Does Toyota lie?

So you think the GPS can tell your speed from a single reading? Silly me, I thought speed was distance divided by time. I had no idea you could calculate distance by measuring a single point.

Calculating velocity isn’t the GPS’s main function. Even it’s main function, which is establishing location, isn’t that accurate. If it were that accurate, terrorists could use them to plan attacks. The government forces the GPS companies to make them somewhat inaccurate.

Does too! Compared to calculating your position, calculating your velocity is a trifling little computation, let alone the computing power needed to run one of those graphics heavy talking car GPS units. The instant reading averages your speed usually every second in order to compensate for the inaccuracy in the position readings and make it so you can read it, but the readings it averages are taken on the order of fractions of seconds-- probably actually far less than .333 seconds. After it has made one reading, it discards the previous position fix and so it starts following a brand new vector. In any practical sense this is identical to a speedometer reading.

No, they can tell your position based on two readings, which they make many times a second. Calculating your speed isn’t the GPS’s main function, but they’re extremely well suited for it because they know your position and they have an internal clock that is accurate to a matter of nanoseconds.

The average uncertainty for most GPS units with a good fix is something like 12-30 feet, and so if you’re going faster than 1 or 2 miles per hour, your direction of travel dwarfs the wandering fix. At highway speeds, the uncertainty is pretty much negligible, especially if you’re doing an averaged reading.

Also, btw, the government stopped scrambling the signal for civilian GPS users in 2000, and so civilian GPS’s are far more accurate than they were in the 90’s. There are other error correcting systems that come on more expensive professional GPS units that can get the accuracy down to a matter of inches.

My Garmin Nuvi and my Toyota Yaris’s speedometer seem to agree quite closely. My Kawasaki ZRX1200R motorcycle is doing a true 66 when its speedometer indicates 70.

Like someone else said, car speedometers are not laboratory precision instruments. You couldn’t afford them if they were. We used to have a precision mechanical tachometer for measuring shaft rpm and that thing costed nearly a thousand bucks.

On the other hand, if Timex can sell me a watch for 30 bucks that loses/gains only a few seconds every week, why can’t a digital speedometer be just as accurate?

This built in inaccuracy feature was true up until about 2 years ago.

You can check your speedometer with a stopwatch and the mile markers. 60 MPH covers a mile in 60 seconds. It works great.

Modern electronic/digital speedometers are accurate.

A recent story told of the accuracy problems of the latest generation of GPS satellites. They no longer are accurate to two feet. Let’s say they are quite a bit farther off.

Ah! Thank you for clarifying, oldschool.

My Jeep came from the factory with 28" tires. The speedometer always read 2-3 mph over my GPS. When I upgraded to 30" tires a few years back, the speedo and GPS became dead on, and have been since. Coincidentally, at the time 30" was the maximum tire diameter offered from the factory…does the Camry come with different tire diameters from the factory? Do manufacturers bother to recalibrate speedometers to accomodate for different wheel/tire options?

My uncle has a Garmin on his Accord and it doesn’t record the same speed.

I just happened to hear a story about the relaxiation in the restrictions on GPS accuracy. I conclude the powers that be dismissed the terrorist angle,but really I don’t know why the GPS accuracy restriction was lifted.

I just traded a car that read 2 mph high relative to our GPS. The car I got reads 1 mph high. Our other car (all are GM) is right on the money with the GPS.

Yes, it could be a plot by Toyota. Engineers think about a 5 cent per unit saving and multiply that by a million units to impress themselves. An odometer a few tenths too fast multiplied by a million cars with faster than normal warranty expiration will save some considerable money. This is no joke.

I hope some teacher prints this thread off and uses it as a question in physics class. This is basic algebraic math at its core and goes all the way up calculus if you’re that motivated.

I’d like to add that, if the GPS device can receive four or more satellites, you get a 3-D fix: lat, long, altitude. In fact, aviation GPSs will warn of altitude conflicts with airspace and/or terrain.

