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Author Topic:   gear ratios
jester548
Dirt Roller

Total posts: 5
posted November 12, 2005 11:50 AM  
I have a 12 bolt 72 monte carlo rear end in my 72 chevelle pure stock. Going to run a 3/8 mile dirt track(Silver Dollar Speedway)with a t350 trans. What kind of rear gear ratio should I look for?

racer17j
Dirt Forum Champ
Total posts: 5472
posted November 13, 2005 08:12 AM  
You probably want somewhere between 6.20 and 6.90 final drive ratio. The tallest gear you can buy for 12 bolt is 6.13, so you might get away with that (also allows you to run in high gear which would be best for your transmission). If you need more gear you have no choice but to run the trans in 2nd or 1st gear thus multiplying the rearend ratio. Most Turbo 350's have a 1.52 second and a 2.52 first. To find out what the final gear ratio is, you must multiply the trans ratio by the rearend ratio. Example: 2.73 rear ratio X 2.52 first gear = 6.88 final gear ratio. Also, for further gear choices, you could swap transmissions. Powerglide two speeds came with 1.76 first gear in V8 crs and 1.82 first in 6 cylinder cars. There are lots of different ratios in standard shift too. Saginaw 3 speeds had second gear ratios of 1.50, 1.68 & 1.89. Four speeds give you even more options. Try searching the web for a chart or table that tells you what ratios were available and how to identify them.

racer s
Dirt Forum Racer

Total posts: 85
posted December 04, 2005 05:53 PM  
I ran a 3 speed with 3.73 on a 3/8 track but not sure about automatic

[This message has been edited by racer s (edited December 04, 2005).]

jester548
Dirt Roller

Total posts: 5
posted December 05, 2005 07:29 AM  
Thanks a lot! That at least sends me in a good direction. I appreciate the Help

Champ2x36
Dirt Full Roller

Total posts: 40
posted December 05, 2005 11:48 AM  
I have done pretty well on a 3/8 track with a 256 rear gear with a TH350 in low gear. This combo gives you a 6.45 overall ratio. Turned about 5900 to 6000 RPM just before I let off for the corners.

Ed

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When the drops the stops!!

My website: www.geocities.com/champ2x36

[This message has been edited by Champ2x36 (edited December 05, 2005).]

racer17j
Dirt Forum Champ
Total posts: 5472
posted December 05, 2005 03:52 PM  
17j is right if you don't have the same rollout you won't get the same result. How many ring and pinions do you want to own?????????

blade
Dirt Full Roller

Total posts: 38
posted December 06, 2005 05:22 PM  
i ran a 72 gto with a 10 bolt ran 4.10 with t350 in 2nd gear with about a 86 roll out, on a 3/8 track didn,t never rev past 6500 worked great. it worked so great that a couple other guys at my track switched once they found out what i had. ran that with bone stock junk yard 350 with no problems.

jester548
Dirt Roller

Total posts: 5
posted December 06, 2005 08:34 PM  
Thanks guys! So roll out is diameter not circumfrence right?

racer17j
Dirt Forum Champ
Total posts: 5472
posted December 06, 2005 08:40 PM  
Just curious about the mathematics of it all. how do you figure in the roll out into the finale drive ratio, or rather what are the effects of a shorter/higher rollout. i ran a 86 crown vic with a stock 302, C4 tranny, 8.8" rearend with 3:73's not sure bout the rollout never heard of it b4, but for an idea i ran a 235/75r15 on LR and a 225/70r15 on RR. i know bigger tire on the inside???? but i ran across the setup in practice and it works well for my style, call me crazy, anyways, if i stayed on the bottom of the corners, it was doggy as heck coming out, but if i stayed high in 1&2 i usally hit 4500ish rpm in 2nd "3rd for those who agree with me that an auto tranny will shift between 2nd and 3rd gear when shifter is in 2ND posistion" going into 3 high, and cut out of 4 low and i usally did ok. but i hate swinging back and forth across the track, but do what you must do right. thanks alot. oh, any idea as to gear ratios for Ford Trannys?

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Rubbin's Racing, just keep the hammer down and the shiny side up.

