Prius Speedo Error

[ticktockmac]

HELP! I bought a one year old Prius Spirit about a month ago and I'm very pleased with it, except that I'm convinced the speedo is inaccurate. Whenever I check the indicated speed against my Garmin Satnav, there is a difference of between 4 and 10%, depending on the speed I'm travelling at. I imagine the gps must be showing the correct speed - am I right? I see in the manual references to recalibrating after a tyre change, but I haven't changed them. Does this mean the speedo becomes progressively inaccurate as the tyres wear? That would be scary. Should I ask the dealer to check it out? If the speedo is wrong, then what about the indicated mileage - and of course the mpg? That could have implications in relation to its value and perhaps even the cost of insurance If it's up to 10% fast, it makes it difficult to hit the right speed when passing speed cameras.

[lininjim]

By law, in this country, the speedo has to read 10% higher than actual to allow for errors that might result in prosecutions for speeding.

If you stick to your indicated speed in your car then not only are you legal but you are driving at a sensible speed. Remember that the speed limit is the MAXIMUM speed, not the minimum (unless indicated otherwise)

HELP! I bought a one year old Prius Spirit about a month ago and I'm very pleased with it, except that I'm convinced the speedo is inaccurate. Whenever I check the indicated speed against my Garmin Satnav, there is a difference of between 4 and 10%, depending on the speed I'm travelling at. I imagine the gps must be showing the correct speed - am I right? I see in the manual references to recalibrating after a tyre change, but I haven't changed them. Does this mean the speedo becomes progressively inaccurate as the tyres wear? That would be scary. Should I ask the dealer to check it out? If the speedo is wrong, then what about the indicated mileage - and of course the mpg? That could have implications in relation to its value and perhaps even the cost of insurance If it's up to 10% fast, it makes it difficult to hit the right speed when passing speed cameras.

[stompe]

Hi ticktockmac,

I also have a 06 T-Spirit with a roadangel camera detector. The speed on the detector is 60mph but on the actual speedo it reads 65mph. I have often wondered about the discrepancy and why they could not have the accuracy of the roadangel bearing in mind the T-spirit uses the GPRS the same as the roadangel to determine the speed and estimated time of arrival on the sat nav. I have noticed on the American Prius forum that they have 15" wheels instead of our 16" wheels and whether that is why ours over reads because of the bigger size wheels. I no doubt someone on our forum will explain why the speedo is not that accurate.

Stompe

[lininjim]

Hi ticktockmac,

... I no doubt someone on our forum will explain why the speedo is not that accurate.

Stompe

Erm...hello!

I think you'll find I did.

[stompe]

A forum is a forum which is open to discussions and to help people answer questions, etc. Obviously you felt your response was sufficient and did not need any one else to add their comments!!

[lininjim]

I don't understand what you are saying, I had already explained why the speedo did not read the same as the GPS, why was it necessary to say "I no doubt someone on our forum will explain why the speedo is not that accurate."

when it had already been answered. Or did you think another answer would make a difference to the fact?

Oh, some people, just drive to the gps speed then and get a ticket, I don't give a damn anymore, any responses aimed at my post will go unnoticed by me.

[japnut]

GROW UP PLANK

[Boro Sera]

GROW UP PLANK

Please refrain from personal insults on this site.

This Speedo issue isn't a Prius problem - as stated above, this applies to just about all cars on the road. I don't know if it's actually a legal requirement by the manufacturers or just a 'gentlemans agreement', but the Speedo indicated speed is ALWAYS higher than it should be. This isn't really a problem - the real problems would come up if the speedo read LESS than the actual speed.

[ticktockmac]

Thanks for your replies. I've known for a long time that speedos mostly read on the fast side, but I didn't expect it to be as much as 10%. I don't understand why the percentage accuracy of the indicated speed varies with the speed. It is out by around 10% at higher speeds, but only 3 or 4% at lower speeds. Surely they don't design that into the car?

If the accuracy of the speedo is directly related to the circumference of the tyre, and needs to be recailbrated when new tyres are fitted, presumably the recorded mileage is affected in the same way. If our recorded mileages are up to 10% high, then perhaps we're all being mislead about mpg, even when we calculate it ourselves from fuel used and indicated mileage. This would mean that we are doing slightly lower mpg than we believe, if I'm not mistaken.

