82Vette'ster

Does anyone know what the stock gear ratios were in the 82 vette's 700r?

I'm looking to put in some go-faster gears. I'm think I want 3.73, but would like to calculate my total ratios and expected rpms at highway speeds.

EDDIEJ82

If I remember right (and I know someone will chime in) that my gears for my 82 were 2.89? 2.98 something like that, they were not 3.08 that was used up to 81. I'm running 3.73 with my 700 R4 and love them. 3.55's won't be enough go with the .73's

Glensgages

1st Gear: 3.06:1
2nd Gear: 1.63:1
3rd Gear: 1:1
4th Gear: .7:1 (over-drive)
Reverse: 2.29:1

82Vette'ster

Thanks that's just the data I needed. I'm going to be brave and post my calculations. Don't laugh, just correct me if I'm wrong. OK, I've had too many beers to drive so I'm figur'in mods instead.

Basic necessary data:

Transmission gear ratios:

1st = 3.06
2nd = 1.63
3rd = 1
4th = 0.7

Differential ratio = 2.89

Effective wheel radius (@ 28 psi) = 12.25 inches
(ground to wheel center)

======================================== ====

At 75 mph highway speeds what is the rpm?

75 miles/hour and there's 60 minutes in an hour so
75/60 = 1.25 miles/minute

The distance the tire travels in one rotation
is the effective circumference of the tire
which should be:
2(pi)r = 2 * 3.1416 * 12.25 = 76.97 inches

The number of tire rotations in 1 mile is:
63360 inches in a mile divided by the tire cir.
63360/76.97 = 823.2 rotations in one mile

If the vette is traveling 1.25 miles per
minute, then 1.25 * 823.2 = 1029 rotations
per minute (rpm).

So at 75 mph the wheels are spinning 1029 rpm and the
drive shaft is spinning 1029 * 2.89 = 2974 rpm

At 75 mph the transmission is in OD (4th gear) =0.7
So the drive shaft is spinning at 2974 rpm then the
engine is spinning at 2974 * 0.7 = 2081 rpm
(neglecting torque converter losses).

This seems about right (approx 2100 rpm). My tach
shows a little less but its a factory 82 tach, not
good enough to measure off of.

======================================== ========

So if I put in 3.73 gears in the rear end the rpm
will go up by: 3.73/2.89 = 1.291

So that 75 mph at 2081 rpm would not be:

2081 * 1.291 = 2687 rpm

I think I can live with 2700 - 2800 rpm at 75 mph.

It'll actually be a little higher since I neglected
torque converter losses, But then I gain a fraction
of that back on tire growth due to rotation.

======================================== ========

The 3.73 will give me a first gear of:

3.06*3.73 = 11.41

I've heard that the target number is 10, but maybe
a slightly higher number will let the the meger
200 ponies pull a little harder.

Ooooh that was tough, I need another beer.

BSeery

what size tires are you running that they are only 24.5 inches top to bottom ???? That is way small. Most C3's run a 27" or even taller wheel.

Check http://www.miata.net/garage/tirecalc.html for your tire sizes.

Also, just rough numbers from my car.
700R4 - 3.73 gears - 25.6" tires = 2800-3000 at 75 MPH.


[Modified by BSeery, 12:39 AM 2/14/2004]

terry82

i wanted 3.55 gears but i think the3.73 would be better .after reading some of the problems that you can get into after you open the rearend up ,i thinki will get one already done .then just bolt it in .how do you spread the case at home?

Glensgages

Thanks that's just the data I needed. I'm going to be brave and post my calculations. Don't laugh, just correct me if I'm wrong. OK, I've had too many beers to drive so I'm figur'in mods instead.

Basic necessary data:

Transmission gear ratios:

1st = 3.06
2nd = 1.63
3rd = 1
4th = 0.7

Differential ratio = 2.89

Effective wheel radius (@ 28 psi) = 12.25 inches
(ground to wheel center)
#1: Don't use the 'spindle-to-pavement' method of figuring tire-height; the lower 1/2 of the tire 'SQUATS' under load.
The 'TRUE' number you're looking-for is 'ROLL-OUT', or circumference.
Jack one corner of your car into the air, and measure AROUND the tire...
My 255/60R15 BFG TA Radials were 85.5" roll-out, or 27.25" tall (85.5 div/by 3.14 [pi] = 27.25)

======================================== ====

At 75 mph highway speeds what is the rpm?

