68hemi

427390HP

I don't believe you calculated the gear ratio correctly. Given that the speedometer is accurate at 80 mph, the tire size diameter is known for a 205/70/R15 (26.3" diameter), and assuming the tachometer is accurate at 3,750, the mathematical calculation for your gear ratio computes to 3.67. So it appears that you have a 3.70 rear end.

Here's the link to check the calculations yourself:

http://www.wallaceracing.com/calc-gear-tire-rpm-mph.php

Terry

Nowhere Man

68hemi

You tire size is different from what I get on the online calculator I used but I think yours is correct.

That is what I assumed I had from the tach readings. My mechanic checked it twice. The original standard tires in 1965 were 25.45" high vs. the 205/70R x15" tires are 27.59" high so of course this will effect both the speedometer reading (which I have corrected with a different speedo gear) and the effective rear end ratio. I just don't know if it would be enough to change it to a 3.70 from 3.36?

AZDoug

Stock tire size was 7.75-15.
https://twitire.tireweb.com/webedit/...on%20table.pdf

That is about a 27.5" tall tire.

If you clanged your speedo gear by 8% for an erroneous tire size change, that is close to the 10 % diff between 3.36 and 3.70.
Doug

toddalin

Yes, your mechanic did it correctly rotating the tire and counting the revs. Based on the math, it would change to a 3.64 equivelent.

27.59 x pi = 86.68"/rev

25.45 x pi = 79.95"/rev

86.68"/79.95" = 1.084

3.36 x 1.084 = 3.64 equivelent gear ratio.

427390HP

EDIT: I originally wrote that "I don't know how a 205/75/R15 tire can have a 27.59" diameter." I meant to type 205/70/R15. All of my references and calculations are based on a 205/70/R15. I apologize for the error.

I don't know how a 205/70/R15 tire can have a 27.59" diameter. Can you post the site where you got that number?

I checked 3 different sites and all showed 26.3" diameter for a 205/70/R15.

Here is one of them: https://tiresize.com/calculator/

Here is another site with tables: http://www.jekylhyderacing.com/HeightofTires.htm

205/70/R15 = 26.3

toddalin

27.11" by the math.

205mm x 0.75 = 153.75mm (profile height)

153.75mm = 6.05"

6.05" x 2 (top above rim and bottom below rim) =12.11"

12.11" + 15" (rim) = 27.11" tall.

AZDoug

215/75-R15 seems to correspond to same height as factory 775-15 tires.

Doug

68hemi

Well, first of all the information I have is that the 65s came with 6.70 X 15" tires standard and that the 7.75 X 15 was an option and likely used with the KO wheels but I think you could order them without the KOs also.

If you look back at my last post you will see that I said I believe the other poster is correct about the 205/70R X15" is correct and mine is wrong making the correct height 26.3"

68hemi

OK, If the car originally came with 6.70 X 15" tires which are 27.4" high and I now have 205/70R X 15" which are 26.3" high according to the online calculator from the link below the new effective rear end ratio should be 3.50 which does not account for the 3750 rpms @ 80 mph I am seeing???

https://www.intercotire.com/tire_siz...tio_calculator

427390HP

It doesn't matter how the car was originally built. The 6.70-15 or 7.75-15 original equipment tires have absolutely nothing to do with what you are trying to determine right now.

I used the website I linked above and computed the gear ratio calculator, the MPH calculator, the tire diameter calculator, and the RPM calculator all using the following variables:

3,750 RPM
80 MPH
26.3" tire diameter
3.70 Rear axle ratio

Here are the results (the tables would not copy and paste into the forum format, so just the results are shown):

Gear Calculator
Your Gear Ratio is 3.67

MPH Calculator
Your MPH is 79.30 MPH

Tire Diameter Calculator
Your Tire Diameter is 26.53 Inches

RPM Calculator
Your RPM is 3,783.06 RPM

These calculations cross check and show that the four variable are correct. If you would jack the car up and re-check your revolutions VERY PRECISELY, I think you will find that you will have 3 3/4 turns. That's all there is to your question. Anything else is irrelevant.

