Speedometer calibration ?

Yes, I read the entire string, which is why I wrote my response. It doesn't matter how old the post is...people search for answers to their questions, and search results find it in old posts, like I did, and hope it is info that they can use, regardless of how old it is, or if it is thought to be relatively common knowledge. The OP question was never answered. Wrongly citing some regulation doesn't mean that the question was answered.

The UNECE 39 regulation that is mentioned in post #11 isn't a requirement to put an offset bias in the displayed speed of a vehicle as some posts have written. It also doesn't require manufacturers to make a car where it is impossible to modify the speedometer (the OP question), it just sets minimum accuracy requirements for speedometers in cars, for the countries that ratified the code.

Here is the exact text from the regulation:

Requirements
The production shall be deemed to conform to this Regulation if the following
relationship between the speed indicated on the display of the speedometer (V1) and the
actual speed (V2) is observed:
In the case of vehicles of categories M and N: 0 <= (V1 – V2) <= 0.1 V2 + 6 km/h;

(read this as V1 minus V2 is greater than or equal to zero (meaning that the speedometer can't ever read less than your actual speed) AND V1 minus V2 is less than or equal to 0.1 times V2 plus 6 km/h)

For an example, if you are driving at an actual speed of 50 km/h, then your speedometer must read somewhere in the range of 50km/h to 61km/h (50*0.1=5, 5 +6=11, 11+50=61, for the upper limit)

This simply states that the manufacturer must make the speedometer display a speed that is always more than the actual speed, and the speed displayed must not be more than 0.1 times the true speed + 6 km/h. The displayed speed must be within this allowed range. It does not mean that the manufacturer must make the speedometer read 6km/h faster than the actual speed or have any bias built in.

Just a side note, I can't find any indication that the United States has ratified this UNECE 39 regulation, which would mean that cars manufactured for use in the United States don't need to meet this minimum accuracy requirement. They could read anything and still be legal. All of the countries that have ratified the regulations have a number assigned to them, and the label is applied to the car in that country by the manufacturer to tell the consumer that the speedometer meets this accuracy requirement, and there is no number assigned to the United States, and this label is not on any US delivered cars. The regulations have examples of what this label would look like.

All of that being said, no one answered the question of HOW to change the displayed speed, as I wrote in my original post. The regulations are just an agreement that the car manufacturer will install an accurate display of your speed, that does not ever display a value that is less than your actual speed. It even mentions the following:

2.5.2. "Technical constant of the speedometer" shall mean the relationship between the
input revolutions or pulses per minute and a specified displayed speed; (this is the calibration factor, which could be modified if anyone knows how, but no one answered the question). Some people mention ways to change it, for instance changing the diameter of the tires installed, but this isn't actually changing the speedometer display versus your actual speed, but may be adequate for what most people are looking for.

 

I forgot to mention that the category "M" mentioned above means the vehicle is a passenger vehicle, in this case where "M" is: Power-driven vehicles having at least four wheels and used for the carriage of passengers. (passenger cars)

 

For some reason you seem to be ignoring Ken's post which plainly says it cannot be adjusted. It is set at the factory and there is no adjustment on the unit that the manufacturer might have applied to the unit. Maybe you are just overlooking what he said before he cited the regulation. Either way, there is NO adjustment or calibration. That makes the third time your question has been answered. Once by Ken, and twice by myself.

Please refer to post #11 in this string.

Also, I am well aware of how the regulation is worded.

Yes, changing tires etc. is not the answer you are looking for. However the fact remains that there is NO adjustment or calibration that can be done on the Prius speedometer.



Best of luck to you.

 

So, you, or Ken, have checked the programming in the ECU to confirm that there is no way to hack it to change the programming and edit any constants or variables to change the calibration?

Please let me know what you've done, or Ken has done, to try to alter the calibration, that led to the "no way" conclusion, so that I don't waste my time going down the same road if you have already attempted some methods, please let me know so I can go another route. Simply writing "no way" doesn't tell anyone what was investigated to come to that conclusion.

 

I can't speak for Ken. What I am going to say is this. It is my understanding that the speedometer is set by the factory and that there is no end user adjustment on it.

