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I know this is not going to be an accurate question/answer but after purchasing this 1965 Corvette 327-300hp Sport Coupe I noticed the engine was turning relatively fast. The question I have is a ballpark question and I could get more info but I can't right now.
My question is this, my engine turns at 3000 rpm and speed is 70mph (yes I paced the car so the speed is correct), when I installed my 4.10 in my 1967 that was the same situation 70@3000, so do I have a 4.10 gear in my differential?
It's been a lot of years but when I was a kid owned two new cars, a 68 GTO then a 69 COPO Camaro. As I recall 3:55 in the first one, 3000 RPM at 70 MPH . The second a 4:10, 4000 RPM at 70 MPH. Several others after that new and used with the same result. A 3;36 would be very close to a 3:55. However I take note the differential between 14 and 15 inch rims, I owned a 70 Corvette with 3:55 and pretty sure it was also 3000 RPM at 70 MPH. I stand corrected if wrong.
The easiest way to find out what ratio you have is to jack the car up, providing it has a positraction differential ( both wheels turn together).
Turn one wheel clockwise one full rotation and count how many rotations of the driveshaft, this will get you close. There are stampings on
the bottom of the differential that would call out the ratio however there is no guarantee of what is in there now is what it had when it left the
factory but usually low horsepower cars are not altered.
I know this is not going to be an accurate question/answer but after purchasing this 1965 Corvette 327-300hp Sport Coupe I noticed the engine was turning relatively fast. The question I have is a ballpark question and I could get more info but I can't right now.
My question is this, my engine turns at 3000 rpm and speed is 70mph (yes I paced the car so the speed is correct), when I installed my 4.10 in my 1967 that was the same situation 70@3000, so do I have a 4.10 gear in my differential?
Regards,
Ed
I suspect your memory may be fading on that 70@3000 with a 4.10 rear in the 67. Too many cars with too many different rears to memorize exact readings but on my 66 Corvette-4.10 rear but I don’t think I can get much more than high 50-something mph@3000 rpm with tall 215/75 (27.7”) tires.
Could be a 3.42 ratio, but need tire size to confirm. Post your tire size and axle ratio can be calculated. Actually, you need to know the rolling radius of the tire but tire height will get you very close.
If the rear hasn't been futzed with over the last 60 years, it may still have the original gears in the pumpkin. The code for the gear ratio is stamped on the bottom of the housing...easy peasy to read.
The only good and easy way to get an accurate account of rear end ratio is to raise up car rear wheels and rotate 1 turn of wheels. Mark driveshaft and count the turns. This is for POSI rear...........both wheels should turn in same direction when doing this check.
I would also check rear axle stamping as Leif says.
You cold also use a stopwatch (old school I know) and a mile markers on the interstate to get your exact MPH. If your a young techie use GPS and your phone.
Indicated speed by speedometer may be off due to tire size and size of transmission driven gear...........so don't use that number at this time (until you verify it is correct). The mechanical tech in our cars should be fairly accurate.
From the 1965 Sales Brochure. Graph and Table below show speed and RPM for each ratio. I used this prior to raising it up and spinning the wheels to confirm my 4:11
Good info............but assumes that the speedometer driven gear in transmission is correct for the rear axle ratio. This is not certain in a 50+ year old car.
Good info............but assumes that the speedometer driven gear in transmission is correct for the rear axle ratio. This is not certain in a 50+ year old car.
Larry
For people not sure how accurate their speedometer is reading, I'd recommend taking a ride with a free speedometer app.
Dennis
Rear axel retio
) and a mile markers on the interstate to get your exact MPH. If your a young techie use GPS and your phone. 
