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Rear Gear Choice

1969 beep beep

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Hi everyone,

I am currently rebuilding my rear end in my 69 Roadrunner. The car weight is 3900 lbs with no driver. The car is used as a street car, and might see the track once a year. The current motor is a stroker making 600hp 630tq. I am running a 28 inch tire. 60 mph rpm is 2850, with a factory 4 speed. No traction issues with the current 3:91.

I originally had a factory 3:55 with the original 383 engine, and replaced it with a 3:91. I then added the stroker motor. I am in the process of rebuilding the entire car to factory specs. I would still like to keep the stroker motor.

I am thinking of replacing the gear set to either 3:55, 3:73 or keep the 3:91. If I chose the 3:71 it should bring the rpm to 2725 @60 and with the 3:55 it should be around 2600 rpm @60.

Light to light is really great with the 3:91 but after driving on the freeway for 2 hours at 2850-3000 rpm it does getting annoying. The car is usually driven around town and the occasional long road trip.

Any recommendations to which gear set I should stick with? I like the lower rpm with the 3:55 but I don't want it to be a dog out of the hole. I am sure the stroker will help with that. Cam operating rpm range is around 3200-6500. The other question I had was which gear is easier on the drive train components? I am guessing the 3:91.
 
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The other question I had was which gear is easier on the drive train components? I am guessing the 3:91.

A 3.55 geared car will be easier on the drivetrain components, compared to a 3.91 gear car. You answered your own queston, I am running a 28 inch tire. 60 mph rpm is 2850, with a factory 4 speed. No traction issues with the current 3:91and with the 3:55 it should be around 2600 rpm @60.. running your car at 2600 instead of 2850 at 60 mph will be easier on the cars drivetrain.
 
The other question I had was which gear is easier on the drive train components? I am guessing the 3:91.

A 3.55 geared car will be easier on the drivetrain components, compared to a 3.91 gear car. You answered your own queston, I am running a 28 inch tire. 60 mph rpm is 2850, with a factory 4 speed. No traction issues with the current 3:91and with the 3:55 it should be around 2600 rpm @60.. running your car at 2600 instead of 2850 at 60 mph will be easier on the cars drivetrain.

Would a 3:55 not get better traction then a 3:91?
 
that's a loaded question when you start to talk about traction, you have to take in how much horsepower a car makes, the tire size, the gearing, suspension work and how much the car weighs. Gear size is only one component of the question.
Yes on the same car 3.55 gears will get better traction then the 3.91 gears, it will spin the tires easier with the 3.91 gears compared to the 3.55 gears
 
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that's a loaded question when you start to talk about traction, you have to take in how much horsepower a car makes, the tire size, the gearing, suspension work and how much the car weighs. Gear size is only one component of the question.
Yes on the same car 3.55 gears will get better traction then the 3.91 gears, it will spin the tires easier with the 3.91 gears compared to the 3.55 gears
I was thinking the less tire spin would put a greater load on the trans, driveshaft and rear end.
 
I run 3.55s with a 28 inch tire like you and I think it's a great all around ratio. I go to the track a little more often though and would like to get a little lower set. You have much more hp/tq than I do and I still run 12.70s with a best 60ft time to date of 1.83. I would stick with what you have but I don't think the 3.55s will hurt your performance that much if you switch. JMO
 
If you have a 600 hp motor you don't have to worry about it being a dog out of the hole, especially with a 4 speed...
 
My rule is if the car spends most of it's time on the road and little on the track, 3.23 is the best gear set. Getting into the 3.5X or bigger range is really cutting into your mileage and not worth the cost unless you're spending a lot of time at the track, and going lower than 3.23 is just no fun to drive on the street.
 
