1956 Speedometer reads about 10mph slow

[seatramp]

My speedometer reads about 10 mph slow according to my GPS (and the local sheriff) . It has a 312 with an early air cooled FM automatic tranny and I was told the rear axel was changed at some point but I don't know what gear ratio it is. I saw in the parts catalog there are 4 different gears for the cable depending on the combo. Any way to tell which one I have?

 

[4VE]

You can jack the rear end and while in Neutral, mark and rotate the drive shaft by hand and count how many revolutions of the drive shaft = one 360 degree rotation of the wheel. Also, the size of your tires can change the mix too.

 

[biddle]

My speedometer reads about 10 mph slow according to my GPS (and the local sheriff) . It has a 312 with an early air cooled FM automatic tranny and I was told the rear axel was changed at some point but I don't know what gear ratio it is. I saw in the parts catalog there are 4 different gears for the cable depending on the combo. Any way to tell which one I have?

When I changed my Mustang from a 2.73 to a 3.55 it made the speedometer shows about 10MPH faster, so it appears you have the opposite problem.

 

[jack-in-sac]

My speedometer reads about 10 mph slow according to my GPS (and the local sheriff) . It has a 312 with an early air cooled FM automatic tranny and I was told the rear axel was changed at some point but I don't know what gear ratio it is. I saw in the parts catalog there are 4 different gears for the cable depending on the combo. Any way to tell which one I have?

Yes. remove the end of the cable that goes into the transmission and look at the gear. It is held in with a small bolt that holds a metal piece that keeps the geat in place. The gear is attached to the cable with a clip that is easy to remove. pop off or pop on. You can count the teeth or it will be in small raised numbers on the face of the gear with the stub on it. In your case you want less teeth to speed up the cable. Not complicatwed. The gears are plastic and relatively cheap. at about $10.00 or less, last time I checked.

 

[doug7740]

Thunderbird odometers are calibrated at 1,000 revolutions of the speedometer cable per mile on the odometer. The rate of turn is controlled by the tooth count on a plastic gear at the transmission end of the speedometer cable. If your odometer is correct when checked against known distances, the speedometer cable is turning at 1,000 revolutions per mile and the gearing is correct.

The speedometer head should be calibrated to 60 miles per hour at 1,000 revolutions per minute. I use a reversible electric drill with a top speed of 1,000 rpm as my test fixture. To calibrate your speedometer, adjust the return spring on the speedometer needle. Look closely and you will observe that the return spring is pinned to the housing at the outer end. Using a small screwdriver you can gently rotate the pin location to increase or decrease the tension on the spring. Make small adjustments until you zero in on 60 miles per hour.

After the speedometer is back in the car you will see the results of your efforts.

doug7740
1955 Thunderbird Blue

 

[hotrodbuilder]

My speedometer reads about 10 mph slow according to my GPS (and the local sheriff) . It has a 312 with an early air cooled FM automatic tranny and I was told the rear axel was changed at some point but I don't know what gear ratio it is. I saw in the parts catalog there are 4 different gears for the cable depending on the combo. Any way to tell which one I have?

You need to remove cable from transmission and count the teeth on the gear. A gear with more teeth will slow the speedometer down. Fewer teeth will speed it up. Each tooth makes about 5 mph difference.

 

[seatramp]

My speedometer reads about 10 mph slow according to my GPS (and the local sheriff) . It has a 312 with an early air cooled FM automatic tranny and I was told the rear axel was changed at some point but I don't know what gear ratio it is. I saw in the parts catalog there are 4 different gears for the cable depending on the combo. Any way to tell which one I have?

Thanks for all the responses. I finally got all the Honey-Do's finished and got to the upgrades I wanted. After rewiring from the dash forward I replaced the floor insulation and sound dampening which gave me the opportunity to access the transmission speedometer gear from the shifter opening. It turns out I had a 21 tooth gear and replaced it with a 19 tooth. (thanks for the tip Hotrod Builder). I am now 2-3 mph slow at 65 which should keep me and the sheriff on friendly terms from now on. ?‍♂️?

 

[Tracey Brown]

How can I get the catalogue you are writing about? What is its name and publisher?

 

[seatramp]

How can I get the catalogue you are writing about? What is its name and publisher?

Concours Parts and Accessories, concoursparts.com . If in the US you can request a catalog and they'll send a hard copy free. it's also downloadable

 

[hotrodbuilder]

Thanks for all the responses. I finally got all the Honey-Do's finished and got to the upgrades I wanted. After rewiring from the dash forward I replaced the floor insulation and sound dampening which gave me the opportunity to access the transmission speedometer gear from the shifter opening. It turns out I had a 21 tooth gear and replaced it with a 19 tooth. (thanks for the tip Hotrod Builder). I am now 2-3 mph slow at 65 which should keep me and the sheriff on friendly terms from now on. ?‍♂️?

Hi, you are welcome. Glad you are comfortable with the change.