Anybody got the formula to figure out what gears you have?????????

Black1987Stang

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Aug 22, 2004
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I used to have a formula we used on our dyno at my school to figure out a cars rear end ratio....by running the car in 4th gear (1:1 ratio) at 2,000 rpms, see what mph your at, multiply the speed to the rpm, then multiply that to the tire diameter, then devide it to 1270 or something like that...Anybody know??? I might be changing my rear gears soon but dont even know what gears I have, (previous owner says 373 but i dont believe it)...5th gear...3,000 rpms...75mph...:bs: .....and have to drive a 14 hour drive in a couple weeks from tn to NJ and I dont want to be getting like 5mpg and wearing out my freshly built motor from cruising at the high rpm for soo long.
Also: help me decide what gears I should get...im already sold on FRPP...tell me what all of you have (gear ratio) and where your rpms are at at a certain mph in 5th gear...ThankS :SNSign:
 
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Wow you went way damn overboard on trying to figure out what gears you have dude...

Jack the rear end of the car up. Take a marker and mark a place on your driveshaft and your rear end. Rotate the rear wheels one full revolution and count how many revolutions the mark goes by the mark on the rear end.

3.5 revolutions of the driveshaft are 3.55's
3.75 revolutions are 3.73's etc...

here is a graph that was used on Stangnet all the time a few years ago.

grc.jpg
 
Look Its Aaron said:
Wow you went way damn overboard on trying to figure out what gears you have dude...

Jack the rear end of the car up. Take a marker and mark a place on your driveshaft and your rear end. Rotate the rear wheels one full revolution and count how many revolutions the mark goes by the mark on the rear end.

3.5 revolutions of the driveshaft are 3.55's
3.75 revolutions are 3.73's etc...

here is a graph that was used on Stangnet all the time a few years ago.

grc.jpg

yea i no about that way im just soo busy right now with work and school to do it....thanks for the graphs, theyre pretty cool, according to them I have like 450 gears or something which I doubt I have that steep gears, in 5th at 3,000rpm im doing 75mph. I guess ill just check it and see for real what I got
 
Black1987Stang said:
yea i no about that way im just soo busy right now with work and school to do it....thanks for the graphs, theyre pretty cool, according to them I have like 450 gears or something which I doubt I have that steep gears, in 5th at 3,000rpm im doing 75mph. I guess ill just check it and see for real what I got

A 4:10 gear with the .68 Overdrive and a 26" tire would be 2,700 rpms at 75 mph. Stock tachs are off about 100 to 200 rpms typically.
 
Bear in mind that using your speedo to figure out what gears you have is gonna be near impossible if you don't have the right speedo gear in there, to begin with. Better to use the driveshaft-and-tire method described above.
 
EMW150 said:
A 4:10 gear with the .68 Overdrive and a 26" tire would be 2,700 rpms at 75 mph. Stock tachs are off about 100 to 200 rpms typically.

yeah, thats about right... if i remember correctly for the ticket i once got!

but as for your tach remark: careful, my PMS is dead on with my stock tach.
 
N8Miller said:
yeah, thats about right... if i remember correctly for the ticket i once got!

but as for your tach remark: careful, my PMS is dead on with my stock tach.
I'd say you are one of the luckier few. The stock tach is often off by a percentage, so as one approaches redline, it's off by a decent bit. The tach on the fox and SN95 are both off on mine (both read high).
 
HISSIN50 said:
I'd say you are one of the luckier few. The stock tach is often off by a percentage, so as one approaches redline, it's off by a decent bit. The tach on the fox and SN95 are both off on mine (both read high).

well, that is till like 3300-3500ish, after that, im usually going too fast to really watch it. BUT, if i set my PMS to rev limit at 6500, and i set my tach to 6500, it will hit the limiter BEFORE the tach goes off. but if i set the tach for 6400, it will shift BEFORE the limiter. so, conclusion= stock tach by 100 rpms.

brin: yeah, pretty simple, huh. sometimes people take the hard road rather than the one that requires less time/work.
 
N8Miller said:
brin: yeah, pretty simple, huh. sometimes people take the hard road rather than the one that requires less time/work.

yes its easy IF you have a jack, jack stands AND free time unlike me....I work all morning and go right to school till 930 at night, get home, eat dinner and i dont feel like messing with my car......i just wanted to get an idea by pressing a few buttons into a calculator which takes what a minute? im gonna try and do the tire thing this weekend
 
im assuming the guy you bought it from told you they have afer market gears, and you are trying to prove him wrong if that is the case than this is a dumb reply but there is a tag on the girdle which has the ratio on it 3L7= 373 ect, i belive it is in the vin number as well.