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Old 05-31-2011, 05:56 PM
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optimal rear gear ratio

Hi guys,

I want to optimize the rear end ratio on my 33 Ford street rod project. I plan on using a Tremec TKO 600 5 speed that has a 1st gear ratio of 2.87:1 w/the remaining ratios of 1.87, 1.28, 1:1 & an OD of 0.87 or 0.68:1. I don't want the rear ratio to be numerically too high since this will make 1st gear almost useless, & I'll be grabbing for 2nd too soon. Are there any guidelines for this ?

Thanks,
Mike.

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Old 05-31-2011, 07:17 PM
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I think ( my opinion) would be a or 4.11 would be pretty ideal, giving a good first for off the line and with a .64 overdrive in high gear, that ends up at 2.63 to 1 .That is high enough to give good fuel economy on the highway.
Be sure to have your driveline balanced well, with a gear that low your driveshaft will be spinning almost twice as fast as normal
tko 500 has a 5th ratio of .68
the 600 has a fifth ger ratio of .64
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Old 06-01-2011, 09:06 AM
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Final ratio in first gear

Hot Rod, IMHO, For drag racing type launch where short shifting to second is not a big concern about 10:1 final ratio in first, for street or road race type launch which gives you a much more usable first gear about a 9:1 final ratio. A 3.50 rear puts you around 10:1. A 3.15 rear puts you around 9:1 final ratio. You have a real low first @ 2.87:1.

To put this into perspective, imagine an old Muncie 4 speed with the popular 2.20:1 first gear. With this old Muncie, a 4.56 rear would be about 10:1 and a 4:11 rear about 9:1.

I hate a first gear final ratio that won't allow you to stop at an intersection, then make a turn without having to shift to second before you finish making the turn, with the engine revving to the moon. Food for thought. Nolan
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Old 06-01-2011, 10:18 AM
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This may help you out,

http://www.5speedtransmissions.com/5sp_comparison.html

Hope this link works
A 3.08 ratio would make 5th gear pretty tall. check the charts you will see .
The richmond may be a better choice.The article may help with an answer to your question.
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Old 06-01-2011, 12:08 PM
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The TKO is a compromise all around. Its very hard to get a completely happy axle/trans/engine rpm arrangement.

As Ol Nolan said; if you multiply the 1st gear times your perspective axle...and try to get say 9.5 (closer to 9 for a street car - 10 for a drag car)

But, you need to be cognizant of your camshafts cruising RPM or you'll have a miserable 5th gear

Also, if you have tall street rod tires, this effectively lowers your axle ratio numerically.
Ergo: .87 OD as I doubt you can keep your motor happy with a .68 as you need a lower axle to play nice with the 2.87 1st
my suggestion is a 3.55 axle ( 3.55 x 2.87 = 10.1 total reduction) and the .87 OD will get you in the 3.08 axle territory (depends on your tires) so you won't have to lug the motor and shift into 4th all the time to keep it happy

Or get a 2.88 T10 or 2.98 Muncie and put 3.08s in the back; you'd have the same performance and in a lighter and smaller package with better thermal efficiency.

Congrats on building your dream car. Keep us posted
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Old 06-01-2011, 04:56 PM
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optimal gear ratio

First of all, thanks to all who have posted I spent some time on the link to 5 speed transmissions & was intrigued by the "distance traveled in 1st gear" calculations.

I need a sanity check. Again the car will never be on the strip. Engine will be 496 & Hp/torque targeted for 550-575 range. Tire size is 31.1 inches (P275-70/16). I cannot change tire size. Car weight will be around 3200 lbs.

On the website www.5speedtransmissions.com, if I consider using a 3.5 gear, the launch ratio is 10.05. The distance traveled in 1st gear will be somewhere around 13 to 19 ft if I shift @ 2500 to 3000 rpm, respectively. This seems to be a very short distance to travel before going to 2nd, but I really don't know for sure. Obviously the distance traveled in 1st gear will be less if I run a numerically higher gear ratio.

On the other end, cruise rpm w/the 3.5 gear is only 1812 rpm @ 75 mph, w/the 0.64 ratio. I'm not sure if I can run that low of an rpm w/carbs. I could go w/the 0.82 5th gear ratio, but that puts the rpm up @ 2322 . So what I am thinking now is a gear ratio of around 3.15 which gives me a launch ratio of 9, & run the 0.82 5th gear which will give me a cruise rpm of 1950 @ 70 mph. Also, the distance traveled in 1st gear is now 18.2 ft @ 2500 rpm shift points.

Does this sound reasonable ?

Thanks again,
Mike.
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