So, the issue about road gradient playing hell with the velocity readout isn’t an issue most of the time.

Speaking of tires changes, many don’t even bother to check the air in their tires…and may get inaccurate speedo readings as a result. What’s a manufacturer to do ? Consider too the slight variations as your tires wear. + or - 2 to 3 mph at higher speeds as the error becomes greater, may have to be acceptable. Police stops for speeds less than 5 mph over the posted speed are vary rare for this reason on the interstates but in 25 mph posted where the error is expected to be less, it may be a bigger concern. Usually only when other factors are involved will doing 70 in a 65 be a problem. The police and the courts get it.

I hope some teacher prints this thread off and uses it as a question in physics class. This is basic algebraic math at its core and goes all the way up calculus if you’re that motivated.

This discussion has to be much more factor specific before you can even compare readings generated by GPS v. speedometer. We are aren’t there yet and the engineer that designs the speed monitoring systems in cars is under no illusion that his system will stand up to that much scrutiny when the unaware buyer drives off the lot. You know, the one that refuses to adjust his tire pressure with elevation changes or as his tire treads wear, which can be significant in deep lugged mud/snow tires over time. All done just to preserve an accurate reading.

6.6" 14.1" 28.3" 88.9" 713/mi 10000mi N/A
6.3" 13.8" 27.7" 87.0" 728/mi 10213mi -2.1%

Just a tread wear as a factor, differences of .3 inches w/o considering tire pressure, accounts for a 2.1 % difference for example.

There is a 3 mile stretch of level straight road outside my city. It is officially designated as an odeometer test strip, and my Toyota with standard air pressure in the tires is almost dead on. Using it to test the speedometer at 60 mph gives a close reading as well.

Car manufacturers usually make the speedo slightly optimistic, since they want to stay out of court in case of a speed ticket dispute.

In the past, speedometers were wildly optimistic. A 1953 Ford V8 had a top speed of 88 mph, just a little faster than a stovebolt 6 Chevy. Since Fords had 8 cylinders and were bought by a more “sporty” set of drivers, the top speed of a Ford showed 100 mph!!!

My brother-in-law had such a car and my sister, then his girlfriend, always told him “not to drive so fast”.

One car rental company years ago cheated by putting smaller tires on the back of their rear drive cars. They gained 4% in miles driven and billed their customers accordingly. This became a celebrated court case with heavy fines for the rental company.

I have a 2000 Camry and the Garmin GPS I have agrees to within 1 mph to what the Speedo says.

As much as I hate the “conspiracy theories” I think there might be some truth to this. Reason being that when I drive down the fwy in my Camry I have noticed that for the same flow of traffic and speed sense my Camry’s speedo reads ~ 5mph higher than the Caravan I have. So I started peeking into the cars driving next to me with the digital odo that is easy to read from outside and well, it seems like the Camry is ~3 mph “faster” than the other cars going the same speed.

Well I was wrong when the unscrambling happened but it did happen. Whitey did you have a problem with me mentioning the unscramble issue? I take it your comment was just a little “dig”.

About the recent Air France crash,it is said that the pilot may not have known his true speed due to icing pitot tubes, why did the pilot not know his speed from his aircrafts GPS system?

About the recent Air France crash,it is said that the pilot may not have known his true speed due to icing pitot tubes, why did the pilot not know his speed from his aircrafts GPS system?

First, this was just speculation, not based on data from the CVR or FDR (the two “black boxes”). As such, it constitutes a “wild-a#$ed guess” of the first magnitude.

Second, loss of the captain’s side pitot would leave (at least) the F.O’s side tube.

Third, SOP for penetrating a storm is: lights to full, maintain pitch and power setting (assuming the crew used common sense and turned off the AP). Thus, an erroneous speed readout shouldn’t have killed.

Considering the weather at the time the plane went down, it seems far more plausable that the turbulance of the storm exceeded the design parameters of the a/c. (Or, in short, Mother Nature >> 2024 Al.)