68hemi

SWCDuke

This question comes up about every month, and I've made countless posts on the issue, but nobody gets it.

The speedo-tach relationship is FIXED, by the speedometer gears, so it doesn't matter what tire or axle ratio is installed. If the tire size is different than OE or a different axle ratio is installed it will result in error proportional to the difference in tire size or axle ratio. Also, you can't calculate tire revs/mile from the inflated OD because pneumatic tires deflect under load, and actual revs per mile are 2-3 percent higher that the "rigid body" calculation using inflated OD.

But you don't have to estimate. Simply look at the "specs" for the installed tire on any tire retailer or manufacturer's Web site. If your specific tire is not listed, look at any other tire of the same size. The various dimensions and other specs will be in a very narrow range as promulgated by the Tire and Rim Association.

Up to '64 Corvette speedo gears were set up for 6.70-15 tire size, which are speced at 760 revs/mile. Beginning in '65 the installed 7.75-15 are speced at 775 revs/mile. Most speedo gearing remained unchanged except for maybe the 3.55 and 3.70 axles. As a result, there is a little more high side error with most axles.

The formula for the correct top gear revs at 60 MPH is:

revs @ 60 MPH = (axle ratio)(tire revs/mile)

and you can linearly proportion to any speed from this datum.

Be advised that most "web calculators" use the "rigid body diameter" and do not correct for the 2-3 percent deflection error to achieve approximate loaded revs per mile, so forget the web calculators and just use a pocket calculator.

The closest radial tire size replacements to OE revs per mile are as follows:

205/75R-15: 766
215/70R-15: 775
225/70R-15: 760

The above sizes will result in little or no additional error compared the the OE tire size. The 205/70R-15 is about 790 revs per mile, so it will add a couple of percent more error than OE sizes.

Speedo driven gears range from 18-24T, which yields about a five percent spread between each. Once you have actual speedo data and an accurate reference, like from GPS you can compute the gain error, and if it's more than about 3 percent you can look at other driven gears to bring the speedo closer to true. Most OE speedos are set up to read zero to slightly high, and if it's around five percent high you can install a gear with one more tooth to slow down the cable about five percent. Or if it's three percent high you can install a gear with one more tooth and it will be about two percent low.

Duke

emccomas

Tire diameter calculations
OD = Overall Diameter
WD = Wheel Diameter
TW = Tire Width
AR = Aspect Ratio

OD = (TW x AR x 2) + WD

So for a tire size of 205/70R15

205 = Tire Width (in mm)
70 = Aspect Ratio (% of width to side wall height)
15 = Wheel diameter (in inches)

Convert 15 inch wheel to mm - 15 inches x 25.4 mm/inch = 381 mm ... WD = 381 mm

OD = (205 x 70% x 2) + 381
OD = (205 x 0.70 x 2) + 381
OD = (287) + 381 = 668 mm, converted back to inches = 668 / 25.4 = 26.29921 or rounded to 26.30 inches

SWCDuke

You don't need to do those calculations, which are based on the nominal size designation, so they are not that accurate. All the dimension data is listed in the tire's spec section on any big retailer Web site like The Tire Rack or the manufacturer's Web site for the specific model and size, and they are usually specified to the hundredth of an inch, and revs/mile are three digit integers.

OA width varies with wheel width. The specs usually list the "measuring wheel width" and the OA width will vary about 40 percent of the increase/decrease in wheel width.

Duke

AZDoug

You can always take the rear diff cover off and count gear teeth, or just look at the side of the ring gear which will have the ring and pinion tooth count stamped into it.

In the example below, 15-41 is 15 pinion teeth, 41 ring gear teeth, 41/15-2.73 ratio.


Doug