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Author Topic:   Rear gear ratio?
fairlaniac
Dirt Full Roller

Total posts: 74
posted June 24, 2002 07:35 AM
We're trying to get the RPM's up at a shorter track (3/8 mile) that we ran at this weekend. we were only able to get 5100 down the straight and normally get 5800 at a 1/2 miler. I think the stock rear in this car is a 3.42:1 ratio ('79 Monte). This is the rear that we were told? We are thinking of getting a second rear and putting in some 3.55 or 3.73 gears or in that neighborhood and use that on the shorter track. What do you think and what gears do most run on a 3/8 miler?

Thanks!

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Doug
"Ford man workin' on a Chebby""

dode
Dirt Freak

Total posts: 417
posted June 24, 2002 08:23 AM UIN: 13209501
What tranny are you running and what gear? Around here, most 3/8 milers run around a 6.00 total reduction.

John

fairlaniac
Dirt Full Roller

Total posts: 74
posted June 24, 2002 09:30 AM
It's a TH350, that's all I know. I'll see if we can figure out what it is. Is there a way to tell on the tranny? any numbers to look for? I'm not used to the GM stuff.

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Doug
"Ford man workin' on a Chebby""

AC156
Dirt Freak

Total posts: 295
posted June 24, 2002 09:40 AM
second gear on a th350 is 1.52 . If you are running a 3.42 rear gear, then your final ratio would be 5.20. I run on 3/8 track and run a th350 in second with 3.73 gear. that comes to a 5.67 final. I turn 6,000 rpm.

fairlaniac
Dirt Full Roller

Total posts: 74
posted June 24, 2002 12:46 PM
It has been about 20 years since I've played with the mathematics of all this. So please bear with me.

You mention "second gear on a th350 is 1.52 . If you are running a 3.42 rear gear, then your final ratio would be 5.20. I run on 3/8 track and run a th350 in second with 3.73 gear. that comes to a 5.67 final. I turn 6,000 rpm."

How does the 5.67 and 6,000 RPM equate to each other? Suppose we'd like to reach optimum PRM 6,000 ex. and were running a 1/2 mile track and with your info above. How do I determine the optimum gear ratio? Even if it was not 3.42:1. Is there a place that has all of the race mathematics? I'd like to set up a formula and plug in numbers. I use to have some hand written formulas that dealt with RPM, tranny gear ratios, axle ratios, tire diameters and would give you the tire speed. It would be nice if there was a web site with these formulas for those of us who have forgotten half of what we learned.

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Doug
"Ford man workin' on a Chebby""

AC156
Dirt Freak

Total posts: 295
posted June 24, 2002 01:06 PM
look in the "toolbox" on this website. there are some calculators in there.
the way that you calculate what your final ratio is by multiplying the trans ratio and the rear gear you have.
example: second gear on a th350 is 1.52. and you said you have a 3.42 in the rear.
1.52 x 3.42= 5.1984 final

[This message has been edited by AC156 (edited June 24, 2002).]

fairlaniac
Dirt Full Roller

Total posts: 74
posted June 24, 2002 01:33 PM
I got the 1.52 x ratio but what does this number do for me? 5.2 (rounded) times what gives me what beneficial information?

Thanks,

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Doug
"Ford man workin' on a Chebby""

AC156
Dirt Freak

Total posts: 295
posted June 24, 2002 02:20 PM
the 5.20 is your final ratio. go to the toolbox on this website and use the gear calculator there. if you want to turn 5,800 on the 3/8 track and you are currently turning 5,100 rpm, then according to the calculator you need to lower your final gear to approximatley 5.91. if you went to about a 3.90 rear gear, then you would be pretty close.
3.90 x 1.52= 5.93

Pickel
Dirt Forum Champ
Total posts: 564
posted June 24, 2002 09:35 PM UIN: 25419780
I run a 538 on a 3/8 mile with a 3 speed standard shift in high gear. I crank 5800. I am going to put a 567 in which should be 6200 I believe.

racer17j
Dirt Forum Champ
Total posts: 1565
posted June 24, 2002 10:54 PM UIN: 54865418
when your figureing your gear you need to take into consideration a few things tire size is a big part of it i run the same gear on 3/8,4/10,1/2 tracks but change the tire size depending on your tires you should be able to run that 3.73 in 2nd gear no problem if you run about a 82-84 in tire on the rear

ray14
Dirt Full Roller

Total posts: 67
posted June 24, 2002 11:02 PM
i run a 3/8 mile track and i use 6.50 gear and run 3rd in tranny which is a 1.00 ratio and 275/60/15 tires and i turn around 6300 to 6500 rpms might be a bit to much for your car tho.

AC156
Dirt Freak

Total posts: 295
posted June 25, 2002 01:59 PM
another thing to take into consideration is that not all 3/8 tracks are the same. banking, lenghts of turns, lengths of straitaways all have and affect on the gear. the track i run is a 3/8 with big wide sweeping turns. it is more of a momentum track. my gears may not work good at a smaller 3/8.

fairlaniac
Dirt Full Roller

Total posts: 74
posted June 25, 2002 02:19 PM
Yeah, I'm taking everything into consideration. The 3/8 track is a sorta tight turn track and the 1/2 miler is a fairly sweeping wide turn.

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Doug
"Ford man workin' on a Chebby""

ray14
Dirt Full Roller

Total posts: 67
posted June 26, 2002 05:59 PM
the 3/8 i run on is tight turns 6.50 helps comeing off turns alot just a thought

93racing
Dirt Newbie

Total posts: 4
posted July 04, 2002 12:44 AM
6.20 gears with a 2 speedpower glide, run it in drive, that should work great for about 5800-6200 rpm, depending on the power of the motor

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