Speedometer accuracy

[popscomet]

PUT a tach on it and drive by that.

Since I swapped out the rear in my Comet and put the 4.11s in it I always drove by the tach, I actually had an adding machine tape I printed that listed mph according to rpm in the ashtray (may still be there) incase I forgot anything.

See Ya,
Mike

COOL !! pop

[Maine Pilot]

One of the first things I do whenever I acquire a different car is to test the accuracy of the speedometer/odometer.

These are the results of my '66 Caliente, (Indicated), as compared to a portable Garmin GPS, (Actual):

INDICATED/ ACTUAL

0-35 Identical

38/ 40

47/ 50

52/ 55

56/ 60

61/ 65

66/ 70

So, as you can see, as you get up above 50 mph, your actual speed increases by approx. 4 mph. The results on your car may vary.

I was told that older speedometers were notoriously inaccurate as compared with today's cars. I've found that to be true as my other three vehicles (2007-2013) are just about dead on accurate.

Knowledge, like this, may someday save you from a speeding ticket.

[poboyjo65]

One of the first things I do whenever I acquire a different car is to test the accuracy of the speedometer/odometer.

These are the results of my '66 Caliente, (Indicated), as compared to a portable Garmin GPS, (Actual):

INDICATED/ ACTUAL

0-35 Identical

38/ 40

47/ 50

52/ 55

56/ 60

61/ 65

66/ 70

So, as you can see, as you get up above 50 mph, your actual speed increases by approx. 4 mph. The results on your car may vary.

I was told that older speedometers were notoriously inaccurate as compared with today's cars. I've found that to be true as my other three vehicles (2007-2013) are just about dead on accurate.

Knowledge, like this, may someday save you from a speeding ticket.

do you have the same size tires as came on the car new? taller tires will give you a lower reading than the actual speed you are running. & generally older cars get taller tires than original put on them.

[Jims65cyclone]

Ah! Vindication! You had me worried with your first post, Ron, that I did the ratio backwards. Fisher.... if you have a smart phone there are plenty of apps you can download for free that will show your speed real time, like a GPS, but without mapping. Some are digital, some analog. I used one to check my speed. A friend uses one with his boat. John beat me to punch on the tire size question. The larger diameter the tire, the more distance traveled per rev, hence a lower indicated speed than actual. Have the wheels been changed to 15"? If so, is the tire size/profile combination close in OD to the OD of the original 7.35/14 tires?
Jim

[Comechero65]

Sorry to throw you off with my first post. Good thing I went back and reread it and caught my mistake.
A gps is one way to check speedo accuracy. I have used the mile markers on the highway and a stop watch to check accuracy. Do they still have mile markers? I know truck drivers used to depend on them. Been awhile since I have travelled across country.
It's not the size of the wheel that effects speedo accuracy but the actuall overall od of the tire itself. You can calculate the size changes from one tire to another to get the correction for speedo gears. You just need to know where to start.
Ron

[fisher]

I am running 215/65/15 Goodyears. Should be a 26" diameter. Running a 3:00 to 1 rear ratio. Doing the calculations If I have a 7 tooth drive gear I'll need a 16 tooth driven and if I have a 6 tooth drive gear I'll need a 14 tooth driven gear. I haven't found any driven gears of the right tooth yet so I may be out of luck for a cheap fix. I have seen the custom ratio drives advertised that can be added on but have not checked into pricing. I'll probably just make up a cheat sheet like others have suggested.

[poboyjo65]

I am running 215/65/15 Goodyears. Should be a 26" diameter. Running a 3:00 to 1 rear ratio. Doing the calculations If I have a 7 tooth drive gear I'll need a 16 tooth driven and if I have a 6 tooth drive gear I'll need a 14 tooth driven gear. I haven't found any driven gears of the right tooth yet so I may be out of luck for a cheap fix. I have seen the custom ratio drives advertised that can be added on but have not checked into pricing. I'll probably just make up a cheat sheet like others have suggested.

I came up with the same calculations. If your car came from the factory with the 3.00 gear ratio & that is the original trans then yours most likely has the black 7 tooth drive gear. If it were me I'd look at the drive gear in the trans to make sure it is black, then I'd take a chance & order the 16 tooth. Everyone agrees (now ) it needs to be less teeth so I think it would at least get closer to right. & since it is the smallest one you can get then it will be as good as it gets,,,,,,for a cheap fix!

If you look at the NPD link, it says black is 7 tooth & pink is 6. it also says typically the black 7 tooth came with the 2.79-3.50 geared rear ends & the pink 6 tooth came with the 3.50-4.30 rears. so yours being a 3.00 gear, is a good sign if it is all original, but I'd have to look in the hole to be sure.

[Jims65cyclone]

It's way more than just your tires. The original tire size of 7.35x14 equates to a current size of 195/75/14.
http://www.tirerack.com/tires/tiretech/ ... nchart.jsp
The diameter of a 195/75/14 is about 1/2" smaller than a 215/65/15, which would result in an indicated speedometer reading of 60 mph when the actual speed is 61.15 mph, which is a long ways from 72 mph.
https://www.tacomaworld.com/tirecalc?ti ... -215-65r15

The speedometer heads in our cars basically consist of a magnet spinning around in a hollow disc that is spring loaded against the direction of rotation. The faster the magnet spins, the more it overcomes the resistance of the spring, and the farther the disc and its attached needle rotate away from the 0 mph mark. Calibration accuracy was never their strong suit. Add 50 years of wear and tear, dust and dirt, and the head could be off significantly. If it's the head, though, I'd expect your speeds to be off significantly through the whole range if it's off by 12 mph at 60. I think it's got to be a gearing issue.
Jim

[lavron]

The speedometer heads in our cars basically consist of a magnet spinning around in a hollow disc that is spring loaded against the direction of rotation. The faster the magnet spins, the more it overcomes the resistance of the spring, and the farther the disc and its attached needle rotate away from the 0 mph mark.

I have always wondered how the speedo worked besides just being magic Learned something new today My belief in magical powers is shrinking

See Ya,
Mike

[fisher]

The car was originally a 289/c4 but is now a 302/T10 4 speed. So the axle appears to be original but from there on who knows. Even the speedo was replaced, as I recall it was an automatic donor car. The car appears to have been assembled from many donors just to resale it. Over 50 years I doubt that there are many cars out there that are absolute original. But it is a solid straight body that is a good driver quality......and I will drive it.....sometimes like I stole it

[StPro]

2 less teeth on the driven gear will correct about 10 MPH. Check it after the gear change to see how far off you are some of that can be adjusted with tire diameter, and air pressure.

[Jims65cyclone]

Just download a free speedometer app to your smart phone and prop it in front of your speedometer when you go for a ride.
Jim