# Representing error: expected versus actual speed

1. Oct 27, 2015

### DaveC426913

I am checking the accuracy of my speedometer.

When my speedo says I'm doing 100km/h, I record an actual speed of 123km/h.
When my speedo says I'm doing 60km/h, I record an actual speed of 68km/h.
I have more data but it corroborates this simple example: error increases with increasing speed.

I will make a chart, but for now...

What is the appropriate way to represent an error like 100 vs. 123?

Do I want to represent it as a fraction of my speedo? (23/100 = 23% error)
Or as a fraction of my actual? (23/123 = 18.7% error)

2. Oct 28, 2015

### Staff: Mentor

IMO, the latter. The displayed speed is about 18.7% under the true speed of 123 kph, and is about 11.8% under at a true speed of 68 kph.

I think your speedometer is going bad -- those displayed speeds are pretty far off. Car manufacturers typically produce speedometers that are within about 5% of the true speed, with a bias toward displaying a higher speed than the actual speed. I think I read that somewhere.

3. Oct 28, 2015

### Guapa

Wrong tire!

4. Oct 28, 2015

### DaveC426913

?
I have been having problems with my leaky tires lately. Are you suggesting a wrongly-inflated tire could have this much of an impact?

5. Oct 28, 2015

### Guapa

It might be affecting the radius/diameter.
But you have an above speed from your odometer, a leaky tire should get smaller and not give you "extra lengths" covers or higher speed, than the odometer.

The size of the tire or diameter is larger than the manufacturer recommendation. Maybe

6. Oct 28, 2015

### Staff: Mentor

Possible, if the tires being used are larger in diameter than the stock tire size.
Not unless the tire was way over-inflated. Larger tires travel farther for a given number of engine revolutions, leading to a greater true speed than indicated by the speedo.

I don't know what kind of speedometer you have, but some mechanical speedos have a rotating cable that is driven from the transmission. At the speedo end of the cable is a magnet that causes another magnet to swing through and arc. The faster the cable turns, the greater the arc the speedo needle moves through. If the speedo is electronic, it could be that there is a sensor that isn't working correctly.

I'm not an expert on speedometers -- just passing on what little I know about them.

7. Oct 28, 2015

### Guapa

I need to change the oil in my pick up, the North west is cold and it is getting colder, It makes it harder to change the oil filter. Does anyone knows if I am going to have a 69-70 degree in the next week or so? I need a good weather spell.

8. Oct 28, 2015

### Guapa

I am completely sure that you know the answer, you just like to pick on people's minds.
Oops sorry, you are not looking for a tire size. Please kindly accept my apologies.

Last edited: Oct 28, 2015
9. Oct 28, 2015

### Guapa

Theory says: There are magnets placed on the axle, rotating under a sensor, which picks ups the pulses of a magnetic field, each time the wheel rotates.
The speedometer uses the time between the pulses and the traveled distance of one rotation of the tires to calculate the speed.

My understanding to find the error, use the two velocities and it will be the aboslute value of the difference between the predicted velocity and actual velocity divided by the actual velocity.
Error is aprox: .18669

0.18 or 18%

Last edited: Oct 28, 2015
10. Oct 28, 2015

### Staff: Mentor

Not too likely. I don't know where in the Northwest you are, but in the Seattle area, the high temp in the next 5 days is forecast to be 60°.

At any rate, your question is off-topic in a thread about speedometers...

11. Oct 28, 2015

### Guapa

DavidC426913 already had the answer way before posting it, he might be just testing your wires or whom ever dares to respond. (well that it is my theory). The data is pretty accurate and in metric, and lot of more data. Montana, it is cold. Montana increased the speed limit to 80 mph, David reading error in Montana will not give him a ticket, he will be doing 76.43 mph. or the other option he could deflate the tire to a smaller diameter and be Interstate legal. His velocity computed is 18.7% off the actual value.
Darn it, it is hard to eat flour and whistle at the same time. But if all the above it is not true, then therefore I have never had a teacher or gone to school. And a Thousand Apologies to DaveC426913.
It's been said : "We are not all there". Sorry!

12. Oct 28, 2015

### Staff: Mentor

Actually, that was effectively a decrease. Back around 2000 or so, and before, the daytime speed limit on the interstate highways (I-90 and I-15 - I don't think there are any others) was whatever speed was "reasonable and prudent." The feds threatened to take away highway funds if Montana didn't adopt a reasonable speed limit.

13. Oct 28, 2015

### DaveC426913

Guapa, I have no idea what you're trying to say. If you suspect something about what I know, why don't you just ask me?

I don't understand what Montana has to do with anything - especially here, where metric is the norm - but you've still come to the wrong conclusion.

If I were in Montana seeing 80mph on my speedo (and if the error keeps increasing at the same rate, it would be in excess of 30%) I would, in reality, being doing something in excess of 97mph.

Finally, please stay on-topic (my topic).

Last edited: Oct 28, 2015
14. Oct 28, 2015

### DaveC426913

Here is my first round of data.
Not a lot of data but, with cruise control, I get pretty accurate readings.

And here is the second round of data. This one has a wider margin of error because my cruise control stopped working, so I had to keep it steady manually.
The 100/123 data ponit is actually six data points super-imposed.

Last edited: Oct 28, 2015