75 miles/hour and there's 60 minutes in an hour so
75/60 = 1.25 miles/minute

The distance the tire travels in one rotation
is the effective circumference of the tire
which should be:
2(pi)r = 2 * 3.1416 * 12.25 = 76.97 inches

The number of tire rotations in 1 mile is:
63360 inches in a mile divided by the tire cir.
63360/76.97 = 823.2 rotations in one mile

If the vette is traveling 1.25 miles per
minute, then 1.25 * 823.2 = 1029 rotations
per minute (rpm).

So at 75 mph the wheels are spinning 1029 rpm and the
drive shaft is spinning 1029 * 2.89 = 2974 rpm

At 75 mph the transmission is in OD (4th gear) =0.7
So the drive shaft is spinning at 2974 rpm then the
engine is spinning at 2974 * 0.7 = 2081 rpm
(neglecting torque converter losses).
#2: Your 700R4 has a lock-up torque converter; at steady-throttle in gears 2, 3, and 4, you HAVE no slippage...

This seems about right (approx 2100 rpm). My tach
shows a little less but its a factory 82 tach, not
good enough to measure off of.

======================================== ========

So if I put in 3.73 gears in the rear end the rpm
will go up by: 3.73/2.89 = 1.291

So that 75 mph at 2081 rpm would not be:

2081 * 1.291 = 2687 rpm

I think I can live with 2700 - 2800 rpm at 75 mph.

It'll actually be a little higher since I neglected
torque converter losses, But then I gain a fraction
of that back on tire growth due to rotation.

======================================== ========

The 3.73 will give me a first gear of:

3.06*3.73 = 11.41

I've heard that the target number is 10, but maybe
a slightly higher number will let the the meger
200 ponies pull a little harder.
#3: I have 'heard' this number 'should-be' 10; based on WHAT, I have no idea.
I've been in and around drag-racing for 22+ years, and OUR rule-of-thumb is throw AS-MUCH gear at it as you can, as long as it hooks.
I ran a 2.75:1 first-gear THM400 in my Z28 with 4.11 gears (11.3:1), and it ran approx a tenth quicker with 4.56:1 gears (12.54). I woulda run 4.88s, but I was restricted to a 28" x 10" slick in the stock wheel-well, and traction with the .56s was marginal...

Ooooh that was tough, I need another beer.
Sober-up, and use THIS method to save head-aches; it's MUCH easier.
To figure RPM at a given MPH with varying gear-ratios, you'll need to know the following items:
over-all tire height (circumference divided-by pi (3.14);
if the transmission has over-drive;
if the torque converter is a lock-up unit...

Formula is:
MPH x proposed GR (gear ratio) div/by 1/2 TH (tire height) div/by .006 (nothing more than a 'factor', taking into account 'pi'...)

If the above example includes a non-locking converter, divide by .9 (simulating a 10% RPM loss);
if the above example includes a n OD-ratio of .7:1, multply by .7 (which will DECREASE the total RPM).

In YOUR instance ( and MINE, too-see my sig ):
75 MPH x 3.73 =279.75 div/by 13.625 = 20.53211 div/by .006 = 3422.0183 x .7 = 2395.4 RPM

I feel these numbers are very accurate, as my tach reads JUST below 2500 RPM at a speedo reading of 75 MPH.

Using this formula in other configurations (backwards?) can be used to figure what gear to choose for a given MPH/RPM ratio, too.

Hope this helps!!!:thumbs:

82Vette'ster

Thanks for correcting my mistake.

My error was in assuming that the effective tire radius was taken from where the tire met the road, and the dimenssions of the tire when its not in contact didn't matter.

I think I'm going to go with the 3.73.

Thanks again for all the help.

Glensgages

For what it's worth.....

In my other-wise stock '82 Cross-Fire, my ETs as-delivered where 15.904 @ 86 MPH, and I got 23+ MPG the only time I 'really' checked my mileage with the 2.87s.......
to 15.203 @ 90.89 MPH, and I got 17+ MPG without even 'trying' on a 430+ mile cruise (approx 'only' 175 of it highway) with the 3.73s.

With the 11.4:1 effective ratio in low gear, even the anemic Cross-Fire can boil the hides, and the 3.73 makes it MUCH more responsive...

You'll like it, but I'd prefer 4.11s...

82Vette'ster

BSeery, Can you recomend someone here in town to put gears in my Dana 44? I wouldn't mind learning how to do this myself some day.