Does Toyota lie?



I am not usually paranoid, but… Is it possible that Toyota really wants us to think its cars are going faster than they really are? My GPS device (Garmin) consistently report a speed 2 or 3 mph slower than the speedometer on my 2006 Camry. Also, why does Toyota want me to think that the outside temperature is 3 or 4 degrees warmer than all other thermometers report. Is this some sort of insidious plot; are they seeking revenge for losing the war?


Your GPS measures velocity, and your speedometer measures speed. Velocity is your speed in a particular direction, so it changes quite often. Your overall velocity is based on vector addition. Speed is how fast you are moving, and ignores the direction in which you are moving. Your overall speed is calculated by simple addition.

The thermometer in your Toyota measures the temperature of the road. Don’t ask me how. I think they put the sensor under the front bumper. The other thermometers measure ambient temperature.

This is no insidious plot.


Automobiles in general read “high” on the speedomenter; it’s not a Toyota-only thing. This can also be affected by the tires; size, pressure, and wear all can affect the readout.

Realize that auto thermometers are unreliable unless the vehicle is in motion. (Also, friction with the air can cause a false high reading, but I believe this isn’t really an issue at automotive speeds.)


Thanks, that helps


Thanks, that makes me feel better.


You should probably consider the speedometer reading as a “ballpark” number. Many speedometers are off by a few mph.

Same with the thermometer. These are not precision instruments.


As has been suggested, none of the gauges on cars are what I would call “laboratory grade” measuring instruments. Whether we are talking about the gas gauge or the speedometer, or anything else, these devices more or less indicate what they are supposed to, but rarely are they extremely accurate.


Your first paragraph is making my head hurt. My car only travels along one vector at any given time, but maybe you’ve got one of those newfangled transdimensional ones.

The GPS gets “fixes” on your position a few times a second and finds your speed by measuring the distance between these fixes. Because there is usually a margin of error in these fixes between around 12-30 feet, the momentary speed readings aren’t all that accurate but over time and at highway speeds it should all even out. The only real difference between this and how a speedometer measures it is that the most car GPS units only record your horizontal speed and ignore any vertical gain, whereas your car’s speedometer which basically just counts how many times the wheels go around doesn’t differentiate. So if you’re drive up or down a 5% mountain pass at 50 mph, the GPS will only record you as going at 47.5 MPH.

But really how many 5% grades do you drive up? Usually the difference is negligible. Speedometers do generally read high, ostensibly to err on the side of obeying the speed limits. To add to your insidious plot theory, I will also point out that a car with a fast speedometer will appear to get better gas mileage than it is and the warranty will run out faster.


Your speedometer reading will vary as your tire diameter changes due to tire wear and inflation pressure (which will change some with temperature). Automakers (all of them) calibrate the reading so that it will never read slower than you are going, so usually it reads a little high. They are just helping you avoid a speeding ticket.
It’s hard to make an outdoor thermometer read correctly on a car. They have to deal with heat from the engine when not moving and heat from the road at all times.


If you live in LA and want to go to Chicago, you can drive or fly. The flying distance is considerbly shorter than the driving distnce, for obvious reasons.

The speedometer measures your SURFACE speed, while the GPS measures how fast you are nearing your destination as measured by satellite up in the sky.


For me the only value of a device that tells the outside temp. is when it can warn me of icing conditions.

Not to be cynical but I don’t think the car manufactures care if I get a speeding ticket. Here in Tucson the photo radar only snaps when you are 11mph or more over the limit. So the cops are giving you 11 already why should the manufactures give you 2 or 3 more.

Now back in the double nickle days I have heard of people getting 57 mph tickets so the 2 or 3 that the manufacture supposedly builds in could be relevant.


Unless the roads you travel are completely straight, and you never change lanes, and you never move from one side of the lane to the other side, your vector will always be shifting slightly. As you drive through a curve, your vector is constantly shifting. It is almost never truly constant, especially if you have cross winds. Vectors don’t bend. When your car changes direction, even slightly, one vector ends and another begins. The end result of vector addition will also be a represented by a straight line.

The easiest way to explain vector addition is with right angles. Please see the diagram below. If your car travels from point A to point B, your speed is represented by the sum of the lengths of the two purple lines. Your velocity, however, is represented by the length of the red line.


Similarly, in this diagram, which represents a curve, your speed is represented by the length of the purple curve, while your total velocity is represented by the length of the red line. This is why your speed will almost always be larger then your velocity. Your car almost never travels in a true straight line.


I guess the part I don’t understand is how the GPS would be measuring your speed relative to any vector other than the one you happen to be traveling in in that particular fraction of a second.


It measures one vector for each two “fixes” on your position. Each fix only tells your position at the time, not your velocity. It needs more than one fix to calculate your velocity. Your GPS not only needs to know at least two positions, it also has to know the time between the two measurements.


Vectors, sectors…You guys are rattling my brain.

Speedos calculate speed based on the signal from the vehicle speed sensor, which simply measures the speed at which the output shaft is turning. Most speedos read alightly high. I suspect that’s because the rolling circumference of the tires is slightly smaller than the outside circumference, which is probably the one used to calculate speedos.

GPS systems don’t measure vectors. They measure as Greasyjack said. And while the math might be fun, inclines have an immeasurable effect. Think of the distance of the satellite, the difference in that distance over a split second on even a steep hill, and do the math. Assume the satellite is directly overhead and it’s arc speed and direction is keeping it directly overhead…for simplicity. How much difference delta would be necessary to cause a 3 mph variance? I dare you.

And the thermometer does not measure the temperature of the road. It’s a simple thremocouple. In my car I have to wait until I’m moving on a hot day to get an accurate reading, because radiant heat from the car affects the reading.


So other than the issues of accuracy and of elevation, this speed reading would differ from a speedometer reading how?


In the diagrams above, the speedometer measures the purple lines or the purple curves. The GPS measures the red lines.

When looking at the curve, adjust the scale of the drawing in your mind until it makes sense. The measurements taken in that diagram by the GPS could be 0.333 seconds apart. In that case, the measurements would be pretty close together. Only on longer distances would the discrepancy be noticeable.


You’re not going to get an amount of vertical travel within a .333 second measurement big enough cause error that is going to be observable by GPS or speedometer. Not in an automobile anyways!

So on the scale that we’re talking about the bunch of tiny vector velocity measurements is for all practical purposes the same as the speed measurement, and thus does not explain the disparity between the two readings.


Your GPS doesn’t have enough computing capacity to adjust your velocity every 0.333 seconds. Even if it did, you would not be able to read it. It shows your average velocity based on data measured over several seconds. Watch the display on the GPS while someone else drives, and you will notice the velocity doesn’t change rapidly enough to be based on 0.333 second measurements.

If you had a computer in your dashboard that could tell you your average speed over an hour, you would notice a real difference between that speed figure and your GPS’s calculation of velocity for the same period. Like I said, the difference at 0.333 second intervals will be small. They do add up though, and the figure you see on the readout is the based on a longer period.