1979 c3 rpm

Roscobbc

Roscobbc

Moderator
Best tell us a bit bit more about your Vette - manual/auto? - have you checked OEM spec' as to what rear axle ratio you have? - what size tyres are you running?
 
Chuffer

Chuffer

CCCUK Member
I have a 1980 with TH350 three speed auto and 3.08 : 1 which was generally the norm for later model year C3`s when fuel consumption and emissions became more important than outright `off the line ` performance . In the early 1970`s I believe there were more ratio choices available on stock cars for performance buffs . What diff ratio do you have ? I run 295 /50 / 15ins on the rear and 65 mph sees about 2600 rpm
 
curious bystander

curious bystander

CCCUK Member
275x75x15 are oversize tyres which would affect ratios I think.
Originally a 79 came with as standard 225x70x15 or 255x60x15 on the alloy wheels
Need to check the overall diameter of your tyres to compare with the standard tyres (26.5" near enough for the 255's)
 
teamzr1

teamzr1

Supporting vendor
If not knowing what ratio the rear diff is, use the old fashion test method :

Determining a Rear Axle Ratio of a Posi Rear
  1. Safety first! Chock the front tires, so your car won't slide off the jack stands, then raise the rear of the car with a hydraulic floor jack and place heavy-duty jack stands on either side of the rear axle housing. Lower the car onto the jack stands.
  2. Turn the driver's side tire a quarter turn counterclockwise to remove the slack in the drivetrain.
  3. With a piece of chalk or masking (painters) tape, make about a 6" line along the driveshaft that can be easily seen from the side of the car and make another mark on the tire. It's good to make this mark in a place that is easy to line up, like the edge of the wheel well or the 12 o'clock position.
  4. Rotate the tire counterclockwise while counting the number of times the driveshaft rotates until the tire has completed one complete turn. (You may need a helper if you can't rotate the tire and see the driveshaft at the same time.) Be as accurate as possible.
  5. The number of driveshaft rotations will help you determine your rear axle ratio. For example, if it turned 3 and half times, it's a 3.50:1 ratio. If it turned 4 and a quarter times, it's a 4.25:1 ratio.
  6. Remove the tape/marks from the driveshaft and tire, raise the rear tires again and remove the jack stands before lowering your car back to the ground. Remove the chocks from the front tires.
Determining a Rear Axle Ratio of a Non-Posi Rear
  1. Jack one wheel off the ground, put the transmission in neutral and take the parking break off.
  2. ***Be sure to use jack stands under the axle for safety precaution***
  3. Mark the wheel and the drive shaft with chalk or masking tape.
  4. Turn the wheel TWO revolutions and count the number of driveshaft turns.
  5. The number of driveshaft rotations will help you determine your rear axle ratio. For example, if it turned 3 and a half times, it's a 3.50:1 ratio. If it turned 4 and a quarter times, it's a 4.25:1 ratio.
  6. For better accuracy, turn the wheel 20 times and divide by 2 for your rear axle ratio.
If you are unsure whether you have a Posi Rear or not

Jack both wheels off the ground and turn one wheel.
  • If the opposite wheel turns the same direction or doesn't turn at all, then the rear axle is posi or limited slip.
  • If the opposite wheel turns the opposite direction, then it's probably an open rear axle (non-posi).
 
Roscobbc

Roscobbc

Moderator
275/75/15 is 31" diameter. Stock diameter of OEM fitted C3 tyre is generally 27". That size is going to make a significant effect on your gearing.
 
S

Silverblack

Regular user
Wow thanks lads great info,
Ive got the diff off at moment,got some grinding noise when warmed up!!!. My taco must be reading wrong cos at 60 I'm showing 6000 rpm.
I think my gearbox is a th350 also.
 
teamzr1

teamzr1

Supporting vendor
If having the diff apart and gear is not marked as to ratio then use this simple calculator

Diff Gear size

Then knowing that and then needing to correct speedo use the charts I gave in other post that show what color(s) gear you need to
get correct output of tranny to speedo and auto tranny
 
