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Author Topic:   how to find gear ratio?
speedster
Member
posted May 31, 2001 10:32 PM
can anyone explain how to find out what your final gear ratio is .... i'm trying to fiqure out what gear i need in the rearend to run a certain rpm using my present trans in second gear instead of third wich i cant get enough rpm out of and i cant go any higher on the axle gear???? you know all that stuff about size of tire distance of track this gear that gear stuff i need the formula?? thanks


racer17j
Member
posted June 01, 2001 12:36 AM
if you could fill us in on what you are working with now we could give you a good answer


BK19
Member
posted June 01, 2001 06:05 AM
tranny gear x rearend gear = final gearing


WesternAuto17
Member
posted June 01, 2001 09:09 AM
first, why do want to race in second? you should be trying to pick a gear which will let you run in 3rd (if its an auto, 4th in a manual). You want to run in the "straight-thru" gear with a 1:1 ratio.

Anyway, the following website is a drag racing site which is the best thing I've found for selecting rear gear ratios.
http://www.prestage.com/carmath/calc_gears.asp


speedster
Member
posted June 01, 2001 11:42 PM
ok what i have is a 10bolt chevy rear end with 4.56 gears wich is the most gear you can run in that housing i run 3rd gear in a saginaw 4 spd with three rings on the input shaft so 3rd gear ratio is 1.64.that gives me a final of around 7.38final with a 82inch roll out tire the proublem is that i only turn 5000 rpm and my motor is built to turn 64-6700rpm so i dont have pull or the horses i need to get out of the corners. my dyno sheet shows i'm only making 375horse@5000 instead of the 430horse it makes at6400. thats a real waste of power that i need. so you see thats what i'm trying to figure out wich rear gear i need to get that extra 1400rpm's with out going to far!!! thanks


racer17j
Member
posted June 02, 2001 01:41 AM
can you drop to a 78-79 roll? now i have a question for you guys i went to that carmath link how do you mesure the tire height? we alway just use roll out i put my gears into it and it came out at 26.88 tire height


sixwillwin
Member
posted June 02, 2001 09:56 AM
tire diameter(height) x 3.14(pi)= tire rollout(circumference)


cmiller51
Member
posted June 02, 2001 10:45 AM
What size track are you running on? That kind of gear should definately come of the corner like a rocket and run out of your rpm range quickly. The easiest way to lower the ratio would be to change trasmissions. 3-speed trannys have a wide variety of 2nd gear ratios--1.64, 1.75, 1.84, 1.88.

I'd say that if your running in a track any larger than a "true" 1/4 mile gear ain't your problem..Hope this helps.



BK19
Member
posted June 02, 2001 11:51 AM
the 10 bolt you have ,is it the 8.5 ring gear?,cause i have the 10 bolt with the 8.5 ring gear and i have 5.57 gear in mine


speedster
Member
posted June 03, 2001 11:55 AM
the track is just under a true 1/4 mile its like .225. yeah i stand corrected on my statemant about 4.56 being the largest gear to fit in that carrier it is indeed the 5.57that is the largest gear to fit in there my mistake. i think we found the combo it looks like a 3.73 rear gear with a 2.47 2nd gear in the tranny with a 82inch roll out on this track will turn a rpm of around 6250 and that would be right in the ball park that gives me a final ratio of 9.21 that should get the job done!!! i hope if anyone has used this combo let me know what you think...


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