2 MPH Speedo error

<div class='quotetop'>QUOTE(wileec @ Aug 1 2007, 06:54 AM) [snapback]488585[/snapback]</div>

About what I'ver noticed also, 203 mph. I have 195/60 Good year assurance tires, so still pretty close to stock and or your size.

 

Me too!
Stock size tires.
Compared to hand held GPS, 2 MPH off.

 

This is typical for all cars, not just the Prius. Most speedometers read between 1 and 10% fast.

It is highly dependent on tires: size, age/wear, inflation, temperature.

 

Interesting, I presumed mine was off by 2 MPH (at 60 MPH since I can use a stopwatch and the mileage markers on the interstate) because my 190/60/15 TripleTreds have 863 revs per mile and the OEMs have 855. Can my (OK YOUR) trusty Toyota dealer can adjust the speedo?

 

<div class='quotetop'>QUOTE(wileec @ Aug 1 2007, 09:54 AM) [snapback]488585[/snapback]</div>

same noticed here.. I have the hydroedges as well..

Now I have 114K miles..
How many extra miles is that?

 

It seems all vehicles are set up that way, with speedometers reading 2 mph faster than actual.

It might have something to do with a law or regulation prohibiting speedometers from ever reading too low.

My Prius speedometer is also 2 mph too fast, based on those radar signs along the road.

Harry

PS: you can be 2 mph over the speed limit and not have to worry about a speeding ticket. This is not the kind of issue I am going to worry about. Subtract 2 mph from your speedometer for your actual speed, and adjust your behavior accordingly.

 

In my case, I have noticed my speedometer indicates 2 mph faster than the signs by the side of the road to give feedback on the speed.

The error, in terms of per cent, is greater at lower speeds. However, I don't think this is a significant problem. I have not noticed errors in odometer indication; the odometer agrees with highway signs, over long distances.

 

<div class='quotetop'>QUOTE(oxnardprof @ Aug 3 2007, 10:40 AM) [snapback]490018[/snapback]</div>

Hmm, my speedo always matches exactly the speed signboards on the side of the road. I haven't done a odo test though.

 

<div class='quotetop'>QUOTE(Earthling @ Aug 4 2007, 02:35 AM) [snapback]490016[/snapback]</div>

There is "UN-ECE Regulation No. 39".
http://www.unece.org/trans/main/wp29/wp29regs/39rv1e.pdf

Ken@Japan

 

My speedo is within 1mph of the speed reported by my Garmin GPS. Usually they match.

 

My 2005 matches my GPS within tenths of a MPH, but most countries mandate that speedometers shall not read slower than you are going ("The speed indicated shall not be less than the true speed of the vehicle"). See para 5.3 on Ken's attachment.

I've seen analog speedometers read 4 MPH higher, so it could definitely be worse!

 

<div class='quotetop'>QUOTE(JimN @ Aug 3 2007, 10:13 PM) [snapback]490290[/snapback]</div>

I also checked mine against my GPS, and they usually matched, or sometimes off by 1 mph (the GPS reads in tenths.) A GPS unit can lag in readings, and also can be inaccurate on curves due to how they measure speed, so I made sure I was on a long straightaway, and had been maintaining a constant speed before checking. I don't recall if the speedo ever read less than the GPS, though I'm going to try it again when I get a chance and this time make note of that.

 

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<div class='quotetop'>QUOTE(catgic @ Aug 4 2007, 05:57 AM) [snapback]490442[/snapback]</div>

GPS is quite capable of this accuracy. Not all receivers are created equal, but ever since Selective Availability was turned off, there is fractional MPH accuracy available from GPS. The following paragraph from the public GPS User equipment document should make this clear:

3.3 RECEIVER VELOCITY ACCURACY
GPS receivers typically calculate velocity by measuring the frequency shift (Doppler
shift) of the GPS D-band carrier(s). Velocity accuracy can be scenario dependent,
but 0.2 m/sec per axis (95%) is achievable for PPS receivers. SPS velocity
accuracy is the same as PPS when SA is off. When SA is on, SPS velocity
accuracy is degraded. The amount of degra dation of the velocity is classified.
However, although not guaranteed, SPS velocity accuracies around 0.4 m/sec 95%
have been observed by civilian users for the typical level of SA associated with
normal peacetime operations and 100 metres 95% horizontal position ing accuracy.
Velocity accuracy can be effectively tested in a laboratory environment, but field
testing can be difficult since a tracking system with 0.05 m/sec or better accuracy is
required. The reader is urged to carefully consider the methods of testing if
velocity accuracy is an important mission requirement.

Bottom Line. GPS can provide better than 0.45 mph accuracy.

 

<div class='quotetop'>QUOTE(oxnardprof @ Aug 4 2007, 02:40 AM) [snapback]490018[/snapback]</div>

I think your observation is very correct.
The speedometer dispaly is intentionally biased and the odometer display is another story.

Ken@Japan

 

Has anyone compared their speedometer reading to the readings of a ScanGaugeII?
In my current car (a 2005 Peugeot 2005, I'm still waiting for my Prius) I have noticed that my speedometer is about 4% off, while the ScanGaugeII seems to be within 1 KPH of my Garmin Nüvi 360.

So there may be that the speedometer is knowlingly "corrected" to show too high speeds, while the car itself, and possibly the odometer, knows quite accuretely how fast and far you are going.

Just my thoughts,
Stein

 

My speedometer is a full 3mph off the pace at 30mph, and 6mph at 60mph. Toyota say there is nothing that can be done to recalibrate it. It seems odd that there should be any variation between Prii given it is a digital system once you get past the circumference of the tyres. I have the standard Bridgestones at 40psi front and 38psi rear.

The odo is pretty much spot on, within 1% of the expected distance. Both my GPS units, Garmin Geko 201 and a TomTom satnav, give the same speed reading as my Scangauge. The only problem with the GPS is the lag between speed and readout.

Those speed indicator signs are certainly not accurate, at least over here in the UK. Like the speedo they too are also set to read high, although I have found the ones in Oxfordshire, operated by Speedwatch to be spot on, ie matching my GPS speed and not my Toyota speed. If you want some fun phone the highways department and ask them about how they calibrate the signs.

I am sure that having a fast speedo just adds to the number of people tailgating me.

 

With OEM tires, my speedo matches the roadside signs at two different 5 mile test locations, and the odo looks like it is close also. I expected them to indicate higher than the actual speed/distance. (Tests done at 60mph with a stopwatch.)