Roscobbc

Roscobbc

Moderator
curious bystander said:
That being the case I'm surprised those tyres fit under the arches tbh.:unsure:
Click to expand...
I'm with you on that one - presumably the 275 75 series tyre is on a stock 8" rim.....if so its tread width relative to height/aspect ratio will mean its probably 'skinny' enough in an old school way to fit 'in' the wheel well......just. If he is running a 3.08 diff....with thst tyre it must be equivalent to a 2.5 axle. It'll certainly be a bit sluggish off the line....
 
teamzr1

teamzr1

Supporting vendor
For 1979 C3

Rear Diff codes :
OM: 3.36:1 RATIO
OH: 3.55:1 RATIO
OJ: 3.70:1 RATIO

Either L48 or L82 engine
Net Brake Horsepower @ RPM:195 BHP @ 4000 RPM225 BHP @ 5200 RPM
Net Torque (lb/ft) @ RPM:285 lb/ft @ 3200 RPM
280 lb/ft @ 2400 RPM (Calif.)		270 lb/ft @ 3600 RPM

Auto Tranny

utomatic Transmission​


Transmission Type:
3 Speed Automatic Transmission L48 & L82​
Order Code:
MX1​
Type:Automatic hydraulic torque converter with compound planetary gear system – three forward speeds and reverse.
Case Material:Aluminum
Parking Lock Type:P (Park)-R (Reverse)-N (Neutral)-D (Drive)-L2 (Low 2)-L1 (Low 1)
Selector Lever Location:Center Floor Console
Lubricant
– TypeDexron II (at time of release. Dexron III may also be used.)
– Capacity20 Pints (Dry) / 8 Pints (Refill)
Clutches
– Type:Four, multiple disk
– Material:Steel with bonded organic facings (Drive Plates) / Flat Steel (Driven Plates)
– Forward Clutch:5 each – Drive and Driven Plates
– Direct Clutch:4 each – Drive and Driven Plates
– Intermediate Clutch:3 each – Drive and Driven Plates
– Low & Reverse Clutch:5 each – Drive and Driven Plates
– Release SpringRadial row steel coil
Planetary Gear Set
– Reaction Carrier Assembly:
4 Steel Pinion: gears​
– Output Carrier Assembly::​
4 Steel Pinion: gears​
– Intermediate Band:​
Circular Steel with Organic Lining​
Range / Gear Ratios:​
– D (Drive)​
2.52:1 (first gear) – 1.52:1 (second gear) – 1.00:1 (third gear)
– L2 (Low Two):2.52:1 (first gear) – 1.52:1 (second gear)
– L1 (Low One):2.52:1 (first gear)
– R (Reverse):​
1.93:1​

Rim Size:15 x 8.00

Stock Size and Ply Rating:P225/70R15

Tires (Optional)
– Construction:Aramid fabric belted radial
– Size:P255/60R15
 
teamzr1

teamzr1

Supporting vendor
Comparing stock and Option 1979 Tires to what C3 has on it now

Stock tires versus what is now on car is about 13% difference in tire rotation and would require
Changes to gear(s) in auto tranny off the speedo cable in tail of tranny

C3tires.jpg
 
Last edited:
Chuffer

Chuffer

CCCUK Member
Silverblack said:
Wow thanks lads great info,
Ive got the diff off at moment,got some grinding noise when warmed up!!!. My taco must be reading wrong cos at 60 I'm showing 6000 rpm.
I think my gearbox is a th350 also.
Click to expand...
Defo got a tacho problem if reading 6000rpm at 60 mph . If you were really running regulary at 6000 rpm you would have pistons coming through the hood by now ! Presumably your engine doesn`t sound like it is running at 6000 rpm ? You will have a TH350 in your car if it`s a 3 speed auto .
 
S

Silverblack

Regular user
Its definatley not screaming at 60 hardly got my foot down at that but taco reads similar to speedo.
They are after market alloys & tyres stick out of arches but dont rub
 
Roscobbc

Roscobbc

Moderator
Be VERY careful if tyres that large are sticking out of wheel arches - if you are two-up and for any reason go over a speed hump or bridge too fast AND tyres contact rear fender is could mean disaster.........not just wheel arch lips.......rear fenders, deck and more could disappear.
Not quite the same thing (burst tyre) - but end result could be the same.
img_0379_1ed47880e0067a331441a55e800af34383e0a245.jpg
 
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