i heard someone say that you can leave it in D and not OD, cruise at a certain speed (around 70ish) and your rpms tell you your gear ratio: 2730 rpms = 2.73's

anyone know this mathematical equation and why it works?

what speed is it? 70?

 

You can use the calculators on this website. All you need to know is your tire size, transmission geat ratio(1:1 for 3rd in an auto, or 4th in a manual, and RPM at a given speed. From there, you can use the equations to figure out your rear end ratio.

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89 Transam GTA, 350TPI, auto, 3.27 rear, dual cats, gray.
Magnaflow cat-back,
K&N open element,
Best E.T. 14.63@95mph

 

It's 70 mph, as long as you have stock tire height. But even with different tires it won't be off that much. I'm not sure why it works other than the fact you're in 1:1 gear ratio.

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"Putting any amount of money into a RICE BURNER is like wiping your a$$ with a turd. "
'86 IROC
T-TOPS, TINTED WINDOWS, BRAKE LIGHT BLACKOUTS
GM GOODWRENCH 350
EDELBROCK TES HEADERS
3" Hooker CatBack w/Aero Chamber muffler
EDELBROCK 600CFM CARB.
KN AIRFILTER
ACCEL HEI DISTRIBUTOR
160* Stat, just switched to 180* b/c of winter coming and going to college in the mts.
3:73 Posi
Rebuilt 700R4
B&M Megashifter, 5" Autometer Tach w/shift lite

[This message has been edited by Mark A Shields (edited November 08, 2001).]

 

I always end up screwing up those formulas. it's easier to just count the revolutions or remove the cover and find the numbers stamped on the gears.

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Kyle Osterholt
Okarche, Oklahoma
ASE Master Certified
86 T/A 383 TPI
89 TTA #1002 T-top/Leather
89 TTA #1358 Hardtop/Leather
80 T/A Pace Car
73 Opel GT
73 bronco
2000 Ducati 996 Monoposto

 

i'm with kyle on this one, pull the cover and count teeth and do the math or find the ratio stamped on the ring gear and do the math. it doesn't huet to take a look inside and refill with fresh gear oil and posi additive if you have posi.

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ICON Motorsports
1st & 3rd
MM Black Diamond 538 F&AM

 

RPM/ gear ratio X tire dia /336 = MPH
or MPH X 336 / tire dia X gear ratio =RPM
This is for manuals or lock up autos with no slippage or tire growth. If you have a TH350
with a normal converter add about 5% for converter slippage.
Actual tire diameter is measured on the car
and will vary with car weight and pressure.
Measure the tire/wheel effective radius from the ground to the centre of the wheel and multiply by 2. this is your actual effective diameter. More accurate result... Have a buddy with a stock vehicle pace you or use a vericom or g-tech to test your speed and rpm
cause your speedometer may be out to lunch.

[This message has been edited by F-BIRD'88 (edited November 08, 2001).]

 

the reason i ask is because it's one of those neat tidbits of info to know, and makes you look all wise to the others ;-)
plus, it works on any other car with a gear at 1.00

i checked it today, and it does indeed look like 70 mph - i realize i could use the formulas, but being able to do it on the fly is fun too
later yall - i gots a date

 

I vote for just changing the fluid and counting the teeth, it probably needs changed anyway. Over a pit, you can roll the car one revolution of the tire and count the driveshaft rotations. That will equal the gear ratio.

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350 with stealth intake, holley carb, 470 lift cam. 700r4 with .5 boost valve, vette servo, tci lock-up kit, B&M megashifter. Richmond 3.73 gears, powertrax locker, timkin bearings, synthetic lube. Custom 3 inch single into 2 2.5 pipes. 1 1/2 drop springs, 1 5/16 solid front sway bar, 1 inch rear bar, custom subframe connectors, custom LCA relocation brackets. Kobel ground FX, currant red metallic paint. Lots of other stuff...
82camaro