Calibration of Speedometer

[Bill Stoneberg]

I would like to know how to check my speedometer in my 47 Buick for accuracy. I want to run a Time/ Distance rally and I cant use my odometer in the calculations at all.

Would the easiest way to go to the Interstate and run at 60 MPH and see how long it takes using the Interstate milage markers ? Are they accurate ?

Is there a easier or better way ?

[The Old Guy]

Bill,

I have used the interstate mile markers for years, and they are usually very close. I usually try to figure out what MPH on the speedo will take me a mile a minute, and that gives me the amount I am off. If you find it quite a bit, go to a truck shop and get a ratio adapter. they have them for almost every ratio.When I changed rear axle ratios in my 40 ,I had to use a ,72 to 1

[Mark Huston]

If you are interested in doing a time/distance run you are looking to find out how acurate your speedometer is. The best and most accurate way of doing this is going to cost money. You will need to locate a speedomoter shop that has the equipment to test your speedometer with the car running. They put your cars drive wheels in rollers and then get it up to speed. When I had this done on my '28 Studebaker they got my car up to 60mph according to their machine and then we looked at my speedometer to see how far off it was. And yes it was way off. The most recent quote I just got for this kind of calibration here in Sacramento was 75.00 to 150.00 depending how far off it was and also what is needed to make the speedometer accurate at the same time while checking it.

[another old car]

Bill

this is the easiest way I know and it seems to work with the newer trans that have a lot of gear ratios ( speedo gears ) take new car out with you and two cell phones , that way you will have speed on both cars at same time , you also could have a ratio adapter made if needed be

[critterpainter]

Gee according to the local portable radar units that post your speed my new cars (89 97 02 ) are more inaccurate than my old cars

[Guest Hal Davis (MODEL A HAL)]

Bill,

There's nothing wrong with using the interstate mile markers. To be more accurate, do your test over the course of several miles. This will minimize any inaccuracies in your timing measurement as well as mile marker placement. A mistake of one second over the course of ten miles will have less impact than a mistake of one second over the course of only one mile.

[Guest stude8]

Before you condemn the old Buick speedo for an error be sure your modern tires are the same OD the factory called for in 1947 and tire pressure is correct. Most modern production tires are a compromise from the original 80+ profile ratio that was common in the 40's. Prewar ratios were a real departure, my 1930 President side mounts never sat as deep in the fender wells because the contemporary 6.00x20" Denmans were larger than the prewar 30x5 standard. Stude8

[Guest Albert]

Why not the next time you see a local speed trap, ask the officer to check your speed, and see how far you are off.. just don't do 80 in a forty zone though..

[Bill Stoneberg]

I am not condeming my Speedo at all, I just want to see if it is accurate.

No, I will not go through the local speed trap and see if I get caught. 10 mph over is $ 120 down here and I can use the money for other things.

The idea with cell phones and another car sounds good.

Thanks for all the ideas.

[ex98thdrill]

Bill,

Being from Texas I don't know how things are done, but my advice to you is check with someone from one of your local law enforcement agencies and academies. I will list some information below that applies to New York State, so if the information is the same as Texas, this will help you.

1. Police Departments - In New York State any police officer who is certified to operate radar must be able to pass a written exam as well as a practical exam where they have to be able to visually estimate the speed of a vehicle at actual speed plus or minus 5 miles per hour. This Certification must be renewed every 3 years. Part of the radar requirement is that the radar unit must be calibrated at least twice a day. (usually at the beginning and end of each shift) and must be send out to another agency at least once a year to be calibrated for accuracy. I can't remember if it's quarterly, semi-annually, or annually, but I know in my county, they have to send the radar units to Albany for calibration.

2. Police Academies - Most of your police officer academies have a portion of the school which includes radar certification in which the cadets have to learn how to run radar.

Suggestion - See if Texas is like that, and if they are, either have a patrol officer clock you on radar at a time when they are slow, or check with either a police department or academy and have your car calibrated when they're doing their radar training. If Texas has the strict regulations requiring radar calibration like New York does, you'll get your speedomoeter calibrated accurately and it won't require you to have to jump through hoops to do it.

[Guest]

Accuracy of a speedometer may be checked by utilizing interstate mile markers. Maintaining a constant speed, {any speed}, divide the number 3600 by the number of seconds to run the mile. That is your actual speed. Try it several times to average any disacrepancy. Try many speeds, as the speedometer will be more accurate at low speeds than at high ones.{60 seconds X 60 minutes = 3600 seconds per hour}

[Restorer32]

A very nice police officer in Port Jervis, N.Y. recently helped me calibrate the speedo on my Dodge 3/4 ton Turbo Diesel. According to my speedo I was moseying along at 72 mph (in a 65 zone) on I 84. According to his radar I was doing 84. And he only charged me $110 for his services.

[Guest]

One method not discussed here is the use of a handheld Global Positioning System (GPS) receiver such as made by Garmin or Magellen. The speed resolution is down to the tenths of a MPH as well as distance traveled. I would suggest using the remote magnetic mount antenna on top of the roof. If you don't have one or don't know of anyone who does, I would suggest looking in your area under scavenger hunts. There are groups of people who "hide things" and only list the GPS coordinates, giving hikers or hunters a challenge to find the "prize". They may like the challenge of using their GPS for something different.

[Guest stude8]

The GPS idea is very good, we have used it to clock speed in race cars which have no speedo or odometer to observe. Why didn't I think of that? Sorry system won't allow me to attach a .jpeg taken from in the racecar on Bosch test track.

Stude8

[Guest imported_PackardV8]

go to the local discount autoparts store and buy a cheap tachometer. If your 6 volt it doesn't matter hook it up anyway, it will work just fine. NOW, measure the diameter of your rear wheel/tire assembly. Find your axle ratio and compute a little table taped to your dash with RPM/MPH. If u have O'drive u need to figure that in too.

If u cannot do the math let me know and i will give the formula here.

Speedo calibration will not hold very long and most factory speedos will loose or gain at hi-speeds and very low speeds. The mile markers are ok but sometimes holding a steady speed for one mile is not easy to do either.

Just get the tach and use it. Thats what i use on my 56 PackardV8.