. Thanks!
 

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The only reason to go with the 31 spline axles is for a larger choice of differentials. If you have a 35 spline differential picked out and you are sure that you want to stay with it then don't even consider 31 spline axles, go with 35. There is no way that you could notice any power loss with the larger axles.

You are going to have more than enough torque for a 3.73 with the better 1st gear in the 4L80E.
 

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Discussion Starter · #3 ·
The only reason to go with the 31 spline axles is for a larger choice of differentials. If you have a 35 spline differential picked out and you are sure that you want to stay with it then don't even consider 31 spline axles, go with 35. There is no way that you could notice any power loss with the larger axles.

You are going to have more than enough torque for a 3.73 with the better 1st gear in the 4L80E.
Thanks for the reply! As far as the rear gear.....was just curious if going a little taller, maybe like a 3.50 or 3.25 would help a little with traction/mpg. My guess tho, is that I'm gonna be blowing the tires off no matter what lol.
 

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There are many things to consider when choosing a gear ratio. Electronic fuel injection makes things easier, but you still have to consider the cam profile, torque converter, tire size and a few other things so that the whole car works together. You can put a 3.00 in it if the cam and converter will work with it, but if the cam overlap is too great then the engine will not be happy with the low cruise RPMs in overdrive. I'm guessing that this is a lock up converter, so converter slip won't be an issue, but stall at take off might not be enough to get the car moving with a 3.00 gear. The overdrive ratio and tall tires might be too much with a 3.00 gear. I just used a 3.00 gear as an example. I doubt that you would want to do that anyway, but you probably get what I'm trying to say. Pick something that will work with your engine's best cruise RPMs and get the car moving easily. You can use this to see what your cruise RPMs will be with different gears and tire sizes. https://www.ringpinion.com/calculators/Calc_RPM.aspx
 

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Discussion Starter · #5 ·
There are many things to consider when choosing a gear ratio. Electronic fuel injection makes things easier, but you still have to consider the cam profile, torque converter, tire size and a few other things so that the whole car works together. You can put a 3.00 in it if the cam and converter will work with it, but if the cam overlap is too great then the engine will not be happy with the low cruise RPMs in overdrive. I'm guessing that this is a lock up converter, so converter slip won't be an issue, but stall at take off might not be enough to get the car moving with a 3.00 gear. The overdrive ratio and tall tires might be too much with a 3.00 gear. I just used a 3.00 gear as an example. I doubt that you would want to do that anyway, but you probably get what I'm trying to say. Pick something that will work with your engine's best cruise RPMs and get the car moving easily. You can use this to see what your cruise RPMs will be with different gears and tire sizes. https://www.ringpinion.com/calculators/Calc_RPM.aspx
Thanks again! I'll check out that link!
 

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How tall is the rear tire going to be?

Andrew
 

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25.5" tall is all
With that short of a tire (why so short?) I would run a 3.08 gear. That will put you at right around 2100 RPM at 75 mph. The power will be fine with your modified LSA.

If you went with a taller tire, like 27" I would then go with a 3.36 or 3.42 max. That will put you at you at 2173 rpm and 2211 rpm @ 75 mph, respectively.

Andrew
 

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Discussion Starter · #10 ·
With that short of a tire (why so short?) I would run a 3.08 gear. That will put you at right around 2100 RPM at 75 mph. The power will be fine with your modified LSA.

If you went with a taller tire, like 27" I would then go with a 3.36 or 3.42 max. That will put you at you at 2173 rpm and 2211 rpm @ 75 mph, respectively.

Andrew
I'm running a 17" rim with 285/40ZR17 tires. Would love to go taller/wider but we know how awful it is trying to fit a tire under these chevelles without cutting. I'm not rolling in my 1/4 panel. Not an option.
 

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I'm running a 17" rim with 285/40ZR17 tires. Would love to go taller/wider but we know how awful it is trying to fit a tire under these chevelles without cutting. I'm not rolling in my 1/4 panel. Not an option.
Its really not hard to get a big tire under a Chevelle, you just have to get the right back spacing. 25" tire is really short (too short), my opinion.
 
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