what's my rear end ratio

[Don M.]

I have a 64 327 365 hp, 4 speed with the factory 7:75-15 tires, at 60 mph I am turning 3500 rpms, almost exactly. Does that sound like a 4:56??

[Mike M.]

Re: what's my rear end ratio

either 4.11 or 4.56. without doing the math, i'd favor 4.11.mike

[Don H.]

Re: what's my rear end ratio

Jack up the rear end, mark the drive shaft, and put the car in neutral. Then count the revolutions of the drive shaft to one revolution of a rear tire. Just over four turns = 4.11. Just over four and one half turns = 4.56. Don H.

[Duke W.]

Re: what's my rear end ratio

Revs @ 60 = tire revs/mile X axle ratio.

The speedo is set up for 760 revs/mile tires, but the OE 7.75-15s are typically 775.

It looks like you have a 4.56.

Duke

[Wayne M.]

Re: what's my rear end ratio

I have a 64 327 365 hp, 4 speed with the factory 7:75-15 tires, at 60 mph I am turning 3500 rpms, almost exactly. Does that sound like a 4:56??

The 1965 dealer (showroom) brochure has a handy chart and the following figures (not allowing for slip or tire expansion):

4.56 ratio = 17.0 mph per 1000 rpm (calc's at 59.5 mph for your 3500 rpm)
4.11 ratio = 19.2 ........................... would be 67.2 mph for same rpm.

So looks like you have 4.56 gears, with your 7.75 tires.

[Duke W.]

Re: what's my rear end ratio

The speed-RPM relationship is determined by gearing and design tire revs/mile.

This relationship holds regardless of the installed tire revs/mile. However, installed tire revs per mile will result in percent speedometer error equal to the increase/decrease percent difference in design tire revs/mile.

The design revs/mile for C1-C3 speedometer gearing is 760 for the 6.70-15 tire size. When Chevrolet went to the 7.75-15 and later F70-15, both of which are about 775 revs/mile, they just accepted the additional two percent high error.

The later GR70-15 and 225/70R-15 sizes improved speedometer accuracy because they have the same revs/mile spec, 760, as the old 6.70-15, but the later optional 255/60R-15s are 775 revs/mile.

Somewhere along the way - it might have been 1968 when a number of FMVSS went into effect - a federal speedometer accuracy standard went into effect, which allowed a few percent on the high side error, but no error on the low side.

Duke

[Don M.]

Re: what's my rear end ratio

thanks guys, I agree I 'm pretty sure it's a 4:56. I used to love all that gear ratio when I was young. It sure skoots, but it is annoying to cruise for a while. Don

[Dennis B.]

Re: what's my rear end ratio

I have a 64 327 365 hp, 4 speed with the factory 7:75-15 tires, at 60 mph I am turning 3500 rpms, almost exactly. Does that sound like a 4:56??

Rear Axel Casting codes
CA 3.36 Non Posi
CZ 3.08 Non Posi
CX 3.70 Non Posi
CJ 3.08
CB 3.36
CC 3.55
CD 3.70
CE 4.11
CF 4.56

[David L.]

Re: what's my rear end ratio

thanks guys, I agree I 'm pretty sure it's a 4:56. I used to love all that gear ratio when I was young. It sure skoots, but it is annoying to cruise for a while. Don

Don,

I have a NOS 7.75 X 15 spare tire in the tire carrier of my 1966 Corvette but it would be a pain to measure the diameter since the car is now in storage. However, I believe the P205/75R15 tire is very close to the same diameter.

Using a P205/75R15 tire:

205/25.4 = 8.07" wide
8.07" X 0.75 = 6.05" height
Tire dia. = 6.05" + 15" + 6.05" = 27.10"
Circumference = pi X D = 3.14 X 27.10" = 85.1" = 7.10 ft.

If you drive 60 miles in one hour you travel 60 miles X 5280 ft./miles = 316,800 feet.
316,800 ft./ 7.10 ft./rev. = 44,620 revolutions

Driving @ 3500 rev./min. for 60 minutes = 210,000 revolutions

ratio = 210,000 rev./44,620 rev. = 4.7 ( w/P205/75R15 tires in 4th gear)

Assuming your ratio = 4.56
210,000 rev./4.56 = 46,053 rev.
316,800 ft./46,053 rev. = 6.88 ft.
6.88 ft./ 3.14 = 2.19 ft. = 26.3"

Do your 7.75 X 15 tires measure about 26.3" in diameter? When I get a chance I will measure my NOS 7.75 X 15 spare tire.

