GPS (Not NAV) MPH vs Speedo MPH

<div class='quotetop'>QUOTE(Rufaro @ Jun 17 2006, 07:07 AM) [snapback]272648[/snapback]</div>

The Automobile Club of Southern California offers a speedometer calibration test. It is performed on a dynamometer (which probably means you can not do the test on the Prius on account of the traction control). I never owned a car that tested to have an accurate speedometer. Most of them read 3-6% above the actual speed.

 

i have never seen a difference betweeen my GPS ( Holux BT 232) and the speedo on my car OR the portable speed monitor used by the Lacey Police department. im sure one of the 3 have been a mph off here and there, but i specifically dont remember anytime time or what was high or low.

considering the MUCH MUCH tougher job the GPS does and the much documented inaccuracies reported here on portable GPS'es i would believe the Prius first

 

This thread might be of interest:

PriusChat Archive

 

I've noticed a similar "too high" speedo reading, even without a separate GPS unit. A few of the radar "Your Speed Is" signs imply my speedo reads a tad higher than reality.

Display Rounding
The speedo, the NAV, and an outboard GPS are all rounding your speed readout to the nearest 1mph. For example, one unit could be rounding down from 59.4 to 59; the other might round up from 59.6 to 60. You'd read a 1mph error when it's only 0.2mph. In this example, the actual error is multiplied 5x because of the precision of the readouts.

Hysteresis
Another factor is hysteresis built into the readout. Hysteresis is a deliberately slowed reaction of the display. If, for example, your actual speed is 59.99mph, you don't want the display wildly flipping back and forth between 59 and 60. The software "smoothes" the instantaneous changes that happen with road conditions. The hysteresis will thus tend to avoid changing the display rapidly. Note that the speedo, NAV, and outboard GPS units may have different hysteresis, so their displays may not change the same way.

Tire Revs Per Mile
The speedometer gets its speed value by counting tire revolutions. The faster a tire turns, the higher the reading. There are a couple of ways to tweak the tire-revolution speed.

Running higher tire pressures will help bring the speedo reading down slightly, which helps the problem. It won't move much, but at least the change will be in the right direction.

As your tires wear, they get slightly smaller, so they rotate faster. The speed will read slightly higher than it did with new tires.

When it's time to replace your tires, choose one with a slightly lower revs-per-mile figure. A slower-turning tire will result in a lower speedo reading. Rev values are available in the data from www.tirerack.com and from individual manufacturers.

 

<div class='quotetop'>QUOTE(Rufaro @ Jun 17 2006, 09:07 AM) [snapback]272648[/snapback]</div>

Your gps receiver is much more accurate.

Some say it is within .5 mph or better
http://gpsinformation.net/main/gpsspeed.htm

Others .7kph or better
http://groups.google.com/group/sci.geo.sat...b61b2330?hl=en&

 

the prius speedo runs a few mph fast. no need to worry, they're all like that. most if not all toyotas overestimate speed, rather than underestimating.