Speedo Reading

[zorro_96]

When driving along at 70mph indicated by the speedo in the dashboard, my tomtom only shows my speed at 66mph, i assume the speed shown on the tomtom is correct as it is calculated using gps.

Can anyone shed some light on this.

[timson]

When driving along at 70mph indicated by the speedo in the dashboard, my tomtom only shows my speed at 66mph, i assume the speed shown on the tomtom is correct as it is calculated using gps.

Can anyone shed some light on this.

Just a wild guess........... your speedo is fast, like 99.9% of cars

[zorro_96]

Yeah i was finding it weird because on the stretches of motorway where the average speed cameras are, i would be doing 50mph indicated by my speedo and every one else is going way faster. I knew that speedos do have a little discrepancy but not by about 4-5 mph

[Q.S.]

Yeah i was finding it weird because on the stretches of motorway where the average speed cameras are, i would be doing 50mph indicated by my speedo and every one else is going way faster. I knew that speedos do have a little discrepancy but not by about 4-5 mph

They would be going faster than you, the speed limit on a motorway is 70mph. :)

I presume you were in a roadworks section with a 50mph limit posted?

As for speedo discrepency, speed cameras allow 10% discrepency for speedos, i.e. 5mph at 50 so in theory you can get away with 55, however, if you are unlucky enough to have a speedo reading slow, 55 by your speedo will trigger the camera so I don't advise trying it out.

Tyres also account for speedo error, new tyres are slightly bigger than worn tyres. 10mm on diameter of a tyre = 31mm bigger in circumference so every 1 turn of the wheel goes 31mm further. How many turns of the wheel for 1 mile? Speedo is set up for new tyres.

[zorro_96]

Yeah i meant through roadworks limited to 50mph :!Removed!: . Thank you for clearing this up.

[cabcurtains]

All cars as standard usually show a higher speed, if they didn't then the speeding issue would be worse than it is. Most Police Panda/Non Traffic cars have calibrated Speedos so when they follow you and your speeding they can match your speed and see if your speeding. Then you may get a tug and a word in the ear!

I have had many cars fitted different GPS systems all had a tolerance from memory, Honda Accords had 2MPH at all legal speeds (careful near cameras) and some Ford Focus's had more than 10% .

[caveman_si]

As I understand it the speedo on cars are not allowed (by law) to under read the speed at any point. Ie actual speed = 50mph then the speed has to read 50mph as the minimum. Due to techinical reasons that may result in under reading eg tolerance in the mechanical and electrical systems that make up speedos, manufactures build in a buffer to ensure that minimum is always met.

Also GPS isnt as accurate as people think. The only GPS you can use to measure speed accuratly is a differental GPS. Regular GPS has too many innaccurracies.

[zorro_96]

So can any one else confirm what kind of discrepancy their avensis speedo shows, i just wanted to know if it was the same for every single car from the same model or differs individually by a bit.

[irishjay]

So can any one else confirm what kind of discrepancy their avensis speedo shows, i just wanted to know if it was the same for every single car from the same model or differs individually by a bit.

when im doing 60mph on my speedo my gps reading is around 56mph

edit: i have a tom tom not an toyota spec sat nav

[ptri]

At an indicated 70mph (under cruise control and over several miles), the average speed readout (after being reset) gives 65mph. At an indicated 60mph it's hardly going faster than lorries. So it appears to be under-reading by ~7%. Mr T seemed to think this was OK. It's just one of the many things that don't work particularly well on an Avensis and not good in these day when you need to trust your speedo.

[Dunkelmann]

My speedo reads 10% over the speed indicated by my TomTom GPS and GPS Dash.

It is an interesting point about "domestic" GPS not being accurate. I can understand that statement in that it is de-tuned to be accurate to say a 30m radius of position. However when travelling, if the same positional inaccuracy is maintained consistently it wouldn't matter to the speed calculation. Even if it wasn't consistently maintained but averaged out the speed accuracy would still be high.

Worst case is 60m/S seconds - i.e. GPS reads 30m ahead of where you really are at the start of measurement and 30m behind after S seconds, (or behind then ahead). Given S is probably no greater than 1 sec, i.e. readings of position are taken at 1sec intervals, the error would be 60m/sec or 216000m/hour!

I suspect the accuracy of position doesn't change very much in S seconds because S is very short. Hence as far as taking the difference between two positions P1+e1 and P2+e2 in S seconds, the true speed (P1-P2)/S and the measured speed (P1-P2)/S + (e1-e2)/S is negligible since position error e1 approximately equals e2.

So if you didn't get all that, I'm saying it is unlikely to be true that GPS can't measure speed accurately, even though they don't have high positional accuracy, since the short measuring interval means the postional accuracy doesn't change significantly in that interval.

[Beagh1958]

Just out of interest have you had new tyres fitted, might be an idea to go into settings on your Sat/Nav & select new tyres just to see if you get a different calculation for your speed????????????

