Back in the day a friends dad drove an unmarked police car. It was a 60's something car with a big straight across speedometer marked to 120mph. It had a piece of tape going across the original numbers indicating "true speed" His dad said factory speedometers were not very accurate and most wildly exaggerated speeds from about 80 on up. He said the Police Department made and put the tape over the speedometer so they could have good legal grounds for issuing a speeding ticket when pacing a car. I guess in car radar wasn't as common in the 60's and many were maybe this way. Do any of you know more about this, I'm just curious to know how they arrived at the calibration. --- Steve ---
In the mid 80's the patrol car speedo was close enough to moving radar speed to work in court. Difference was each number on the speedo was not there. On the moving radar it showed the number.
AAA use to come out and check the speedo calibration. Then they would give a printout showing true speed and indicated speed. These are used in court when a speeding ticket is based on a "pace". Officers that use radar have to attend a radar class and be certified to be accurate in a visual speed estimates within 5 mph. It has been my experience that everyone is accurate within the 5 mph range.
I think motorcycles are the worst offenders of "un-calibrated speedo". I have checked several against cars and they are always 5mph faster at around 60mph. At 90 in the car the bike reads a cool 100. My guess is they do this so guys brag to their buddies "my bike does blah blah blah..." Kelvin
I rode in several cars back in the 60's that buried the speedometer past 120 when they certainly didn't have the real horsepower to reach that speed. The old cop car (64 or 65 Ford I believe) I mentioned, had 105mph on the calibration tape just about where the factory 120mph mark was.
I can remember riding in my father's '57 Ford station wagon with the speedometer showing over125 MPH! I think factory speedometers were way optimistic before Ralph Nader..
I am sure the cops just timed their cars over known distances at certain speeds. I can't recall seeing one in a few years, but there are/were sections of highway that had speedometer calibration signs posted. There would be a sign that would say "speed calibration marker ahead", then in a little while a sign saying "start speed calibration" and after a mile, "end speed calibration". Now days just use a flat road, cruise if ya got it, and a smart phone with a gps speed app. You can take apart your speedo and adjust the torsion spring if you are brave. Kelvin
on the APP STORE ULYSSE SPEEDOMETER My daily driver is an 07 Crown Vic Police Interceptor Police Package. The speedo is calibrated to Police specs. I've tested this app against it, it is SPOT on. I highly recommend this APP. Just turn it on and drive its GPS based.
I have the same app, have gotten a few customers out of tickets when I proved their speedo was out of calibration and that it has been corrected. Used the defense that they were driving at the speed the speedo showed and had no idea that the factory speedo was off by that much. The older cars that used cables, 1 tooth = approx 5% change in the speedo reading. The newer cars with electric speedos can be calibrated to almost dead on. But I have found that a lot of them tend to start reading a little fast once you get above 60-70 mph even if they are dead on at lower speeds. I had to "recalibrate" my GF's speedo in her Shelby truck to read fast.... she hasn't gotten a ticket in a long time now... she doesn't know it.
A buddy of mine had an old Glendora CA cruiser (69 Fury) we turned into a street machine. The speedo went to 140 and said "certified" across the bottom. Guess they had some way to adjust them by that point.
Dont forget that your speedometer is calibrated for the factory sized tires...so changing to a taller or shorter tire will throw off the speedometer reading. I got a customer out of a ticket a while back...she was borrowing a friend's truck to move some things, and she got a ticket on the highway for going 72 in a 60...she swore she was only going 60-62 tops (indicated.) I took one look at the truck, lifted with big giant tires on it and told her that was the problem. The bigger tires cover more ground in one revolution thus making you go faster than indicated speed. I actually found a few formulas online that can show you just how far off your speedometer is given the overall height of the tire. The judge said she had never heard of such a defense, but backed up with the math she let the ticket go. All of that was said because as hot rodders, we love to stick non-factory wheels and tires on our cars, which will throw the speedometer off. I use the GPS speedometer app and find it to be extremely accurate. Just something to consider...
was a speedo shop around the corner from where I worked that did the state/county cars , it was a drive on unit ( still use it for Vascar calibration and rolling radar ) , they also had a machine to fix the heads of them , I was in there one day , got to see the guts of a old style unit ( cable ) and it had a bunch of adjusting screws to tension the spring and there is a dampner on the cup too. they also did the meters for taxis as they have to be calibrated so they do not over or under charge the customer on milage figures , anyone remember the old fords that had the big flat speedos with a pin that stuck out from the one side that they put a wire seal thru ?? that pin is a friction device that rides on the needle cup .. a freind of mine does the radar gun calibration ( with the wave guide tunnel, not the improper way using a tuning fork ) , now thats neat
Generally speaking, factory calibration spec is +/- 10%. Vintage speedometers operate off of a balance between magnetism and hairspring counter tension (pointer balance also plays a role). At the factory, they were moving units off the assembly line as fast as possible and you can find quite a few "issues" that would effect calibration even with new old stock units still in the box. But like all things, time takes it's toll: magnetism decreases, mechanical wear, so on and so forth... so most speedometers are not very accurate by the time we see them here in the shop. The same is true for other gauges.
I worked in a speed shop in the late sixty's where we would check the speedometers of all the local police cars annually on our chassis dyno. They provided us with a form that had to be filled out that listed the results at a number of different speed points. They used this document to validate the accuracy of their speedometers in court cases. In most cases they were within 2 to 3 mph of our reading. If the variance was greater than that they would replace the speedometer and bring it back for a retest.
Correct. Been awhile since they used AAA though. Now it's all calibrated by the radar unit in the patrol cars. At least in California it is. And a side note on GPS. It is not always accurate. I live near two big Navy and Air Force bases and nothing works when your within a few miles. Or if you don't have good line of site due to tree cover or driving mountainous roads or tunnels. Just a heads up is all, it won't get you out of a ticket or a win in court. Posted using the Full Custom H.A.M.B. App!
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