Speedometer calibration ?

Consumer GPS systems are not noted for accuracy, and are not to be used to measure speed. There are some specialty GPS tracking devices that are more accurate, and may be used.

There was an article somewhere recently about some kid who was cited for speeding, parents took it to court to fight it (they had installed one of those "minder" GPS systems in the car). Court allowed that it was accurate (as it's one of the few special ones), but the article emphasized that the consumer grade GPS systems are not accurate. At least, not accurate enough.

 

Many European cars are required by law to fudge the displayed speed. I recall that the number is some place around 10%. I'm not sure if this is the case with the UK model, but it wouldn't surprise me.

Consumer GPS units do okay with speed as an average, but the instantaneous speed jumps all over. It will be fast for one sample period, slow for the next. There is a lot of jitter. Twisting roads and hills will also upset the GPS speed. The units measure point to point straight line distance, so a curving path will be ignored between samples.

Tom

 

Well in my case (not sure if this is Normal) Often when I slow the car to stop. It will show 6..5..4..3..(my car is COMPLETELY STOPPED at this point) then 2..1..0. Looks like the display is about 1-2 second behind.

Added: note, that's in KM/H

 

You must be driving so fast that your speed can't catch up with you.

Seriously, that's pretty weird. There is a bit of a delay in the speed calculation, but it shouldn't be that long. Sounds like something is bad in that department.

Tom

 

Using two possibly inaccurate GPS units is not the answer to proving a speedometer is wrong. Try finding (if they still exist) a section on Interstates that have a 5- or 10-mile speedometer test. Start at marker 1, try to drive at a consistent speed through the section, time your run and figure the average speed. Your state's transportation department may be able to locate one for you.

Alternatively, if your community's police sets up speed radar units that flash your speed as you pass by, run through them a few times at different speeds. There's no guarantee the police units are dead-on accurate, but it's a good test source.

I've found our speedometer is within a mile or so of the police units where we live, and the variation could be due to the difference between a whole number and fractions on either side.

 

You don't need a reference to tell that the speed is off at zero. All he has to do is look out of the window and see that the car is not moving, then look at the speedo display and see that it shows 6. This is a pretty good indication that something is fishy.

Tom

 

No way.
The speedometer is intentionally biased to meet the UN ECE Regulation #39.
http://www.unece.org/trans/main/wp29/wp29regs/39rv1e.pdf

Ken@Japan

 

Specifically, that regulation says the displayed speed has to be between the actual speed and 10% of the actual speed plus 4 km/h.

So at an actual speed of 60 km/h, the displayed speed has to be 60 to 70 km/h. Although it's allowed to exactly display the true speed, because of manufacturing tolerances the car maker can't guarantee that it won't go below the actual speed, and that's not allowed. So they add a fudge factor.

 

My speedometer is actually 2 mph faster than my speed. I've checked it against every police speed radar (the ones that show you your speed), and I've been 2-3 mph over their speed. And that's in different towns/states, same results.

 

Mine reads one mph faster than the speed on the radar signs. That's with my Micheline MXV4+ Energy tires, which have slightly larger circumference than the OEM tires.

Tom

 

So none of you have that problem? I thought it is kinda normal... Humm. maybe I should go back to dealership have it check.

BTW I am not driving fast.... often happens when slowing down at stop signs/red lights/traffic.

 

Yep,that happens to me also but i use my Prius as a taxi and when you drive 350km-400km a night a little glitch like that doesn't bother me one bit,it's big bangs and rattles that i'd worry about.
In relation to Dozzer's original question,i've noticed that [using the signs in different areas that flash your speed]if the sign flashes 60kph my speedo shows 56kph,i just hope the hand held speed guns the police[Garda]here in Ireland use are calibrated in the same manner....or!!

 

Do any questions ever get answered on this PriusChat Forum? The original question was, "Is it possible to adjust the spedometer?" All I see are others complaining about their speedometers also not being correct, others saying that the speedometer is required to be inaccurate, others claiming that they can't be inaccurate by law, others claiming the Earth will be destroyed if you adjust your speedometer (my exaggeration). Can anyone just answer the question, "Can the speedometer be adjusted by some simple means, like press this button 8 times, while standing on your head in the rain, and it will put you into a super secret calibration mode, where you can change the calibration factor number, so that your speedometer reads whatever the hell you want it to read, so that you, personally are happy with it, regardless of what anyone else THINKS your speedometer should read?"

 

Please have patience, it has only been 5 years. Someone is working on an answer.

 

ANSWER: No adjustment, factory set. And the question was answered in post #11 five (5) years ago. Did you not read the entire string? If not, look at post #11 above.

 

The speedo cannot be reset simply, but changing the tyre size to 205/55/16 from the OEM size of 195/55/16 reduces the error by about 50%.
This has other advantages as there is a larger choice of tyres at this size, the RR is lower, and they are at leased 25% cheaper.
I have the UK Prius Tspirit and that drops out of EV at an indicated 31 mph. My speedo indicates 1 mph faster than roadside radar speed indicators at 30 mph with the 205/55/16's.

John (Britprius)