3:55 or 3:91?

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Grinder said:
**** just go 4.56's and get an overdrive.
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Haha that is awesome.
Is it tough to put in an overdrive??
(i say go 3.73 but I do not have a lot of knowledge in this area)
 
4.10 x 803(tire revs per mile) = 3292 rpm @ 60 mph. Kinda high for the
street but great for the track. FYI when racing 100 mph= approx. 5486 rpm.
You can do the math yourself. Tire revs per mile times any given rear ratio
will give you rpm at 60 mph. Hope is helps.
 
mullinax95 said:
This will help you choose the correct gear you want.

http://www.bigblockdart.com/techpages/speedtire.shtml
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1969GTS said:
I'm running 391's with a 28" tire on a 15" rim with a 3500 stall. At 70 MPH I'm running about 3200 RPM so long trips would get expensive. Here are some great tech calculators that are easy to use.

http://www.moparsunlimited.net/tech_calculators.htm
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Hell, I'm running that with my 3.23's and 14" wheels!

Back in "the day' we used to have 2 sets of wheels, 15" tall tires for cruising
and 14" 50's or 60's for tracking. Might be cheaper in the long run!
 
Back in the early '70s, I ran 4.89s behind a built 340/727 combo on the street with my old '69 340 Dart Swinger. Did not drive on the freeways with it, but what a rush coming off the line with that combo!
 
I had 3.91's with the 3 spd auto...SUCKED driving on the highway, so I went with 3.55's, but I only race 2-3 times a year
 
68fastback said:
I never thought about that. I should try to find a taller tire.
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Your 235-60-14 are just a little shorter than stock its far from super short. A 28" tire on the other hand is tall. A 68 Formuls S tire was an E70-14 at around 25 1/2" tall.

I run E70-14s with 3.91s and I think its easy to live with.
 
Orv Grote said:
Your tires are 25.1 inches tall and turn 803 revolutions per mile. 803 x 3.91
equal 3139 rpm at 60. Kinda high. 803 x 3.55 = 2850 at 60mph. Liveable.
803 x 3.23 = 2593rpm at 60. This is assuming you don't have an OD trans.
FYI: 3.91 at 70 would be 3662. I guess it depends how tight you want to
twist it.
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It doesn't sound like much but this information is dead on. It is also the reason I went from 3.91 to 3.23. I drive the hwy, alot and the 3.91 was just to tight for me. Go 3.55 the difference isn't that much.
 
go with 391 or 410 and run the tallest tire that fits and run a 26 inch slick at the strip. im probably not the best person to give advice tho. im gonna run 4.88s on the street. i can already hear the whine
 
idrift said:
Thats cool but check this website out!

http://vexer.com/68rt/speed.html
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Man that is one cool calculator but I don't believe it compensates any for slippage in an auto trans. torque converter. For a stick it is right on but for an auto you need to add some RPM's for the slip of the converter.

EDIT: I overlooked the features on the top that lets you select an auto trans and adjust the converter efficiency. That's a very good calculator.
 
fishy68 said:
Man that is one cool calculator but I don't believe it compensates any for slippage in an auto trans. torque converter. For a stick it is right on but for an auto you need to add some RPM's for the slip of the converter.

EDIT: I overlooked the features on the top that lets you select an auto trans and adjust the converter efficiency. That's a very good calculator.
Click to expand...

How would you know the efficiency of a trans or torque converter?
 
if you really want it right 430 for the strip an 355 for the street 391 best all around gear . i keep a set 323 for a long ride an others to play with .
 
mullinax95 said:
How would you know the efficiency of a trans or torque converter?
Click to expand...

The trans. itself shouldn't slip any so it's just the converter efficiency that's in question and you figure that up by using a calculator to determine what rpm you should be running at a given speed. Then you go run the car at that speed and see what rpm it really is running. Then divide the rpm it runs by what it should be running and that's the converter efficiency. Of course to be accurate you have to measure the actual loaded tire height and use that in the calculator and use a gps or some other means to make sure your running the proper speed and an accurate digital tach or large face tach is also necessary.

Here's an example of mine.

3:73 gears with 255/60/15 tires.

Loaded tire height (I measured the run out and converted to tire height) 26.45"

Using my gps I set the speed on 60 mph and the observed rpm was 2960.

According to the calculator my rpm with no slippage would be 2797. Divide that number by the actual rpm observed which was 2940 and I find the converter efficiency is 95.1%
 
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