Speedometer Accuracy

Sacramento tends to have a bunch of these portable radar displays on the road.

[Broken External Image]:http://www.papd.org/graphics/smarttrailer1.gif

Every time I see one and look at my speed, they are the same.

I have yet to see one in a 55 or higher zone, so I can’t vouch for the Prius speedo at that speed.

Once you start really flying . . . say near the speed of light . . . I’m positive the Prius speedo is highly inaccurate. That dang time shifting thing really screws things up. :x

 

According to my Kenwood navigation its within .5 mph its almost always the same. I'm sure running 42/40 psi in my tires is the difference between the two.

 

When I changed tires, I was supposed to be 2% off. I am now supposedly going faster than I think, but people still pass me. I believe it has more to do with following the speed limit (or not) than mechanical inaccuracy...

 

As compared to my hand held GPS unit, my Pri was showing 2 mph less than actual.

 

My Garmin GPS unit shows that on my Classic, the speedometer is dead-on (up to the limits of a whole number speed readout) up to around 90 km/h, and then a slight discrepancy starts to show up, so that at 100 km/h, the GPS actually shows 102-103 km/h. I've got the 185 sized Michelin Hydroedge tires on the car, which would account for this. I haven't gotten around to checking what my recently installed nav system says, but I'm betting it would be the same as the Garmin GPS..

In contrast, on my old Corolla, the speedometer was likewise dead on up to about 80 km/h, and thereafter, it read high, so that at 100 km/h, I was actually only going about 98, and at 130, the car was only doing 120..

The old 94 Jeep Cherokee we use at the barn is dead on with the GPS throughout its range..

I think that most speedometers are built to have most accuracy in the low speed range (0-80 kph/0 - 45 MPH)

 

I don't know how the US models are adjusted, but it looks the Japanese 2004 Prius was built following "ECE Regulation No. 39".
http://www.unece.org/trans/main/wp29/wp29r...regs/39rv1e.pdf

On the last page, it says...
0 < or = display error < or = (real speed)*0.1 + 6km/h

The Japanese Prius displays about 6km/h higher than real speed when the vehicle speed is 40km/h or higher.

Ken@Japan

 

According to my Garmin when I drive 60 I am really going 59.
This 1 mph difference seems to hold from about 30 to 70 at least.

 

<div class='quotetop'>QUOTE(Tideland Prius\";p=\"105317)</div>

Come on now. I have a BMW 740iL and have timed it at exactly sixty seconds when traveling at what it claims is 60mph on a measured mile.

 

My handheld GPS shows my speedo is reading 1 mph fast at 70mph (cruise control on) and the radar gun speed signs show the same error at 50mph.

 

<div class='quotetop'>QUOTE(genalex\";p=\"105315)</div>

[font=Comic Sans MS:0c2f9c81bf]So there are a lot of inaccurate speedos out there, almost all showing a higher speed that reality. This means, IMHO, that all those people passing you are SPEEDING. If you have any questions about this, call up you local highway patrol and ask what the "real" average speed is on your local freeway.

Just cruising along in my Silver Cloud....[/font:0c2f9c81bf]

 

Hey! The speed displayed on the diag screen of the NAV system and the speed on the dash is way off! Almost 5 mph faster on the dash at 65 mph!

Someone else try this with their NAV system and get back to me.

Nate

 

<div class='quotetop'>QUOTE(genalex\";p=\"105324)</div>

We checked our Prius at 45 and it was dead on.