Speedometer accuracy

Yesterday I passed by one of those speed signs posted by the police that show the speed you are doing on the road. It showed one mph slower than what my car was reading, so I would say that the speedometer is accurate

 

I did a 10k (6mile) test and at the end of 10k it read 9.9k so that would be within a 10th of a mile in 6 and probably about 5.94 miles. So I'd say fairly accurate. I haven't had a direct speed read like the other post but I don't think it's inaccurate enough to throw off the mpg readout.

 

Hey, today I drove by a police thing too and it too showed one mph slower than what mine showed. It read: 34 while the Prius sp. read: 35.

 

if that's the case we can go 1mile over the speed limit without them knowing it! :mrgreen:

 

Ya, that would be pretty cool, but would really be going the speed limit. 8)

 

Please keep in mind those police "thingies" are notoriously inaccurate. Also, both are digital, meaning a filtering algorithm is rounding up and down per an equation which is often not intuitive. Even a radar gun has 2% error when functioning correctly. The car speedo is only required to read between 0 and +4 MPH actual speed at 55 MPH in the US. At any other speed, accuracy is not guaranteed and many suppliers do not even test them at other speeds before shipping them to customers. In other words, your speedo will generally always read fast, with more error in this direction as the tire tread wears, adding another 3% or so to the full tread error.

Your best test method is to use a tachymeter over a 2 mile measured distance and double the result to get your actual speed. This will still have error from the tachymeter, but if you are within 1 or 2 MPH of the display, you are pretty accurate. For comparison, our F150 reads 5 MPH fast at 70 MPH and just meets the 4 MPH requirement at 55 MPH. The wife always wondered why others were flying by her all the time. Our other car is within 1 MPH for speeds greater than 40 MPH. I have not yet checked my Prius. Digital displays are harder to measure as the speed can be up to 3/4 MPH faster or slower than indicated without the display rounding up or down.

 

The other good method is to use a GPS to check the speed.

Dave

 

I have a portable GPS that we use on our boat. I will take it for a ride with me today and see how it turns out.
Brian