Dave

[Duke W.]

Re: what's my rear end ratio

You made the all too common mistake of computing the "rigid body" tire revs/mile. This will work for a steel railroad wheel, but not for a pneumatic tire, which defects significantly under load.

How does one determine tire revs/mile? Very simple. Rather than using your calculator you simply look up the manufacturer's specification at a tire retailer Web site like The Tire Rack or the manufacturer's Web site, which list various technical data for each make, model, and size.

Manufacturers actually MEASURE tire revs/mile at design load and pressure. Actual on-the-car revs/mile will vary very slightly (usually less than one percent) depending on ACTUAL load and pressure.

For a give tire size, the range of various specifications must be in a narrow band as defined by The Tire and Rim Association, so if you cannot find data for your particular make and model tire, any data for the same size tire, even from a different manufacturer, will usually be within one percent.

As a general rule of thumb, the loaded revs/mile of a tire is about 3 percent greater than the rigid body calculation number, and tire revs/mile increase another two percent when the tread depth is down to the 2/32" wear limit.

The average revs/mile for the 205/75R-15 of 766 just about splits the difference between the 6.70-15 and 7.75-15.

Speedometer driven gears for manual transmissions are available in about 5 percent increments, so one should be able to pick a driven gear that yields accuracy within +/- 2.5 percent, which is 1.75 MPH at 70 even if installed tire revs/mile are significantly different than OE.

Duke

[David L.]

Re: what's my rear end ratio

You made the all too common mistake of computing the "rigid body" tire revs/mile. This will work for a steel railroad wheel, but not for a pneumatic tire, which defects significantly under load.

How does one determine tire revs/mile? Very simple. Rather than using your calculator you simply look up the manufacturer's specification at a tire retailer Web site like The Tire Rack or the manufacturer's Web site, which list various technical data for each make, model, and size.

Manufacturers actually MEASURE tire revs/mile at design load and pressure. Actual on-the-car revs/mile will vary very slighly (usually less than one percent) depending on ACTUAL load and pressure.

For a give tire size, the range of various specifications must be in a narrow band as defined by The Tire and Rim Association, so if you cannot find data for your particular make and model tire, any data for the same size tire, even from a different manufacturer, will usually be within one percent.

As a general rule of thumb, the loaded revs/mile of a tire is about 3 percent greater than the rigid body calculation number, and tire revs/mile increase another two percent when the tread depth is down to the 2/32" wear limit.

The average revs/mile for the 205/75R-15 of 766 just about splits the difference between the 6.70-15 and 7.75-15.

Speedometer driven gears for manual transmissions are available in about 5 percent increments, so one should be able to pick a driven gear that yields accuracy within +/- 2.5 percent, which is 1.75 MPH at 70 even if installed tire revs/mile are significantly different than OE.

Duke

Duke,

We are not building a watch here and accuracy to the nth degree is not necessary. We are just trying to determine if Don's axle ratio is 4.11 or 4.56. I think that my calculations are accurate enough to determine that the axle ratio is 4.56.

I just measured my NOS 7.75 X 15 Goodyear tire and the circumference measures about 85 5/8" (or 7.135 ft.) using a cloth tape. This would calculate at 5280 ft./7.135 ft. per rev. or about 740 revolutions per mile. Since the actual is about 3 percent greater as per your research then the actual revolutions/mile would be about 1.03 X 740 or 762.

3500 rev./min. X 60 min. = 210,000 (60 mile trip @ 60 miles/hr.)

762 rev. per mile X 60 miles = 45,720 rev.

Theoretical axle ratio = 210,000 rev./ 45,720 rev. = 4.59 (very close to 4.56 so I would say this confirms that Don's car has a 4.56 axle ratio)

Dave

[Duke W.]

Re: what's my rear end ratio

Simply looking up the revs/mile spec for the design tire is both more accurate (within one percent) and easier than measuring an actual tire and running the numbers through a calculator to get a rather inaccurate (about 3 percent) answer.

Since some vintage OE sizes are out of production, here's data from tire spec sheets that I've been collecting for 40 years.

6.70-15: 760 (This is the "design" size for most C1-C3 speedometer gear setups)

7.75-15, F70-15, 255/60R-15: 775

GR70-15, 225/70R-15: 760

Duke

[Don M.]

Re: what's my rear end ratio

Thanks guys, I'm convinced it's a 4:56, I put a set of 2:15/75 15 radials on it today. Cut down the revs a hare, it sure handles nicer. Don