If the recorded mileage is that much higher than the true figure, it could even have a small effect on the price a buyer is prepared to pay when we sell, if he or she thinks the car has done a higher mileage up to 10% higher than the true one. My insurance company asks what mileage I do in a year each time I renew. If I give them a higher reading than the true figure, it could even affect insurance costs.

Having said all that, I'm still a huge fan of the Prius after just four weeks. I do find it amusing, though, that the DVD supplied with the car to show how to use the Intelligent Park Assist had no such contents. Instead, somebody had recorded a couple of episodes of Top Gear on it!

[Habu]

Having said all that, I'm still a huge fan of the Prius after just four weeks. I do find it amusing, though, that the DVD supplied with the car to show how to use the Intelligent Park Assist had no such contents. Instead, somebody had recorded a couple of episodes of Top Gear on it!

Speedos are built to a minimum cost and are designed in a way that any Prius speedo from the big box of Prius speedos in the factory will slot straight into any Prius and not read under the legal limit, hence the usual c.10% difference. The manufacturer has to allow for people running their cars with over (and under) inflated tyre pressure by accident or otherwise and a host of other factors which can affect the accuracy of the speedo. In any given normal situation the speedo will, at best, be accurate and worst case read over your true speed for safety reasons. As your speed increases so the difference between your GPS speed and Prius speed expands. For example, GPS/Prius 30/33, 50/55, 70/77, 80/88. It's crazy when you see people slamming on their brakes at 75mph on the motorway 'cos there's a copper ahead. The copper will be getting your true speed which at 75mph is probably only in the high 60's.

I remember having the above discussion with an idiot who said the above wouldn't apply to his car as he has a Beemer X5. I explained that his speedo will be innaccurate and is probably made by the same company that makes speedos for cheap french cars.

This'll help you with the nav as it's quite a good video to explain all the weird and wonderful features.

http://www.toyota-eshowroom.com/partner/To....html?ref=t.com

[hertsnminds]

The most accurate speedo I have had on a car was the digital display speedo on my Citroen C4 always within 1 to 2 MPH of my Garmin GPS reading. All the cars I have owned with needle and dial speedos have never been spot on, usually read over at low speed and under at higher speeds most accurate around 40MPH.

I used to put the inaccuracy down to the way speedos used to be driven by a spinning cable in running into the back of a wound electromagnet, but now they are driven by readings taken from sensors passed through the ECU it must be due to the reasons explained earlier in the thread.

[timberwolf]

What is the accuracy of your Garmin GPS? :o

The Garmin Nuvi GPS car sat-nav specifications are tucked away on the web-site, and I would guess that those figures are for optimal reception conditions.

Positional accuracy is often quoted as <5 Meters typical. (Comparing with my Garmin hiking GPS of <15 Meters 95%, I've assumed that "typical" means for 95% of the readings, the other 5% of the readings can be outside that range).

The speed has a quoted accuracy of 0.5 Meters per second RMS (why Root Mean Square?).

So if your Garmin GPS has a good view of the sky and the satellites are nicely arranged in the sky at that time, I think your speed accuracy at best is going to be something like +/- 1.11 mph on average.

The specifications doesn't say what the accuracy is going to be like when there is a less than the optimal reception of the satellites.

[hertsnminds]

So if your Garmin GPS has a good view of the sky and the satellites are nicely arranged in the sky at that time, I think your speed accuracy at best is going to be something like +/- 1.11 mph on average.

Does that mean the speedo on the C4 was correct and the difference was down to satellite reception on the GPS?

Before I had the Nuvi I had the eMap for about 4 years (much less accurate) but of the 4 cars I have owned since I have had a GPS to check against all of the dial speedos where indicating the 5% - 10% difference described above.

Another (and more exoensive way) I can think to check is to run a GATSO in a 40mph zone with your speedo reading 50mph and see what the indicated speed on the photo is Or go to a track day where they can perform some accurate timings against known distances.

[Jonazap]

I had same problem with a 1 year old Prius T3. Speedo is reading about 9 to 10% higher at most speeds compared to Garmin Satnav Nuvi 310.

I contacted local Toyota garage and they said that the Prius was fine and Satnav faulty. Then said I could go in and get it checked but they would charge me for it if no fault found - even though still under warranty.

They later rang back apologising that they were wrong and that an engineering note had been issued by Toyota saying that Satnavs were accurate and their speedos could be up to 10% out (reading high) and since it was within government guidelines that was OK. I asked if it could be adjusted and they said not.