That does not mean that someone like yourself might find a way to hack the system if they spent time, effort and money to do it. If it means that much to you, I suggest you find the means of doing it which will involve research into the product and trying to figure out how to hack it, alter it or whatever else without causing problems elsewhere.

The question though really is this. Is a one or two MPH discrepancy really worth it?

Best of luck to you and I hope you find your answer you are looking for.

 

This speedometer issue isn't critical, it just opens the door to manipulate everything in the Prius, such as the discrepancy between the calculated fuel economy and the measured, seeing the process variables of the engine management, manipulation of the speed at which the gas engine comes on versus just the electric motor, etc. I'm interested in far more than just the speedometer calibration. I would like to build some screens on the graphic display to show more than just the cartoonish information provided by the standard Toyota graphics. It's a long term hobby.

 

What you are saying has a interesting concept. A long term hobby might be cost prohibitive unless you have a lot of funding to back you. If you are interested in profiting from your endeavor, perhaps you should present your ideas to the auto industry as a whole. Engineers are a special breed all their own.

Best of luck to you.

 

I wish only people with an answer to this would post. There is little value in speculation and opinion. Either say how to calibrate the speedometer or don't post. I too would like my speedometer to be accurate and not read 1 to 2 mph higher. I'm tired of going 65 and having pickup trucks three inches from my bumper because I'm actually only going 63 mph. I drive several cars, so I never get used to calibrating this in my head, I just assume the speedometers are accurate. I too will continue to look for a hack and will post once I figure it out.

 

2 MPH is not going to make a lot of difference in the real world. The pickups on your tail are probably not too worried about your speed, more likely one of those sick-in-the-head bully-boy types that want to hassle you because you drive a Prius. Just pull over and let them pass. Or just continue to slow down until it is no longer fun for them.

 

Simple, simple solution: go 67 (as displayed). And do you think the tailgating truck driver will then back off, lol?

 

It is unreasonable people like you who make it so pleasant to work in the service industry.
NOT.

Given that tires with a slightly different diameter can affect the speedo calibration........would you like to rebuild the speedo every time you change tires ???

If this concerns you SO MUCH and your memory is SO BAD, I suggest that you stick a little piece of masking tape in the center of your speedo display with the correction factor written on it.

Or maybe you should concern yourself more with driving safely than with complying with the letter of the law.
IF the flow if traffic is +5, then stubbornly sticking to the posted limit is not the safe thing to do.

 

Sam Spade- Doesn't it change even as the same tires you have on the car wear down? A person could go crazy constantly trying to keep the speedo calibrated perfectly.

The tip about going +5 helps most of the time; it doesn't help if you have "out-of-state" plates and you're traveling through a small town that depends on ticket revenue for city operations. I have seen copies of tickets for as little as 2 mph over.

Venix - Rather than constantly recalibrating the speedometer, we often use a Garmin GPS unit on our cross country travels because it's much better than the GPS on our Prius. Besides map, directions and traffic it displays current speed as well. We use it to verify the accuracy of the speedometer. The drawback to it, though, is that the GPS unit is only giving a flat "point-to-point" speed and doesn't account for hilly terrain.

 

I was in the car with our daughter a few weeks back, going through the residential street beyond our place. I usually don't go this route, prefer to go the other way, up to the main arterial route, stay out of the neighbourhood's "hair".

But anyway, short notice route change, we end up going down this little secondary street. It's got 90 degree turns every 100 yards or so, properties with hedges, lots of driveways, a daycare, occasional kids/dogs. In short: a place where you want to take it easy, so I'm averaging maybe 30 kmh (20 mph), slowing on the corners.

My daughter out-of-the-blue asks something, can't remember the exact phrasing, but something like: "Aren't you concerned you're going to get hassled for going under the speed limit?". I pointed out some of the points of the second paragraph, and that the speed "limit" is the upper limit, but how you should never be driving faster than your visibility, ability to stop safely. I'm not sure if any of it filtered through.

 

Love the majority of your answer, but must object to your last addition. Speeding is bad. Being a speeding lemming is stupid.

