1955 Ford speedometer reads too fast??

S

steve fritz

Well-known member
So if my speedometer on 55 Ford is reading too fast, do I change the speedometer cable lower end gear to fewer teeth or more teeth?
The manual calls for a 22 tooth gear , thats what I have in it now. But the speedometer reads approx 10 mph faster than my actual speed. Is there any rule of thumb on the number of teeth change compared to the desired reduction wanted in MPH ?
 
Have you changed rear ratio or tire size? Trans came in car is original to the build?

What is your exact application?
 
ok , heres the deal...
original 1955 Ford Ranch Wagon 6 cyl , 3 speed O.D.
Had 47/11 ring and pinion , which is 4:27 ratio
replacement tires are 205R 75-15.
Speedometer reads approx 10 mph too fast with original 22 tooth speedometer cable gear.
 
The speedometers tended to read a bit high anyway. I guess it was a marketing ploy???
Changing the tires to a set with a slightly smaller overall diameter accentuates the problem.
Are you checking it against a known correct speedometer? Mile markers on the hiway? GPS or ?
Has the speedometer been cleaned and serviced recently? If not, it may just need attention.

Adding a tooth to the driven gear on the lower end of the speedo cable will slow the speedometer approx 5% (it's been a very long time since I did the calculations to change one) so a replacement gear with 24 teeth instead of 22 may be about right.
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My 55 T-bird with Fordomatic, original 3.31 rear gears, and 205/75r15 tires was a minimum of 10% fast at highway speeds. It had been my father's car, but I never drove it until I inherited it. Everyone, including truckers, passed me until I kicked it up a bit. I can't recommend a gear, other than it should be larger than what you have. I had 2.72 gears and an Auburn limited-slip put in my differential during rebuild, and the Fordomatic is being replaced with a C4/w Dynamic Racing Transmission's wide-ratio gear set with numerically higher 1st and 2nd gears.

In the meantime, you shouldn't have to worry about speeding tickets.
 
steve fritz said:
ok , heres the deal...

original 1955 Ford Ranch Wagon 6 cyl , 3 speed O.D.
Had 47/11 ring and pinion , which is 4:27 ratio
replacement tires are 205R 75-15.

Speedometer reads approx 10 mph too fast with original 22 tooth speedometer cable gear.
Click to expand...

Here is what I found. There is a conversion chart(s) for calculating needed driven gears. The only problem is finding tire specs for older tires.

Supposedly, the car came through with 6.70 X 15 which has been replaced by 205R-75-15. The diameter of the original bias ply is 28.58" while the diameter of the replacement residual is 27.1". That is enough to throw the speedometer off.

FYI - The driving gear is 7T. The supplied driven gear for your application was 22T.

When you use a speedometer calculator, it call for 22T (B5A 17322-D - BLACK) with the updated tire size. This was also used for assembly with the bias tire. So FORD averaged out speedo gears.

For reference, here are two different gears in the same gear series -

21T - B5A 17322-B (GREEN)
23T - MC 17322-B (YELLOW)

When you decide on the needed gear, use the mile markers on Interstate to confirm accuracy.
 
You can only go so far with gear and tire changes and the drive and driven gear when gear teeth are rolled on the output shaft. The muscle cars of the 60/70s had speedo gear reduction boxes added to make up for low ratio's. My 58 had an original rear around 2.75? and changed it to a 3.56?. Never did find a speedo gear to make it accurate. I wish you luck. If you are dedicated to the ratio and tire size you can have speedo/cable place make up boxes and cables for the percentage you are off. I never had the money at the time.
 
I have what is supposed to be (and truly appears to be) an all original car that I bought from the original owner. My car has the exact same problem. I GPS compare, and the factory speedo is almost 10MPH fast!
 
Car manufacturers do calibrate speedos to read faster than actual speed, why is anybody's guess, possibly legal liability. I use a GPS app on my phone. I found that my Merc reads about 5 mph fast up to about 60 and becomes pretty accurate up to about 85 which is as fast as I've gone in it so far.
 
The way that I would solve this, is by using %.

If your speedo reads 65, when your actually going 55, it's 1.181818 off, if you multiply 22 teeth X that, you will get 26 teeth and that should get the right mph.

I have rarely had the right size tire on any of my cars or trucks and what I did, in the olden days, was use a stop watch, over a 1 mile marker. 60 mph = 1 mile per minute or 60 sec. If you clock yourself at 55 sec. 60 sec/55 sec = 1.0909. 1.0909 x 60 mph = 65.45 true mph. Once you know the percentage that your speedo is off. in this case we'll call it 109.1% you can calculate the rest of your speedo readings. i.e. 20 mph x 1.091 = 21.82 mph or 100 speedo reading would bw 109.1 mph.

For my truck, it just happened to be a easy percentage ~110%. Holding the speedo at 60, I covered 1 mile in 54.5 sec. 60/54.5 = 1.100 x 60 = 66.055 mph so, I would just add 10% to my speedo reading.

The other way I do it, is with gearing and a tach. rpm/final drive gear x .002975 x tire dia = mph. 4000 rpm/4.11 = 973.24 x .002975 = 2.895 x 28 tire dia = 81.07 mph.

You can correct for the speedo gear, with any of these %. In the last case, if you speedo said 100 mph at 4000 rpm, 100/81.07 = 1.2335 x 22 teeth = 27.137, so a 27 tooth gear, would get you very close.

Lastly, if your speedo is accurate and you change tires. You can calculate how much your speedo is off or calculate the speedo gear teeth, needed to bring it back, as close as you can. In that case, we'll say you go from a 215 60 15 (25.157 dia) to a 235 60 15 (26.10 dia). Here you have to speed the speedo up to match the speed increase and need a gear with less teeth. 26.10/25.157 = 1.038 or 103.8%. 22 teeth / 1.038 = 21.205 so, a 21 tooth gear would get you close.
 
Has anybody found a source for speedometer gears with more than 24 teeth? Most I have ever seen is 23 teeth and they don't seem to be available.
 
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