Speedo Gears

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wheelsport

wheelsport

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My '65 Barracuda speedo is off by 5 MPH at 60. 60 MPH is actually 55. The car left the factory with 170/13 tires which are no longer available outside of vintage sellers. The closest I could find were 185/70r/13. Is there a scale to figure out the correct speedo gear for my tires to give the correct speed?
 
The 185/70-13 tire diameter is approximately 23.2" tall. I've used this chart that I found online and it was pretty close. It sounds like you need a speedo gear that has 1 less tooth than you have now so that it spins a little faster. I don't know which rear end gear you have. You'll need to know that to use the chart. Or just take the speedo gear out and look at the tooth count and subtract one tooth. Good luck.

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wheelsport said:
My '65 Barracuda speedo is off by 5 MPH at 60. 60 MPH is actually 55. The car left the factory with 170/13 tires which are no longer available outside of vintage sellers. The closest I could find were 185/70r/13. Is there a scale to figure out the correct speedo gear for my tires to give the correct speed?
Click to expand...

Not positive, but I think 2 teeth equals 5mph.
So for example if your current gear is a 34, then you probably want a 32.
 
After searching the correct tire size for my car, I find I bought the wrong tires. My car came with 700/130 tires which converts to 205/70r/13 which has a 24.3 dia. and according to the cart calls for 16 tooth gear. My tires have a dia. of 23.2 which would call for a gear that possibly dosn't exist.
 
55 vs 60 is a small error, around 9%. Simply pull your gear out, count the teeth, and just get one with one with 9% less teeth. That will be as close as you'll ever get it.

Actually, on second thought, first verify that your speedometer is reading low. I think that's what you are saying in the O.P. To be exact, you'll need a gear 9% smaller if you want the speedometer to speed up. .
 
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They go all the way down to 21 teeth... you will be able to get the right one
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No, they don't, and no, he probably won't. We're talking about the pre-'66 speedo drive pinions. They're much smaller than the '66-up pinions, and even when they were easy to get—which they haven't been for many years—there weren't many options. Smaller pinion diameter + coarser tooth pitch = very narrow range of available tooth counts. Specifically, the range with the pre-'66 pinions was 16 to 21 teeth (six options). Compare that to the '66-up range of 21 to 45 teeth (twenty-five options, that's over four times as many).

Perhaps the OP will be lucky, and when they pull out their speedo pinion, count the number of teeth, and multiply that number by 1.09 the answer will be within the range of 16 to 21 (likely it'll be halfway in between, like 17.44, which means picking a little fast with a 17-tooth pinion or a little slow with an 18-tooth pinion). Then comes the fun of trying to find the needed pinion; good luck.

OP, you may wind up needing a ratio box to get the cable to spin at the speed you need.
 
I found out when setting up my car each pinion from the chart covers about 3 sizes of tires and you can only get so close. Brewers is where I got my pinion from and they sell them used but got lucky they had one for 20 tooth pinion. 25.6 tall tire and 3:23 gears.

I never knew they had ratio box's so I may give that a try. Thanks Dan
 
I just measured a 7.00-13 Wards Power Cushion tire mounted on a rim that came as a spare with one of my cars and the diameter is right at 26 inches
Those old bias tires had tall sidewalls, more like what would be equivilent to a 90 series today.
 
slantsixdan said:
No, they don't, and no, he probably won't. We're talking about the pre-'66 speedo drive pinions. They're much smaller than the '66-up pinions, and even when they were easy to get—which they haven't been for many years—there weren't many options. Smaller pinion diameter + coarser tooth pitch = very narrow range of available tooth counts. Specifically, the range with the pre-'66 pinions was 16 to 21 teeth (six options). Compare that to the '66-up range of 21 to 45 teeth (twenty-five options, that's over four times as many).
Click to expand...
I'm so used to the "newer" stuff I never even realized that he was talking about the old/small style. My apologies (and I have removed the erroneous info from my post).
 
I'm assuming that the tire Grim measured is a belted tire. I got the 700/13 and 205/70r/13 diameters (which both have a 24.3 dia.) from the tire manufactures sizing charts. I assume my car has the standard 2.93 ratio rear end. These sizes call for a 19 tooth gear. My tires have a 23.2 dia. It looks like the lower the ratio the lower number of teeth required. The highest number of teeth is 20. I would need at best a 21 tooth gear which doesn't exist.
 
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