 

No it isn't.
There are a LOT of things more important than being sure you don't go 1 MPH over the posted limit.
Like keeping safe distances, both ahead and behind.......and when possible on the sides too.
Turning yourself into a traffic hazzard is more dangerous than keeping up with the flow that is a few MPH over the limit.

But you can believe whatever you choose.

EXCESSIVE speed is bad; coupled with lane surfing and no signals is worse.

The important point is: You should not be passing a lot of cars on the freeway. You also should not be BEING passed by a lot either.
Both situations are dangerous.
It is mostly pointless to debate which is worst........because it depends on specifics.

P.S. Since you live in or near Chicago.........
What speed do you drive when on I-295 (Tri-State) ??
The posted limit on most of it is 55.
If you actually drive 55, ALL of the other traffic will be passing you ......going about 20 MPH faster.
Do you really think doing that somehow makes you safer ???
Hint: It doesn't.

 

Just curious Sam, what speed do you do when there's little or no other traffic. Do you then abide by the posted limits? Or?

 

Most of the time, yes.
But I still pay attention to whom is overtaking who and adjust accordingly.

If I'm in my motorhome, it usually is -5 to-10 in the right hand lane.......and most everybody seems to understand and respect that.

 

Excessive speed is defined by the signs that read, "SPEED LIMIT". It is not your special, god given, civil right to speed and endanger others. The point of speed limits is to reduce accidents!

No one is arguing about "1 mph" or "a few" (or 5 or 7). Remember this: last year's newspaper report found that Police set their radar at 8mph over the limit, so that should teach you something very important. But 20 over the SPEED LIMIT is an extra expensive ticket, AND a genuinely stupid danger. We should agree that people who choose to drive over (or under) the posted LIMITS are "turning themselves into traffic hazards." But, your personal twist on a good rule, "You also should not be BEING passed by a lot either" is a crock of poop--I bet you don't say that when you get stopped for speeding!

FYI: most of the highways around Chicago (including I-295) have 60-65mph limits that change appropriately with road conditions. Since Priuschat has documented that 61mpg is the Prius' golden mpg number, I often set the cruise there. But, because Chicago is a BIG nice person city, with lot's of traffic, regardless of time of day, I rarely use the cruise until I get into the far outer suburbs.

 

And then there's those that use their cars to "demonstrate their opinion", doing sudden deceleration, acceleration, various stunts. Everything BUT driving responsibly. That feller from Norway for example, can't recall his name now he's on my ignore list. Seriously: 17 years old and already such a chip on his shoulder. He'll be lucking to make it to twenty without seriously injuring someone or ending up in jail. The latter might be for the best.

 

Folks need to remember that this is very strongly a function of local driving culture. I learned to drive in a time and place where a habitual 5 mph over the limit would typically lead to excessive points and loss of driver's license in about a year. Nearly a half century later, a relative is now a county judge there, and receives plenty of speeding ticket appeals from out-of-staters who didn't slow down when leaving the nicer/wider/straighter roads surrounding the region, didn't notice how many local plates they were passing, and felt they have a birthright to higher speeds. Sorry, appeals denied. That region suffers from limited resources for road maintenance, and the fatality rates show it.

In a different state, my home of the past three decades is near a similar zone in a neighboring town. Some call it a 'speed trap'. My travels often take me through other similar, off-interstate zones, usually semi-rural, and I do witness other drivers around me getting pulled over.

Road culture tolerance for vehicles obeying the posted speed limit also varies enormously. In my region, while the average speed is certainly above the limit, there are so many drivers obeying it that no driver can reasonably claim that the limit is outside of the normal speed distribution. But vehicles with out-of-state plates are sometimes slow to realize this.

With modern steel belted radial tires, it doesn't change nearly as much as you'd expect from applying the simple circle equations to the amount of your tread thinning. The sidewalls flex, so the patch on the ground is flat, not round. When inflated and loaded to the normal operating range, the rolling circumference is set mostly by the length of the steel belts beneath the tread, and these don't change noticeably with wear or inflation pressure. In fact, manufacturer tire specs generally call out an 'RPM' spec - revolutions per mile.

You can use a single calibration figure for the entire life of your tires.

This applies only to modern steel belted radials, not to the old style bias ply tries. Note that common bicycle tires are not steel belted radials.