Depending on how strong your stroker is as many cars with strokers dont need as much gear as you would think. I remember Mopar Muscle tested a B-body with a stroker eng and it went as fast with 3.73's as it did with the 4.30's. It makes enough low end grunt that it did not need all that gear. I have 4.30's behind my 493 but I run a 30" tall tire and I really feel it would be faster on a little less gear. I had the 4.30's behind my old 440 and I just went to the taller tire for now but I may drop to 3.91's with the 30" tires. Good luck to you. Ron
 
This depends on so many factors....all I will say is that on my most recent car, a 68 satellite with a 440/ 230 deg cam @ .050, 2400 stall, on 28"rear tire I went with 3.73s for the first time ever and I gotta say I'm real happy...it's a street driven car with rare track use but lots of fun around town (many of the streets around here have 45-50 mph speed limit) rpms don't drive me crazy and oh boy it goes...by the time the 'new' muscle cars catch up to me it's time to slow down for the stop lights anyway
 
This depends on so many factors....all I will say is that on my most recent car, a 68 satellite with a 440/ 230 deg cam @ .050, 2400 stall, on 28"rear tire I went with 3.73s for the first time ever and I gotta say I'm real happy...it's a street driven car with rare track use but lots of fun around town (many of the streets around here have 45-50 mph speed limit) rpms don't drive me crazy and oh boy it goes...by the time the 'new' muscle cars catch up to me it's time to slow down for the stop lights anyway
I run a 3;73 geared dana 60 w/ 29 1/2" tall drag radials, ain`t too bad anywhere, approx. 700 h.p., auto, 3500 convertor .
 
Depending on how strong your stroker is as many cars with strokers dont need as much gear as you would think. I remember Mopar Muscle tested a B-body with a stroker eng and it went as fast with 3.73's as it did with the 4.30's. It makes enough low end grunt that it did not need all that gear. I have 4.30's behind my 493 but I run a 30" tall tire and I really feel it would be faster on a little less gear. I had the 4.30's behind my old 440 and I just went to the taller tire for now but I may drop to 3.91's with the 30" tires. Good luck to you. Ron
Agreed, since I have a 6 speed I built a set of 4:30s for mine and hated them (I know it's a bit steeper than your talking), it was so quick at going threw the gears it really slowed the car down. I'm running a 456 stroker and it loves the 3:55s. Now instead of shifting constantly it pulls longer and feels much faster. Torque doesnt need the gearing to go fast.
 
747Mopar, What cam are you running?

not so famous Bob, & Ron which tires are you guys running?

Replacing the rear 28 inch tire to something taller is another option if I keep the 3:91.
 
I run the Hoosier radial slicks that are 30 X 9. I drive on the street with them also. I have 4.30 gears and I drive it the 90 miles to Carlisle doing 60 mph on the freeway. It turns about 3200 rpm at 60 mph which is humming right along for it. Ron


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I forgot the original question.....but my '70 Road Runner (back in 1987-90) had a mild cammed 440 with barely 9:1C/R, 4 BBL Holley 750vac-sec, 4-speed manual gearbox, 3.23:1 rear end and a sure-grip. Tyre size was 235x60x14, and a true speedometer readout was checked....as accurate as possible.
I can honestly say, that I wound that car over the 150mph on the speedometer and back to Zero. It was an honest 150-155 mph car. One trip we made to a weekend drag meet, I was towing the Club BBQ trailer, and I totally left behind a '66 427 SS Chevelle & '69 428 Mustang Cobra Jet. they were getting smaller in my mirror....no BS. The trailer broke on the way home, but that's another story :D
Anyway, my car while being very mild mannered around town, not loud, but capable of phenomenal speed on the open road, still made a flat 14.0 second quarter mile.
But it could also do this.......

I wish I still had that car. :(
 
I was thinking the less tire spin would put a greater load on the trans, driveshaft and rear end.

To me, the hardest load on a drivetrain is when spinning tires suddenly grab traction. I always thought spinning tires slowed you down. So if you are spinning, wouldn't you want a lower (numerical) gear ratio?
 
droptop you made a good point. When the tire spins then hooks there would be a lot of load on the drive train components.
 
Kiwigtx, very nice video. I am sure you burned up a set of tires.

Ron thanks for the pics. That car looks amazing!
 
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