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Road trip musings

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IRstuff

IRstuff

Aerospace
Jun 3, 2002
44,753
For our road trip to Heavenly Valley, we rented a BMW X3 and Google Map's speed indicator showed us going 4 mph slower than the nice digital speedometer claimed. My three cars all have speedometer readings slower than GPS claims, by 1, 2, and 2 mph. The X3 was the worst, but maybe that's because it's being driven by a BMW driver. ;-)

According to the speedometer reading could be as much as 10 mph faster than actual speed when at an indicated speed of 65 mph is displayed. OK, so maybe that explains some of those slowpokes on the freeway driving at 55 mph.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! faq731-376 forum1529 Entire Forum list
 
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Not a musing; our first day at Heavenly was wet and miserable, compounded by my ski boot's toe breaking off. They're 30 years old, and Salomon apparently had the bright idea to make the toe and heel removable/replaceable, but apparently the material suffered some fatigue and broke off.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! faq731-376 forum1529 Entire Forum list
 
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Year ago, I had read that Japanese car makers deliberately make their speedometers show a faster speed than the car is going. Supposedly, when the Japanese car makers started really gaining market in the US, they were faced with a US regulation that stated that the speedometer / odometer had to be accurate to within 3 percent. To the Japanese, this was a joke. They could easily make theirs accurate to within ½ percent. They quickly realized the value of using the full limit of the regulation. With the speedometer showing the car speed faster than it was going, the driver was likely to be more satisfied with the car. He would think he was driving faster. If he calculated his gas mileage, it would show better fuel economy. He was less likely to be pulled over or ticketed for speeding. But, to the car manufacturer’s advantage, the cars warranty would expire sooner. My last two Japanese cars had speedometers that were off by almost exactly 3 percent. But, our newest Subaru has a speedometer that is dead on accurate. So, I wonder if something has changed in the regs or otherwise removed the incentive for the car makers to fudge with this. Maybe the fact that everyone sees their speed on their GPS or navigation app, creates a new incentive to be viewed as higher quality by being more accurate.

Johnny Pellin
 
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The biggest issue in any speedometer / odometer which uses the rotation of the tyre is that the tyre keeps changing.

Different tyre manufacturers have slightly different OD / deformation under load.

Load in the car changes the circumference as does tyre pressure / temperature and wear.

Maybe not a huge amount, but around the 3% limit.

So accuracy to 1/2% would be impossible over a significant period.

But in general yes, I agree, compared to most GPS systems the car speedo will read higher - usually 3-5% in my experience.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
 
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First thing I do in a new car is check the speed at 100 kph indicated with my GPS. We can get fined and points for 3 kph over, which means Victorian drivers spend most of their time watching for cops and cameras or fixated on the speedo. That, apparently, is a road safety improvement.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376
 
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They only have to verify compliance with the regulation under shop test. And they know the exact test conditions. I expect for that test that accuracy within 1 percent would be easy. Brand new tires at shop temperature and exact pressures running a known speed for a known distance.

Johnny Pellin
 
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Here in the US different states have different amounts of leeway. Here in SoCal, unless you're driving really crazy, changing lanes a lot, following too close, etc, they'll give you a lot of slack when it comes to driving over the posted speed limit. As one CHP officer told me, their primary job is to assure the safe flow of traffic. If the traffic is moving 10-15 MPH over the limit but it's moving along with no one messing around and the weather's clear, they won't be stopping people. Now in Northern California, it's a bit difference. One of my only two speeding tickets ever was when I got nailed for doing about 10 over, up North of Sacramento, off on one of the state roads. I was going downhill at the time and the cop was going in the other direction, but he did a U-turn and chased me down. Again it was good weather and virtually no traffics. He was a local county sheriff's deputy, not the CHP, so I suspect he was behind on his weekly 'quota'.

But then you have places like Oklahoma where they have a 'zero tolerance' policy (and they tell you that in big bold letters on a big sign as you cross the border) and will stop people for doing 1 MPH over. I got stopped once doing 3 MPH over on a Sunday morning on the interstate, not a cloud in the sky, and virtually NO traffic. Now he didn't give me a ticket, but he did say that he was putting my name in their system and that this was the only break that I was going to get. I asked him if he would have stopped me doing 1 MPH over and he said yea. I asked him if he ever gave anyone a ticket for 1 MPH over and he said yes, but it depended how the person was acting. I guess I acted OK because I didn't get a ticket.

John R. Baker, P.E. (ret)
Irvine, CA
Siemens PLM:
UG/NX Museum:

The secret of life is not finding someone to live with
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Yeah, in California, the rule of thumb is basically that the closer you get to 15 mph or above the posted speed limit, your chances of getting pulled over goes up exponentially, particularly if you are more than 15 mph over the limit. I got pulled over once for doing about 20 over the limit, and the Chippie was starting to write up the ticket when another car blew past us doing easily 100 mph. Needless to say, the Chippie ran back to his cruiser and started chasing the other guy. Can't remember if he actually told me to wait for him, but I got back into my car and drove with the traffic until I got home.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! faq731-376 forum1529 Entire Forum list
 
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My last 2 cars have been within the width of the needle on accuracy.
 
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My 2018 Camry is < 3% too high. 1-2 mph @ 60 mph.

Good Luck,
Latexman
 
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The article linked in IRStuff's first post has a lot of hand waving about GPS, ending with the incorrect claim that GPS speed must be more accurate because it uses satellites and magic.

In truth, GPS speed relies on GPS position. And GPS position, while usually reasonably accurate, can at times be not so accurate, and this can and does sometimes make the GPS speed not so accurate.

Fire up a map application on your phone. See the transparent circle halo around the sold "you are here" dot? That is roughly the uncertainty of the GPS position at the moment. GPS knows that you are somewhere in that circle. All subsequent position and speed calculations assume that you are at the center of that circle.
 
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I once owned a Toyota van that had a speedo that ran about 10% low. Following a speeding ticket and a dyno test, Toyota informed us it was within tolerance.
 
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