Need help with choosing a good rear axle ratio

Willybill32

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Jul 16, 2019
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I have an ‘86 GT 5.0, 5-speed. The stock engine has been warmed up a bit (Twisted Wedge heads, E303 cam, roller rockers, BBK fuel rails with adjustable fuel pressure regulator, 24 lb injectors, Bassani shorty headers, BBK 65mm throttle body, BBK 76mm MAF, new Centerforce clutch kit, etc.). I just had it tuned, and it runs well. It also has the stock 2.73 rear end, and in discussion with the tuner, perhaps a numerically higher rear axle ratio would be appropriate. I’m considering both a 3.55 and a 3.73, but I’d really appreciate some real world feedback on these options.

I’m in the senior citizen ranks, and I don’t want to run the car too hard. I admit that cruising on the Interstate at 80 mph at an indicated 1,800 rpm is pretty comfortable for me, but with the modifications, around town driving is a bit difficult and I’m looking at the change mostly for driveability purposes. But my buddy says the 3.73 would really wake the car up! Can you offer any advice?
C53642A7-2ED5-4B05-9517-7EC5102753D9.jpeg
 
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revhead347

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It depends on your goals. The factory 2.73 is definitely hurting your performance, and I definitely agree with your friend that a rear end gear change is the best thing you can do to improve acceleration. There is a gap between drag cars, and street cars. Mathematically, on a drag car, we split the power curve from the dyno, figure out what the max rpm at the end of the 1/4 mile will be, and adjust the final drive ratio to ensure the car goes through the traps at that rpm. In that mathematical rigid understanding, you are looking at at least a 3.73:1, 4.10:1 if not a 4.30:1 raio. For a street car, driving manners matter more. I have always preferred 3.55:1 on my street car. It's going to put you around 2600rpms at 80mph. With a good running car, and a quality exhaust system, that's not really going to bother you much. There isn't a whole lot of performance gap between the 3.55:1 and the 3.73:1. Most people are fine with the 3.73:1, but if in doubt, the 3.55:1 is a sure bet.

Kurt
 

LILCBRA

I started this morning by knocking out some studs
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I have 3.55s in my Cobra II and with 265/50-15s it has decent acceleration and is tolerable at highway speeds. I give them my vote for your car too.
 
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Habu135

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Jan 10, 2019
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I've had four Fox Bodys and have never gone lower than a 3.55. My current coupe has the Trick Flow head/cam/intake combo and I'm very happy with a 3.73. Either is perfect. It doesn't sound like you plan to hot dog the car around town too much so....3.55 gets my vote.
 

02 281 GT

Agreed...My wife has great Boobs
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Agreed on 3.55s or 3.27s. I have 3.73s in my '95 Cobra. It turns nearly 2,800 RPM at highway speed. I'm going a little more extreme and acquired a set of 3.08s to install in it (factory gear ratio).
 

aztecgwynn

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Apr 3, 2015
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any website that can verify what RPM's would be with gear change, tire size etc etc. I am also looking to redo axle but need to verify RPM with gear change, don't want to install and I don't like car revvving to moon on freeway speeds and ask shop don't like gears. Can you change to another gear and he gives you second bill for $1K LOL!!!!
 

Habu135

Advanced Member
Jan 10, 2019
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any website that can verify what RPM's would be with gear change, tire size etc etc. I am also looking to redo axle but need to verify RPM with gear change, don't want to install and I don't like car revvving to moon on freeway speeds and ask shop don't like gears. Can you change to another gear and he gives you second bill for $1K LOL!!!!
The most I've paid for a rear end gear change was $400. I remember paying $150 back in the late 1990s though.
 

CarMichael Angelo

clearly, I’ve got something going on in that hole
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Ok 90 bagillion replies, all pointing to picking 3.55’s as a rear gear set.

DO WE NEED TO TALK ABOUT THIS FOR ONE FREAKIN’ SECOND LONGER????
 

General karthief

wonder how much it would cost to ship you a pair
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Ok 90 bagillion replies, all pointing to picking 3.55’s as a rear gear set.

DO WE NEED TO TALK ABOUT THIS FOR ONE FREAKIN’ SECOND LONGER????
I
Would you rather talk about what tire/rim combo fits without rubbing?
Why an E cam is soooo 90's?
Oh, I know, why stuffing a tooge in a 78 fox platform is a better idea than a coyote!!! :jester:
 
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