Can speedometer be wrong?

[Walla Walla]

long-short story
i was on a highway, in quebec city (speed limit 100kmh(63mph), police tolerate up to 120kmh(74mph) always)

i was on cruise control, 118kmh(73mph) in left lane, 10 pm, and a cop doing radar in the center of the highway, saw me, started (without lights) so i really thought he wasn't chasing me, and then came at me and arrested me.... (and was angry because i took the next exit 200ft later so he wouldn't get out of his car on the side of the highway but in a smaller road... what a prick i am...)

he said he caught me at 129kmh(80mph)

i told him it was impossible... he and he gave me a ticket....
it is not much, 130$... but still....

i always paid my ticket, i'm 29, always took my responsability no matter what ever I did... i still have all my points, never had a car accident (except from a hit and run in a parking lot while i was inside a store) so, i'm really not an angry young teenager hating the police

i want to know, is it possible that speedo can do freegame, and give us bad speed reading? anybody had that problem? (i have different wheels, but tires are 225/40/18 chich is almost same wheel ratio)

i told the police officer that i was really ashamed of his work as it is impossible he caught me at that speed. that i would gladly paid his ticket a 118kmh, wich is the speed i was going...

in the end i know i will pay the ticket, but still... is it possible he was right and my car was wrong? i'm thinking about writing a complain letter, just to let know his supervisor, i'm dissapointed.... (i know it wont change anything... but goint in court and loosing 4h of work at least to save a few buck is just a bad financial decision)

[PSJohnDoe]

The diameter isn't the only factor, tires have a spec called revolutions per mile which accounts for tread, the give of the tread, inflation, etc. Find that stat for your tire model and size, that will give you an idea. Also, keep in mind that factory allows for a few percent margine of error and that tire diameter naturally changes over its lifetime so the rev/mile is just an estimate.

[whaap]

I've never had my speedometer calibrated so I'm not in a position to say he was right or wrong.

[cnk]

Can it be wrong. . sure. . it's possible. With your new tire size, your speedometer reads 1.9% lower than your actual speed. So if you were going 118 kmh on the speedometer, you're actually going 120 kmh.

Honestly what really got you the ticket was probably the fact that you didn't pull over right away and that pissed the officer off. In the end, it comes down to your word vs his and in most cases, you'll lose in court. I don't know how it works in Canada, but in the US, it's almost always better to go to court because you can usually get it knocked down to 0 points on your license for a higher fee. That higher fee is almost always cheaper than the increase in insurance premiums you'd get with points on your license.

[Kelbyat07]

He arrested you? I am a young teen who hate cops so i am going to tell tou that he probably wanted to look good to his boss. Eff the copsss.

[SigmaHyperion]

The only reason cops give, at least almost all of the time, any sort of 'leeway' of ~5-10mph is to account for variation in speedometers and their own detectors. It's not because they're being nice, it's because no measuring instrument like a radar detector or your speedometer is going to be absolutely perfect. Realistically, each is likely to be be 2-3% inaccurate. So they give you the benefit of the doubt, even if your speedo was reading on the low end, and their detector was reporting on the high end, they'll never be more than a combined 5-7% off; so they tend to give you 5-10mph of room before they start issuing tickets.

So, hypothetically, you think you're doing 73mph. But your speedometer is, let's say, 3% off -- so you're really doing about 75mph. Your tire size is different than OEM, so it'll have you reading slower than actual by 2% -- so now you're at about 77mph actual, even while your speedo says 73mph. Now the cops radar is off by a couple percentage and he clocks you at about 80mph. WAY over the limit on a 63mph road.

Anytime you push the upper limit of what they "usually" have decided to "allow" you to go, you are gambling. Because, once you're riding right on the cusp you have very little margin of error to play with.

[Luis_GT]

Simply, yes... Speedos often have a 5% variance right from the factory (+- 2.5% of the current speed) for safety margins... that is the reason cops have tolerances up to a certain speed.

My BMW's Speedo reports 5 miles more than I'm really going, and I tested that with a GPS.

This site should help...
http://www.rimsntires.com/specs.jsp

but accoarding to it, your car will be running 1.8% faster than the speedo reports

[fistpoint]

The speedo being slightly off in conjunction with your larger diameter wheels/tires is what did it, as well as the possibility of the cops radar gun to have been off by 1 or 2. Three things that helped contribute. But it's your tires that take most of the blame.

Stock sidewall height is 96.75mm, you went down to 90mm which is about a 7.5% difference.

A 235/40x18 would have been the closest match without going too wide(94mm). Using a 225/45x18 would still be off(101.25mm). Going really wide with a 275/35x18 would have been almost perfect(96.25).

Anyone running the size you have is also having the issue whether they know it or not. It may not be as bad as the result you got, but it is happening.

[Walla Walla]

ok... so, as i said, i will pay this ticket, and will now keep speed at 115kmh (72mph) and next time i encounter a cop, i will slow down, as it is possible that i go over the speed limit and the speed upper limit...

thanks for the answers!

[Porsche]

About the tires: Tire Rack shows the revs per mile for the OEM tire = 844, while the Michelin Pilot Super Sport in 225/40-18 = 829 revs/mi.

I calculate that you will be going 1.777% faster with the just slightly larger diameter tire on your car. Ignoring any speedometer error, this means that if the speedo reads 118 kph, you're actually going 120.1 kph, or 1.777% faster than the speedo indicated speed.

Your tires, therefore, played only a small role in any speedometer error. This did put you 0.1 kph over their tolerance level, according to you.