OK so at least they admitted they were wrong and there was a problem and I can use my Satnav for accurate speed measurement (providing not using it on a specific journey as speed doesn't then show on front screen on the Garmin), but what annoys me is that the mpg I thought I have been getting (52mpg) is now really less by about 10% (47mpg) ! The toyota chap confirmed that too! I can't even take a manual reading using the mileage and how much fuel I put in as the mileage isn't accurate. So I'll just have to try and map the average percentage difference at different speeds with the satnav and use that as a multiplying factor.

This makes me suspicious that this could have been deliberately set up like that so people will think the car is performing better than it actually is! So how many times have we seen people quoting high mpgs when really they may be up to 10% lower!

Still think it is probably the most economic automatic to drive though, which is what I need to avoid getting sciatica in traffic jams.

[John in Hay]

10% seems a lot - and more than my Prius' error (about 4-5% I think). Satnavs BTW will usually be spot on.

Nobody has mentioned I think that this can be a tyre issue - have Toyota changed tyre spec and not made the adjustment possibly ? The effective circumference of tyres does vary between designs... (And the inaccuracy even goes up with tyre wear - think about it...)

I think the UK law on speedos is / was that they are allowed to exaggerate but mustn't underestimate - resulting in manufacturers playing safe as discussed above.

[timberwolf]

OK so at least they admitted they were wrong and there was a problem and I can use my Satnav for accurate speed measurement (providing not using it on a specific journey as speed doesn't then show on front screen on the Garmin), but what annoys me is that the mpg I thought I have been getting (52mpg) is now really less by about 10% (47mpg) ! The toyota chap confirmed that too! I can't even take a manual reading using the mileage and how much fuel I put in as the mileage isn't accurate. So I'll just have to try and map the average percentage difference at different speeds with the satnav and use that as a multiplying factor.

Although the speedometer and odometer presumeably take the same sensor feed, that does not mean that they are designed and built to the same accuracy specifications. There are UK/EU rules governing the indicated speed and that includes that it must never under read and can over read by upto 10% plus some constant that I've forgotten. I'm not aware of if or what the rules are for odometers in cars.

I think you would need a devise a test, such as travelling a known distance and comparing the cars odometer reading difference between the start and finish points, to prove or disprove you odometer assumption.

This makes me suspicious that this could have been deliberately set up like that so people will think the car is performing better than it actually is! So how many times have we seen people quoting high mpgs when really they may be up to 10% lower!

Still think it is probably the most economic automatic to drive though, which is what I need to avoid getting sciatica in traffic jams.

Could be, although may be the tolerance of the odometer is better than the speedometer.

[Jonazap]

OK so at least they admitted they were wrong and there was a problem and I can use my Satnav for accurate speed measurement (providing not using it on a specific journey as speed doesn't then show on front screen on the Garmin), but what annoys me is that the mpg I thought I have been getting (52mpg) is now really less by about 10% (47mpg) ! The toyota chap confirmed that too! I can't even take a manual reading using the mileage and how much fuel I put in as the mileage isn't accurate. So I'll just have to try and map the average percentage difference at different speeds with the satnav and use that as a multiplying factor.

Although the speedometer and odometer presumeably take the same sensor feed, that does not mean that they are designed and built to the same accuracy specifications. There are UK/EU rules governing the indicated speed and that includes that it must never under read and can over read by upto 10% plus some constant that I've forgotten. I'm not aware of if or what the rules are for odometers in cars.

I think you would need a devise a test, such as travelling a known distance and comparing the cars odometer reading difference between the start and finish points, to prove or disprove you odometer assumption.

This makes me suspicious that this could have been deliberately set up like that so people will think the car is performing better than it actually is! So how many times have we seen people quoting high mpgs when really they may be up to 10% lower!

Still think it is probably the most economic automatic to drive though, which is what I need to avoid getting sciatica in traffic jams.

Could be, although may be the tolerance of the odometer is better than the speedometer.

Hi - I actually got thinking to myself about this afterwards and remembered that there was some odometer reading hidden on one of the screens on the satnav so I reset the Prius trip meter and the Satnav one and watched as I drove comparing the 2. Making sure that the satnav had synced up first I drove off and was amazed at how the 2 matched exactly! The satnav reads to 2 decimal places and the car to only one but it changed at exactly the same time as the each of the .05 increments went round (it obviosuly rounds up). This continued for 10 miles with no difference that I could detect - good enough for me! The car has only done 3000 miles so negligable tyre wear and pressure set to spec.

