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Our local police like to place unmanned radar on the streets occasionally that has a large sign displaying your speed. I passed through one today and although my speedometer read 40 mph my actual speed (according to the radar) was only 37 mph. Correct me if my math is wrong but that is approximately a 7.5% discrepancy. In the old days before ICs were everywhere on a car a cable was used and it wasn't very accurate either. I assume our system is digital and something is reading tranny, driveshaft, wheel rotation??? Anyone have a clue how our speedometer actually works, and what about the odometer. I hope it isn't overeading.

As sophisticated as cars are now, one would think that the speedometer would be dead on.

Gene
 

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1. It is not easy to read 40mph exactly though the speedo unless it is digital. If it is a bit higher or lower means it might be 39-41 mph.
2. The radar showed the point where you were aimed. Unless you're driving exactly the same speed looking at the speedo. You might even driving at 38 - 42 mph (even using curise control doesn't mean it will stay exactly 40mph).

Testing the speedo error with higher speed will miniumize the above errors percentage.

3. Yes, most (if not all) cars are decided to show higher speed although not 7.5% different.
4. The odometer uses different method of measure. http://auto.howstuffworks.com/odometer.htm
 

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dont trust the readings

There are so many variables in your situation, don't put all your faith in what you saw.
1. Most speedometers are off by 1 to 2 mph
2. were you the only car in the radar's cone of influence? perhaps it was picking up another vehicle near you.
3. the radar unit could be off by 1 or 2 mph
4. if the radar unit was not pointing DIRECTLY at your vehicle, it could give a low reading by 1 or 2 mph (cosine effect)
5. if it was off by 2 or 3 at that speed, is it also off by 2 or 3 at 90 mph? or does your error increase with speed? most speedometers when they are off, are the same # mph off across the board.


etc., etc., etc.
 

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I had a 2001 Honda CRV that was 5 mph under the actual speed. I used to wonder why I was always being passed on the highway, especially since I tend to have a heavy foot. So when my wife and I were coming home from somewhere in separate cars I called her and asked her to drive 55. It turns out (using my spedometer, digital display) that she was going 50.
 
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Also, do not forget that actual speed also relies on tire inflation somewhat. The tire is rubber, so underinflation will decrease the circumference of the tire.

The police radar is there to SCARE you so it may show higher speed.
 

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Interesting. On my road trip from Boston to Orlando back in March, I had a Garmin GPS with me. When I had the cruise control set to 75 MPH, with the needle sitting directly overhead of the 75MPH tick mark (I was carefull setting it for testing), the GPS was always 2-3 MPH lower.
 

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The speedo cannot be always accurate in RAV4 as the car comes in 3 different tire sizes that are close but not the same.
The bad news is that if your speedo is off 7.5%, you loose 4500 miles of warranty coverage.
 

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friendly_jacek said:
ic said:
friendly_jacek said:
The bad news is that if your speedo is off 7.5%, you loose 4500 miles of warranty coverage.
The odometer uses different method of measure than speedo http://auto.howstuffworks.com/odometer.htm
How so? Do you care to elaborate?
I didn't learn anything new from your link.
Yes, the link only show how odometer works.

OK, let me try...
Although odo and speedo use the same cable connect to the gear at the transmission to calculate the distance, but the speedo needs the time per rev to calculate the "instant" speed. As the time per rev is in the measurement of milliseconds and mutiple to have the scale of hour, slightly error will be mutipled. As the rule of thumd is to show higher than normal speed is better (think of the other way around if you get speeding ticket), the speedo is designed to be optimatic although I don't think it will be 7.5% different. I heard that it should be within 5% in Europe, please let me know if I'm wrong.
 

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I'm pretty sure the speedo and odo on the Rav4 are electronic and use magnetic or optical wheel and shaft speed sensors to aquire vehicle speed, there no mechanical gears and cables on this type of system.

Tim
 

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Set you cruise control at 60 and drive one mile. It should take you exactly one minute to do this. I've noticed on a few different radar machines that my speed displayed on the board was 2 mph slower than what my odometer read. Most radar devices are calibrated to be accurate within a couple mph and usually show a slower speed than what the car is driving.
 

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:twisted: I love those mobile stand alone radar units they have one always in the same place as soon as I see it its like a green flag.I see how high of a score I can get :lol: .Seriously its on a empty street with nothing around otherwise I dont do it BUT that one everytime man :wink: .I got a high score on my buell one time so high the lights just flashed cause they didnt go high enough.
 
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