Sometimes on downhills the cruise control may allow the car to stray a bit above the set speed. Might that have been an issue here?

Perhaps he gave you a ticket for a false 129 kph thinking that you would go to the court and plead it down to a lower speed, which would then work out to be fair and just. Who knows?

Had it been me and an officer told me he clocked me at, say, 10 mph higher than I believed I was actually traveling, I'd ask to walk back to his squad car and have him show me the reading on the radar unit; it should still be up on the display. If the officer was correct, I'd investigate my car's speedometer. Then again, if the officer appeared to be the type who might beat me senseless for challenging him to provide proof, I might abandon that plan. There are bad police officers, more than we would like, unfortunately.

By the way, if you drive 15 miles at 81.5 mph, it will take you a tick over 11 minutes. If you drive 15 miles at 69 mph, it will take you exactly TWO MINUTES longer. If the distance is less, the time saved by speeding will be even less … but the ticket will still cost as much, the points on your driving record will be the same, and your insurance rates will rise the same.

That can prove to be an expensive TWO MINUTES.

On the Interstates of the U.S., I set my cruise control for four mph over the limit for regular drives. I drive completely relaxed without fear of an Enforcer hiding over the next hill crest, round the next bend, or up on an entrance ramp hidden from my view. During rush hour commutes I "go with the flow" as needed, which could easily be 85 mph out in the left lane. At 69 mph, I'd be trampled out there!

[Porsche]

Originally Posted by Walla Walla

ok... so, as i said, i will pay this ticket,

If you talk to the court, you can probably plead it down to a lower speed for a lesser fine and less damage to your driving record. That's common in the U.S., but I'm unfamiliar with Canada.

One of these days they're going to nail me for going 120 mph out in the country, and I'm going to be crying, too.

[Sportsguy83]

Originally Posted by Porsche

About the tires: Tire Rack shows the revs per mile for the OEM tire = 844, while the Michelin Pilot Super Sport in 225/40-18 = 829 revs/mi.

I calculate that you will be going 1.777% faster with the just slightly larger diameter tire on your car. Ignoring any speedometer error, this means that if the speedo reads 118 kph, you're actually going 120.1 kph, or 1.777% faster than the speedo indicated speed.

Your tires, therefore, played only a small role in any speedometer error. This did put you 0.1 kph over their tolerance level, according to you.

Sometimes on downhills the cruise control may allow the car to stray a bit above the set speed. Might that have been an issue here?

Perhaps he gave you a ticket for a false 129 kph thinking that you would go to the court and plead it down to a lower speed, which would then work out to be fair and just. Who knows?

Had it been me and an officer told me he clocked me at, say, 10 mph higher than I believed I was actually traveling, I'd ask to walk back to his squad car and have him show me the reading on the radar unit; it should still be up on the display. If the officer was correct, I'd investigate my car's speedometer. Then again, if the officer appeared to be the type who might beat me senseless for challenging him to provide proof, I might abandon that plan. There are bad police officers, more than we would like, unfortunately.

By the way, if you drive 15 miles at 81.5 mph, it will take you a tick over 11 minutes. If you drive 15 miles at 69 mph, it will take you exactly TWO MINUTES longer. If the distance is less, the time saved by speeding will be even less … but the ticket will still cost as much, the points on your driving record will be the same, and your insurance rates will rise the same.

That can prove to be an expensive TWO MINUTES.

On the Interstates of the U.S., I set my cruise control for four mph over the limit for regular drives. I drive completely relaxed without fear of an Enforcer hiding over the next hill crest, round the next bend, or up on an entrance ramp hidden from my view. During rush hour commutes I "go with the flow" as needed, which could easily be 85 mph out in the left lane. At 69 mph, I'd be trampled out there!

Thanks for actually posting data. I knew the 18 wheels + 40 height would be very minimal difference than what the stock speedo reads. It seems his radar was not accurate. Police radar detectors try and correct as much as possible for angled signals but there is also a margin for error there. I would take it to court.

[Porsche]

Originally Posted by Sportsguy83

Thanks for actually posting data. I knew the 18 wheels + 40 height would be very minimal difference than what the stock speedo reads. It seems his radar was not accurate. Police radar detectors try and correct as much as possible for angled signals but there is also a margin for error there. I would take it to court.

You're welcome, David.

I always calibrate my speedometer with new wheels and tires by heading out onto the Interstate with my stopwatch. Here in the Midwest we have little green mile markers along the right hand side of the road. I time a number of laps past these markers while maintaining an exact speed on my speedometer, and with those times I can compute my actual speed, comparing it to my indicated speed.

Maybe you have those mile markers, too. Look for them along the sides of the Interstates; having found one, the next one will be along in … one mile!

For those who have forgotten, the formula needed is D=RxT. Distance equals Rate times Time, or R=D/T. Don't forget that you'll be recording your times in minutes and seconds, which you will then need to convert to hours to get MPH, or miles PER HOUR. And Distance "D"=1, 'cuz you're measuring ONE mile, right?


Another point for those who may find this useful: If you choose to go to court to contest your traffic ticket, the officer who ticketed you must show up in court, too. They are often too busy with more important matters. If the officer fails to appear when your case is called, your case will be dismissed, ordinarily. And you're FREE!

Of course, all of this will have required at least two visits to the courthouse, waiting time, and so on, but hey, you might want to pursue this course for a number of reasons.

[cnk]

You can use this calculator to determine speed variances from stock. This is what I used to give you the 1.9% differential above.

http://www.miata.net/garage/tirecalc.html