I can only then assume that either the odometer is working off a different sensor than the speedo or that if they are using the same sensor then there is a fudge factor being added on! If it is the second case then just need to find out from Toyota if they can remove this please!

Now I know the distance measurement is accurate (and therefore the mpg may not be wrong after all - phew!) I'll check on my next fill up to see how accurate the fuel measurement for the mpg calculation is and then can confirm if that is OK finally.

I'll update this when progressed.

[timberwolf]

Great minds think alike, or fools seldom differ

Thanks for posting back with your results of the odometer. I am relieved to hear that the mileage and hence my manual mpg calculations should be okay.

[Jonazap]

Great minds think alike, or fools seldom differ

Thanks for posting back with your results of the odometer. I am relieved to hear that the mileage and hence my manual mpg calculations should be okay.

Your welcome.

Well I've taken 1 reading now of the MPG and done the manual calculations (assuming the garage are measuring their petrol pumps accurately) and the resiltis :-

399 miles 37.79 litres = 8.31 gallons

mpg on prius display - 50.4

Calculated 48.0

Therefore reading high by 5 %

Drove this one quite hard one journey as I was in a hurry but normally getting about 52 mpg on lots of short trips and longer ones at higher speed.

Disappointing. I did take another reading but lost it - it came out about the same. I wonder what they are quoting for the accuracy of that measurement?

Anyway I guess things aren't as rosy as they seem. As mine has only done about 4000 miles hoping to get some improvement in the near future.

Anyone else checked their MPG calcualations?

[Sagitar]

There is a good section in Wikipedia that gives a lot of hard information on speedo standards - look here http://en.wikipedia.org/wiki/Speedometer

[SilverT4]

In the good old days before SatNav and speed cameras, speedos were assumed to be inaccurate to 10% either up or down. The only accurate speedos were on police cars which were specially calibrated. Therefore if you were 'caught' doing 33mph in a 30mph zone, you were unlikely to be prosecuted as you could just claim your speedo was inaccurate.

But of course things have now changed :(

[Mooly]

Around here the police sometimes put up portable displays on a trailer that show your speed in huge digits. My Corolla TS shows 33mph for a "true" 30. The trip computer seems to show the correct speed, drive at an indicated 33 on the speedo and the computer shows 30. 60 is nearer 55/56mph. Tried it many times on these road display things and it seems consistent.

[Sagitar]

In the good old days before SatNav and speed cameras, speedos were assumed to be inaccurate to 10% either up or down. The only accurate speedos were on police cars which were specially calibrated. Therefore if you were 'caught' doing 33mph in a 30mph zone, you were unlikely to be prosecuted as you could just claim your speedo was inaccurate.

But of course things have now changed :(

I have been driving for a long time and I cannot remember a time when the law in the UK allowed speedometers to read a speed lower than the vehicle is actually doing. As far as I am aware, in the time that I have been driving, the tolerance has always been expressed in the form minus 0 - plus a fixed percentage.

If the legislation did actually say plus or minus a fixed percentage at some time, I would be interested to know when it changed.

[SilverT4]

I think the current legislation was introduced in the Road Vehicles Construction and use Act 1986 (or one of its later amendments). The Act specifies that speedos can over read by 10% +4kmph, but there is no allowance for them to underead.

I don't know what the legislation was prior to this, but bear in mind that speed cameras were only introduced in 1992, and therefore a police officer could only rely on his own speedo to determine another vehicles speed.

[Sagitar]

I think the current legislation was introduced in the Road Vehicles Construction and use Act 1986 (or one of its later amendments). The Act specifies that speedos can over read by 10% +4kmph, but there is no allowance for them to underead.

I don't know what the legislation was prior to this, but bear in mind that speed cameras were only introduced in 1992, and therefore a police officer could only rely on his own speedo to determine another vehicles speed.

I agree about the 1986 Act and my recollection is that it connects also to European legislation, but Vascar has been around since the early 1970s - all it needs is a measured distance and a stop watch, though it has been "poshed-up" a lot recently with modern technology. So I suspect that the minus 0% plus 10% bit has been with us for at least 30 years.