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Rear Gear calculation ?

CQRT

Well-known member
Joined
Aug 24, 2003
Messages
268
Location
Scottsdale, AZ
Corvette
2003 50th Anniversary coupe
Greetings from Arizona:
My 74 (origL-48) w/auto turns almost exactly 2500 rpm at 60 mph. Any way to calculate rear gear ratio - - perhaps others can advise from their own observations ? I believe 3.08 was standard, with 3.36 optional- although I'd love to change to 3.55. Also- how can you tell if the car's equipped with the gymkana suspension- it has a new rear spring, so counting the leafs may not be accurate and I'm not curious enough to drop the fuel tank!
Thanks-
RWT
 

Glensgages

Well-known member
Joined
Jan 10, 2003
Messages
757
Location
State-of-Confusion
Corvette
Black '82 Coupe
Formula for gear ratio calculations:
RPM x .006 (this is a 'factor', using, amongst other things, 'pi') x 1/2 tire height divided by MPH = gear ratio.

Use this formula for older 4-speeds (1:1 ratio in 4th), and THM350-C (with a lock-up torque converter & 1:1 high-gear ratio).

If you run a non-locking converter (PG, THM350, THM400, etc.), x by .9 (MINIMUM!) for converter slippage.
If you run an OD transmission, multiply by the OD ratio (.7 for 700R4)

I'll give your 'guesstimate' a shot:
2500 x .006 x.9 ( I'm guessing it's an automatic? ) x 13.5 (Again, I'm guessing a 27" tire? ) divided by 60 (MPH) = 3.0375, which is damned-close to 3.08s.

To figure cruise RPMs @ 60 MPH with 3.55s:
3.55 divided by 3.08 = 1.1525974
Multiply your current 2500 RPMs by 1.152974 = 2881 RPM.
 

CQRT

Well-known member
Joined
Aug 24, 2003
Messages
268
Location
Scottsdale, AZ
Corvette
2003 50th Anniversary coupe
Wow ! Thanks for the education.. guess I should've provided better info on which to base the assessment-- Yes- it's an auto- factory THM 400, rebuilt, standard p225-70-r15 tire.
It'd guessed it was a 3.08 based on my vague recollection of our 73 & 75 L-48's. It appeared that someone had been in the pumpkin- but must have just done so to replace seals & lube when all the universals were replaced.
Thanks !

RWT
 

Glensgages

Well-known member
Joined
Jan 10, 2003
Messages
757
Location
State-of-Confusion
Corvette
Black '82 Coupe
CQRT:
No prob; anytime! I help figger this stuff for drag-racers & circle-burners all the time.
This formula works for my 12-second '79 Z28 that goes 105-107 MPH. I have 4.10s, 28" Goodyears, and a 'shift-lite' set to 'flash' @ 5800. If you use 106 MPH as a base-line, and 'figger-backwards', it looks like this:
106 (MPH) x 4.10 div by .9 div by .006 div by 14 = 5748 RPM.
Sometimes, the shift-lite 'blinks'; sometimes it don't....
 
J

JR-80

Guest
You can figure out which ratio by jacking up rear. Put a mark on driveshaft and a mark on the tire. Then count revolutions of the drive shaft for one revolution of the tire. You can estimate it pretty close. If it's just a tick over 3 then it's a 3.08.
If it's 3 and a half revolutions then its a 3.55. Simple ratios.
 

CQRT

Well-known member
Joined
Aug 24, 2003
Messages
268
Location
Scottsdale, AZ
Corvette
2003 50th Anniversary coupe
Thanks all- am going to measure the sway bars today-- appreciate the expert counsel ! Have a safe Holiday weekend !
RWT
 

CQRT

Well-known member
Joined
Aug 24, 2003
Messages
268
Location
Scottsdale, AZ
Corvette
2003 50th Anniversary coupe
Gents:
Based upon your wise and shared counsel, I have determined that my `74 is but a paltry 3:08 rear gear and non-gymkana suspension-- but at least- Now I know ! Thanks to all.
Regards-
Rollie
 

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