Speedo vs. GPS???

FWIW, speedo error isn't Prius specific. We had some discussion about this at http://priuschat.com/forums/gen-iii...ometer-error-about-1-mph-some-evidence-2.html, where I'd also linked to a great article.

 

With new tyres my 2010 speedo is 4kms out at any speed above 10kph.

 

I've noticed the same thing on mine.

 

yeah it is 1-2MPH off and as cwerdna pointed it is not car specific; with exception of 1 all cars we owned had the same issue.

I believe car mfg do this to account for different tire brands/tire wear; after all the difference could be a couple %.

 

My current 2010 Model III that I traded in on 2011 Model IV both were 1 mph over GARMIN GPS

 

And mine's 1mph under my Garmin figure.

 

I posted in another thread that with the gear ratios of the Gen III HSD, my 195/65R15 tires and tire revs/mile and the fact that my ScangaugeII shows about 7000 RPM for MG2 speed at the dash speed of 60 mph on level ground this calculates out to a true speed of 58.6 mph.

So, right between the 1-2 mph everyone else has posted.

http://priuschat.com/forums/gen-iii...-discussion/99923-gen-iii-mg2-rpm-vs-mph.html

 

Very common, and I've seen this on other vehicles.

 

Speedometers are designed to be a bit optimistic. By international law, speedometers cannot display a speed that is lower than the actual speed, so most manufacturers error on the high side.

Tom

 

Maybe I have a strange way of looking at things, but it's never bothered me that my speedometer is showing a bit faster than actual (this isn't my first car like that). What really bothered me on a different car was that the odometer was wrong - it showed more distance covered than actual by gps, which meant that calculated mpg was off and the car showed more miles on it than I actually drove (I think it was off something like 5-10%). I would have had to buy larger (and more expensive) tires and either had the potential for tire rub or had to put spacers and mud flaps as the larger tires would also have been wider, so I never bothered. I was delighted to check the odometer on my Prius and found it matched the Garmin gps. The speed being off just means I'm less likely to get a speeding ticket!

 

Keep in mind that your speedo error is dictated by speed. At anything over 40mph the error margin increases. So at 35mph your speedo may be correct but at 75mph it may be off. Part of this is due to tire expansion and partly GPS signal strength and updating ability.

It is very common to see 1-2mph too fast vs. GPS on the GenIII and 2mph too fast on the GenII with its shorter OEM tires.

 

Your GPS is measuring speed using the distance traveled in a straight line between two points over a measured time span. Your speedometer is measuring the distance your tires travel over a time interval. There are bound to be differences caused by the different algorithms.

 

Are there still GPS units on the market that calculate speed based on distance traveled? I thought they had all gone to doppler based calculations?

gpsspeed

 

There are a lot of older units still in use. Do we know which model the OP is using?

Tom

 

As Tom mentioned in post 10 it is deliberate. International law, for some strange reason, requires that car speedometers be either right on or fast, but not slow. So virtually all manufacturers err on the side of caution and design speedometers to read a little fast, usually 2% or 3% with OEM tire size.

The MPH on my Scan Gauge reads right on checked against two different Garmins while the speedometer reads to high by about 2 MPH at 60 or 70. My odometer reads a shade high but not to bad.

This version of the regulation is dated 2003 but the original came out in the late 1950s.

 

Note that the countries (footnotes, page 6) covered by this trade regulation are primarily EU block, with some Asian and a few others. But NONE of them are in the Americas, North or South.

The offset in the U.S. is driven by domestic forces, and is generally smaller than the offset in the above markets.