[East]

I have been trying to cross reference factory tire heights with the P255 R60 15 tire height, and 3.08 gear ratio.

The 43 tooth appears to be a close match.

3.08 1362196 43 PURPLE F78 x 14 ( 26.5 to 27 inch height) Drive gear teeth = 18

Could somebody with the same tire size and rear gearing please confirm?

[b-man]

TH350 or 400?

Shouldn’t matter but I have the TH350 with a 19 tooth drive gear and a 44 driven, 3.08 gear and a 26.7” tall 215/75-14 tire. 44 divided by 19 = 2.3157

Yours - 43 divided by 18 = 2.3888

Maybe consider a 42 driven. 42 divided by 18 = 2.3333

Should be plenty close enough.

I’ve checked my speedometer against a GPS and it’s pretty accurate within 1 mph at 50.

Calculator here:

https://www.tciauto.com/speedometer-gear-calculator

Punching in 26.5” tire, 3.08 gear and 18 tooth drive you’ll need 42.15 tooth driven gear (42).

Same for a 27.05” 255/60-15 is asking for a 41.29 tooth driven gear (41).

Safe bet would be the 42 tooth gear as the best pick for tires in the 26.5” to 27” range.

My setup 26.7” tire 3.08 gear 19 drive calculates to a 44.16 tooth driven gear so it’s a good calculator, my 44 tooth gear is on the money.

[rohrt]

I have 68 FB with 235/60 tires, 3:08 gears and the spedo is right on.

I was trying to decipher my own post on the subject. Looks like I had a red drive gear and black driven gear.
https://forums.maxperformanceinc.com...d.php?t=793344

[East]

I should check the drive gear because I am only assuming that it is 18 teeth.

I would be happy to be within 1 MPH.

I was using this chart from Frank's

[East]

1964 Tempest and GTO 2 Speed Only

1965 Full size TH400 All

I don't understand the tire sizes

AI Overview

The height of an 8.25 x 14 tire is approximately 26.8 inches, assuming it's a standard 8.25-14 tire with a typical aspect ratio of around 80. The tire's overall height is a combination of its rim diameter (14 inches) and the height of its sidewall, which is calculated by multiplying the tire's width by its aspect ratio (height-to-width percentage) and adding it to the rim diameter.



How to calculate the tire height:

Convert tire width to inches: 8.25 inches is already in inches.

Determine the sidewall height: In older tire sizes, the aspect ratio isn't explicitly stated but is usually around 80 for this width. So, the sidewall height would be 8.25 inches x 0.80 =6.6 inches.

Calculate the total height: Add the sidewall height to the rim diameter. Since there are two sidewalls, you multiply the sidewall height by two. So, the total height is 14 inches + (6.6 inches X 2)=14 + 13.2 = 27.2 inches.

[b-man]

Forget those charts. Those old tires referenced there really aren’t relevant to your 255/60-15s.

Depending on who made your tires they may or may not be the calculated 27.05” height. Measure your actual tires then use the TCI calculator in the link posted I previously above.

[East]

Originally Posted by rohrt

I have 68 FB with 235/60 tires, 3:08 gears and the spedo is right on.

I was trying to decipher my own post on the subject. Looks like I had a red drive gear and black driven gear.
https://forums.maxperformanceinc.com...d.php?t=793344

The overall height (also called overall diameter) of a P235/60R14 tire is approximately
25.1 inches (637.6 mm)

The overall height (diameter) of a P235/60R15 tire is approximately 26.1 to 26.2 inches.

The height of an F70 x 14 tire is approximately 26 inches in overall diameter, a Firestone F70-14 is listed at 26.4 inches

[East]

Another chart from Frank's for gear colors.

[East]

I removed the speedometer gear housing to replace all the o-rings. I noticed that the housing is marked for 36, 37, 38, and 39.

Because I was replacing the speedometer gear with a 42 tooth gear, I shopped for a housing marked 40, 41, 42, and 43.

While the housing was removed, I decided to count the drive teeth since I could not get a tooth count by the color orange or yellow. I crawled under the car several times because I kept telling myself that I wasn't careful with my counting. After 5 times of counting 19 teeth, I feel sure that is the correct number.

Then I was getting a different number for how many revolutions in a mile with a 27.05 mounted tire. The resulting number would depend on how I worded the question.

The number of teeth is between 43.1621 and 43.5883

I was going to order a 44 tooth gear and call it a day. I was using a 37 tooth gear. So what is going to be the problem going above the speedometer gear housing marked 40 to 43?

I would like to ask b-man if if wouldn't mind one more analysis knowing that I am now operating with a 19 TOOTH DRIVE GEAR

>>>>"The BFGoodrich P255/60R15 tire has approximately
733 to 769 revolutions per mile, depending on the specific product line and load. The overall diameter of the tire is consistently listed as 27.1 inches across various retailers."<<<<<

[b-man]

Put the 44 tooth driven gear in it to go with your 19 tooth drive gear.

I forget where I bought my housing, but I know that I was price shopping for it on the internet and could have found it from a seller on eBay. They’re readily available.

Found one for 40 through 45 tooth gears on Amazon, probably where I bought mine:

https://www.amazon.com/TH400-NP208-S...7507976&sr=8-4

[East]

So I bought a replacement housing that fits 40-45 teeth driven gears, and the 44 tooth driven gear. All of the seals and o-ring are new.

I was installing the housing retainer and I reached for the bolt only to find that it had a nut on it. I didn't notice it before. I get the fact that the nut was on there because the bolt was too long.

Is the bolt supposed to 5/16"-18 x 3/4 ? I have read that metric size is also a possibility.

I removed the housing late one night and was focused on getting the drive gear tooth count. I crawled under the car at least five times to verify that the count was accurate. Then there was the correct housing issue with using 44 teeth driven gear. Now it's the dang bolt.

It's always something.

I also found 5/16NC X 1/2 and 6mm x 1.0 thread. About 15mm long.

[East]

I searched through my bolts and could not locate a perfect replacement. I modified the bolt that I removed even though it looked like it belonged to something else. It took two attempts to shorten it. It was pretty short when I finished it.

I have only to adapt the modulator vacuum line bent for an A body,. I figured that the th400 to engine combo was close enough to work with.