James Ott
Dirt Maniac

Total posts: 160
posted December 07, 2005 06:03 PM  
roll out is circumfrence or to make it simple how far will the tire travel in one complete revolution

racer17j
Dirt Forum Champ
Total posts: 5472
posted December 07, 2005 07:58 PM  
yeah but you can calculate MPH with a memory tach. I can most people are not going as fast as they think they are

jester548
Dirt Roller

Total posts: 5
posted December 08, 2005 12:07 PM  
Do you find a certain roll out size to work well on 3/8 tracks racer17j?

racer17j
Dirt Forum Champ
Total posts: 5472
posted December 08, 2005 05:51 PM  
jester548: There are several things to consider on roll-out. As 17j said, too much sidewall height can cause problems, however a taller tire (one with more roll-out) has a larger contact patch than a shorter one. And of course just changing tire circumference doesn't equate to an increase or decrease in speed. That depends on where your engine likes to run as well. Changing from an 80" tire to an 84" tire should have the same effect as going from a 6.11:1 ratio to a 5:89:1 You could gain 3 mph, or lose 200 rpm, or some combination of the two. You'd be better off to get as close as you can with your axle gearing and transmission gear to your "target" rpm range, and then choose your tires for the best contact patch and cornering. Worrying about roll-out on a pure stock is pretty much a waste of time. In order to ger a MAJOR change in gearing you give up too much in traction. Just my opinion.

Normal Sullivan
Dirt Full Roller

Total posts: 64
posted December 08, 2005 11:48 PM  
Dirty Deed: C-4
1st 2.42:1
2nd 1.42:1
3rd 1:1

Your tires should be close to:

235/75-15 +/- 90.7" roll-out
225/70-15 +/- 86" roll-out

If this is working for you, I assume two things. You need more crossweight. You're not running a locked rear.

Try this: get your rear stagger right, around 1.5 inches more roll-out on the right rear, or if you're running an "open" rear, run them both the same size; and run as much split on the front as you can get away with without rolling the R/F under, ie. larger R/F, MUCH smaller L/F.

[This message has been edited by Normal Sullivan (edited December 09, 2005).]

J3
Dirt Freak

Total posts: 205
posted December 09, 2005 05:20 AM  
How do you use a mem tach to figure out speed?

J3
Dirt Freak

Total posts: 205
posted December 09, 2005 05:23 AM  
quote:
Originally posted by J3:
How do you use a mem tach to figure out speed?

Nevermind, went to the tool box and found it.

jester548
Dirt Roller

Total posts: 5
posted December 09, 2005 06:56 AM  
OK so am I looking for somewhere around 4.10 gears?

john56h
Dirt Full Roller

Total posts: 27
posted December 09, 2005 07:40 AM  
In my opinion 4.10 final gear ratio will never be enough on a 3/8 mile track. If you used the calculation formulas, you probably over estimated your "speed". I wouldn't think that you'd want less than 6.50 on any 3/8 mile track, unless maybe it was asphalt.

racing8r
Dirt Newbie

Total posts: 4
posted December 09, 2005 09:36 AM  
i was in the same boat not knowing what gear to run so all year i changed gears about every other race ,i fineally decided to run a 546 gear with a 350th tranny in drive my tires was 205 70 r15s i was getting about 5500rpm at end of straightaway and about 4500to 4800 comming out of the corners it seemed to work well

Normal Sullivan
Dirt Full Roller

Total posts: 64
posted December 09, 2005 07:34 PM  
jester548: 4.10 gears, and a turbo 350 in 2nd gear gives you a 6.23 final. That should be close.

racer17j
Dirt Forum Champ
Total posts: 5472
posted December 09, 2005 08:23 PM  
Sorry 17j, I wasn't meaning to be disagreeable , it's just been my experience that in order to get a significant rpm change, (500 rpm or so) requires such a major change in tire size that you trade off too much in both forward bite, and cornering traction.

The result *for me* has always been an expected increase in rpm due to better gearing, and an unexpected increase in rpm due to spinning the tires on corner exit.

racer17j
Dirt Forum Champ
Total posts: 5472
posted December 10, 2005 09:23 AM  
not a prob we have just been bound to this rule for so long we go to the tire shop and messure all the tires before we even by them to be sure we get the right roll out

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