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Discussion Starter #1
Looks like I'm looking at new rear gears. It will (eventually) be paired with a WC T5, but for now it'll be with my C4.

Current gears are 3.23. Tires are 25.65" tall.


Speeds on track range from 35mph-120ish.


Thinking either 3.25s or 3.40s should work well. What does everyone else run? Thoughts?
 

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I'm running 3.70's with a T56.

If you look at a final drive ratio of 1:1 (for your C4 or 4th gear on the T5) then your tire height and 6500 RPM yields a top speed with 3.23's of 153 mph which is much higher than your intended usage. 3.70's give a top speed of 134 mph at 6500 RPM.

Are you strictly interested in track performance or also freeway manners?

Also, there are a few tracks out there where you can most likely exceed 120mph.
 

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Discussion Starter #3
I'm running 3.70's with a T56.

If you look at a final drive ratio of 1:1 (for your C4 or 4th gear on the T5) then your tire height and 6500 RPM yields a top speed with 3.23's of 153 mph which is much higher than your intended usage. 3.70's give a top speed of 134 mph at 6500 RPM.

Are you strictly interested in track performance or also freeway manners?

Also, there are a few tracks out there where you can most likely exceed 120mph.

I don't want my engine to implode, so I limit it to 4800. 4000 on the C4 is ~94mph. 4000 with the T5 and with 3.25s would be 149mph, with 3.40s would be 142mph, and with 3.55s would be 136mph. My concern is that 3.55s might be too deep, and I might do too much shifting on a road course, using 5th for just the straightaways...but maybe that's not a terrible thing?? I like having options though lol.



I don't often run it on the highway, mainly because of the C4/3.23 combo it has now.
 

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4800? That is very low.
I lapped CMP in 3rd and 4th with 3.50 rear gear.
I would at lest run 3.70 for rear gears.
 

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I used to open track a ‘83 GT. After I grenaded my T5 and 7.5” rear, I changed to a WC T5 and a 8.8” with 3.73’s. It was OK...needed 3.89 or 4.11 to get out of the turns better. Also depends on the track. Small and technical, or long and speedy. Don’t be afraid of imploding your engine. Should be good to at least 6000 on a regular basis. It’s only o key to be there for a few seconds at most anyway.
 

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Discussion Starter #7
The tracks I run on - Pacific Raceways and The Ridge - are mostly technical, with lots of tight turns and only really a single straight away.
My friend/mechanic who's helping with all this wants me to put in a Toploader...but I don't see why? I know they're bulletproof, but don't they also have 1.0 4th gear like 3rd on the C4? I fail to see his reasoning, but what do I know lol.
I'm still undecided and have a tentative track day on the 23rd. Arggggg.
 

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The tracks I run on - Pacific Raceways and The Ridge - are mostly technical, with lots of tight turns and only really a single straight away.
My friend/mechanic who's helping with all this wants me to put in a Toploader...but I don't see why? I know they're bulletproof, but don't they also have 1.0 4th gear like 3rd on the C4? I fail to see his reasoning, but what do I know lol.
I'm still undecided and have a tentative track day on the 23rd. Arggggg.
If you don't want to 'impolde' your engine, then the most important thing is to make sure you're using a quality baffled Road Race oil pan from someone like Aviad, Armando or Kevko. You don't want a street or drag pan as the baffling won't help you in those tight technical turns.
 

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I'm not trying to insult you but it takes a fairly healthy engine to pull 120 mph @ 4,800 rpm. A 3.80 (8" I assume you have) would be good for Pacific raceway's long straight. Your mechanic is suggesting a four speed for better ratio selection in lower speed corners. It would make you faster that you are with the C4. If you are going to a T5, make sure you get one with the 2.95 low as the standard 3.35 low might be too much rpm in a lower gear and not enough in a higher gear due to the "wider" ratios. That is a technical track for sure.
Randy
 

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Gearing will depend on a few factors. You gave speed, tire height and possible transmission, what motor are you running and where will it make the best torque and hp and what rpm range does it make it? When I was running 289/302 engines, gearing was 456, 411, and 370. Went to a 351 and gearing was 370 and 350. Later went to a stroker and gearing is 350 or 325 all depending on the track.
 

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With the spec's in your post, you will need a 300 gear to give you 120mph @ 4800. fast, cheap, durable ... you can only pick 2
 

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Discussion Starter #12
I'm not trying to insult you but it takes a fairly healthy engine to pull 120 mph @ 4,800 rpm. A 3.80 (8" I assume you have) would be good for Pacific raceway's long straight. Your mechanic is suggesting a four speed for better ratio selection in lower speed corners. It would make you faster that you are with the C4. If you are going to a T5, make sure you get one with the 2.95 low as the standard 3.35 low might be too much rpm in a lower gear and not enough in a higher gear due to the "wider" ratios. That is a technical track for sure.
Randy
No insult taken - I'm spinning closer to 5k to get to 120mph. I just used the 4k as a comparison with the different gears.


Gearing will depend on a few factors. You gave speed, tire height and possible transmission, what motor are you running and where will it make the best torque and hp and what rpm range does it make it? When I was running 289/302 engines, gearing was 456, 411, and 370. Went to a 351 and gearing was 370 and 350. Later went to a stroker and gearing is 350 or 325 all depending on the track.
Warmed over 289. Power band is about 2600-3400.


With the spec's in your post, you will need a 300 gear to give you 120mph @ 4800. fast, cheap, durable ... you can only pick 2
Funny you should mention a 3.00...I'm "borrowing" a friend's 3.00 3rd member this weekend - not to keep permanently, but just to narrow down the cause of the noise, to see how bad of shape the current gears are, and to buy me time to order new ones and get the 3rd rebuilt.


While I would LOVE to get lower gears, the T5 install is a ways off, and I need to run this car with the C4 for the foreseeable future. It sucks, as I would rather do it all at once and be done with it, but I'm still about $1300 short of the T5 conversion. Grrrr. Thanks all for the advice!!
 

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I did A helpful tool for determining gearing a while back. Helpful to get an understanding of top speed and shift point given transmission, rear end ratio and tire size.

Works better on a desktop then phone.

http://www.gtsparkplugs.com/GearCalc.html

Sandy
 

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3.00 gears and a C4 for track use? How did that go? I expect you found going from 3.23 to 3.00 was the wrong direction. Did you run out of power on the top end before you run out of RPM range on the long straight?

My street car evolved from C4 w/3.00 to C4 w/3.55 to T5z w/3.55 all running 225r14-60 tires. I cannot imagine a road course scenario where the 3.00 would be over a 3.55, for a warmed up 289.

Shelby delivered cars with 3.89. A friend that vintage raced a '65 GT 350 typically used a 4.11 on tracks such as Mid Ohio.
 
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