[zorro_96]

Unfortunately, my tomtom doesnt have this option. However its good to hear that it is not just my car that has the discrepancy.Although the discrepancy might save me from getting nicked by speed cameras i think , toyota should make the reading to within 5% of the actual speed rather than 10% as this just causes too many problems especially around the average speed cameras when you think you are doing the indicated speed limit but are holding up traffic behind you.

[Beagh1958]

Sorry, my fault. I meant the Avensis in-car Sat/Nav. My vehicle has the TNS300ETA Sat/Nav & when you have renewed front tyres you should go into settings & select tyre change. I presume it re-calibrates system for renewal of front tyres.

[zorro_96]

I see, thank you for your suggestion anyway

[Jimmy_Lemon]

Have found my Avensis is about 8-10% out on GPS and those road signs that tell you your speed.

Have been tempted before to set the cruise control to 80 on the dial as that is only 73/74mph - still a little naughty but it is good to know you have some breathing space if you accidentally go through a camera at 32 (in a 30 zone obviously)

[Zwart_Verso]

Have found my Avensis is about 8-10% out on GPS and those road signs that tell you your speed.

I have a Verso which I think has the same engine, gearbox, differential and tyres as the Avensis, but not the same speedo. There was a similar post about this here > Verso Speedo Accuracy

As stated there, compared to my GPS (Garmin Quest) the speedo over reads by 7%!

However I think that the tyres have a lot to do with this. The Verso (and I think Avensis) is available with either 205/55 16 (which I have) or 215/50 17 tyres. But these do NOT have the same diameter!

The 215/550 17 is 2.3% larger than the 215/55 16. Which would mean that the speedo would only over read by 4.7% if I had the other wheels and tyres.

I haven’t looked into it yet, but when I need new tyres I am thinking about fitting 215/55 16 these are still 0.6% smaller than the 215/50 17, but would mean my speedo is more accurate, and I am hoping this would give all the advantages of the wider tyres without needing new wheels.

Has anyone here tried this?

Regards,

Andy

[BenN74]

My Avensis 53 T-Spirit D4D also over states speed by 5mph, I never noticed this when using my tomtom, but recently aquired a road angel 2 speed camera/laser detector, and that uses gps to calculate speed. I soon noticed it was always 5mph lower than my speedo read, I had to have my front tyres changed, and it now reads 4mph under the speedo. I now tend to use the Road Angel to tell my speed, as it reads the actual mph and not somewhee between 25-30 on a dial. I suppose this helps however when you may have cut it fine on a speed camera, cause you'll actually be 5mph slower than feared. I was told the police no longer give 10% leeway on cameras due to modern speedo's being so acurate! Obviously not the case!

Ben N

[Dunkelmann]

My Avensis 53 T-Spirit D4D also over states speed by 5mph

I think people need to be clear whether there is a constant difference measured at one speed only. I think it is much more likely that it will be a percentage difference and that people should measure over a wide range. (There's no guarantee it will be a constant percentage difference over the range but it is highly likely.)

It's easy to check - go down the motorway at a steady 80, note the difference then do the same at a steady 30.

My Avensis' speedometer overreads by about 10% i.e. 8mph at 80 and 3mph at 30.

If your speedo is not far out at 80, it's unlikely you'll see any difference at 30.

It's worth knowing because if you are 8mph out at 80 and you drive an indicated 8mph faster at 30 you run the risk of getting done in town.

[Aamir2892]

TomTom is just an estimate. The speedo is connected directly so it will have to give the most accurate reading.

[caveman_si]

TomTom is just an estimate. The speedo is connected directly so it will have to give the most accurate reading.

Sorry but wrong.

What exactly is the speedo directly connected too? Back in the day you are right the speedo cable used to be connected to the wheels in some manner. BUt now is it not as common to have an electically pick up ? So no physical connection?

Also with the speedo lots of things affect how it reads such as tyres, stick bigger/smaller ones have some tyre wear this all affects it.

Mind you the GPS aint always that great too. The accuracy on the tom tom is probably at best 10-30m and as you drive along and pass buildings/ trees/ hills etc you change satalites and that affects the acuracy especailly if you are moving quickly. And as you move the accuaracy changes it could improve or decrease depending on a whole host of variables. Using GPS for work 10-30m errors in accuracy would be unaceptable (probably get me sacked) more like 5-10mm as acceptable depending on the work.

The only truely accurate way to measure the speed of a vehicle is over a fixed know distance with a timing set up similar to F1 or drag racing etc.

BUt for driving along the speedo is fine. Just be aware that it can never under read (ie if you are going 30pm the lowest the speedo can read is 30) and drive for the conditions and everyone will live happily ever after. IF you are worried about 1 mph this way or that way for speed cameras then you are not paying attention to the rest of the road.