Speedometer Accuracy

[JamesT5]

Hi

I'm interested to find out whether there is a standard set up for Speedometers as I've been doing some tests against my SatNav to see how accurate my speedometer is and the results are interesting.....

My SatNav is almost spot on, I know that much because those matrix signs that measure and display your speed matches my Garmin SatNav but neither match my speedometer.

By law there can be an overread on the speedometer of not more than 10% and my car is within that limit. However, I've noticed the following....

At 30mph, the Speedo is around 1 - 2 mph out i.e,. it says I'm doing 30 but I'm actualy doing 28 - 29mph.

At higher speeds, say 60mph it says I'm doing 55 - 56mph on my SatNav and at 70mph on the needle, it says 64 - 65 mph on the SatNav.

Conclusion, the speedometer over reads more at higher speeds. Has anyone else done this test and got similar results?

I'm also curious to see if any Ex-Plod owners have tested their 'calibrated' speedometers to see if they are set to be more accurate.

Anyway, this over reading may well have been the reason why a few weeks ago, I went through a mobile speed camera trap at 68mph in a 60 :sinner: (purely by accident as I hadn't long come off a dual carriageway), and didn't get a ticket, although the tolerance on the Welsh Safety Camera Partnerships is 10% + 2mph, inline with the ACPO guidlines.

At 68mph on the car speedometer, I was probably doing about 64mph, comfortably inside the tolerance threshold. :beer:

[T5frankie]

they all under read at 70 by 4-6 mph fit bigger wheels to get it more accurate, with my 19's on and 35 profile tyres mine now over reads by 1 mph

[M-R-P]

most speedos overread by 4-5% as car manufacturers don't wanna get sued for loads of people getting speeding tickets. from what ive seen, auzzie cars are closer to bang-on than any others in the world but I don't know why.
my speedo, although replaced, was replaced "like for like" and is bang-on with no more than a 0.5 mph overread at any speed. I tested this on 3 different gps systems. This is how I know all the matrix signs on the estates in Dorset are out by 3 mph. (they say 33 when I'm doing 30)

Think of it as your Volvo looking after your license as well as tour safety. :D

[JamesT5]

they all under read at 70 by 4-6 mph fit bigger wheels to get it more accurate, with my 19's on and 35 profile tyres mine now over reads by 1 mph

I quite like the standard Volvo 17" wheels I have, although they need a refurb after 226,000 miles of brake dust, road salt, curbs and water (not guilty on any count!).

But 19" wheels, seriously? I'd cry at the tyre bill and the wheels scraping the wheel archs would drive me insane. :insane:

I think I'd rather take it to a specialist and get the speedometer calibrated like the Police do. :D

[p fandango]

Bonny's is miles out, as is the miles to empty on the trip computer lol

[T5frankie]

I quite like the standard Volvo 17" wheels I have, although they need a refurb after 226,000 miles of brake dust, road salt, curbs and water (not guilty on any count!).

But 19" wheels, seriously? I'd cry at the tyre bill and the wheels scraping the wheel archs would drive me insane. :insane:

I think I'd rather take it to a specialist and get the speedometer calibrated like the Police do. :D

they dont rub mate

[JamesT5]

they dont rub mate

I still like my 17" standard Volvo wheels. Calibration shop for me. :D

[foammanmark]

Using the same satnav, I've noticed that my recently sold p2 V70 said I was going 2-3 mph faster on the speedo than the satnav, but then with my p1 V70, the speed on the satnav is exactly the same as the speedo reading.

I now make sure that I'm smack on the speed limit if I'm in the p1 V70 when passing a camera, whereas in the p2 I would be slightly over the speed limit according to the speedo.

[JUDGENINJA]

My V70 is actually over reading on my tyres.

[JamesT5]

Using the same satnav, I've noticed that my recently sold p2 V70 said I was going 2-3 mph faster on the speedo than the satnav, but then with my p1 V70, the speed on the satnav is exactly the same as the speedo reading.

I now make sure that I'm smack on the speed limit if I'm in the p1 V70 when passing a camera, whereas in the p2 I would be slightly over the speed limit according to the speedo.

To be honest, I'd always drive under the limit near a safety camera rather than 'smack on', it's just not worth it. You'll absolutely not get a ticket if you deliberately drive 4 - 5 mph under the limit.

The fact we have to be so pedantic just in case is really silly. I can understand in built up areas and around schools, but on Motorways and dual carriageways I can't believe we have to worry about being a few mph over just incase we have an overzealous safety camera partnership.

Anyway, at least with an over read we can worry less about being a few mph over because there's a built in 'safety margin' within our speedometers.

[jbconno]

most speedos overread by 4-5% as car manufacturers don't wanna get sued for loads of people getting speeding tickets. from what ive seen, auzzie cars are closer to bang-on than any others in the world but I don't know why.
my speedo, although replaced, was replaced "like for like" and is bang-on with no more than a 0.5 mph overread at any speed. I tested this on 3 different gps systems. This is how I know all the matrix signs on the estates in Dorset are out by 3 mph. (they say 33 when I'm doing 30)

Think of it as your Volvo looking after your license as well as tour safety. :D

Yours would have been calibrated from the factory as its an ex plod car. On the older 850/P1 V70s there is a small serial number sticker in the middle of the speedo with "Calibrated by IRS Notts" on it. As part of the Volvo police preparation package the speedos come calibrated from the factory. As a result your speedo should be spot on.

[S1mon]

To be honest, I'd always drive under the limit near a safety camera rather than 'smack on', it's just not worth it. You'll absolutely not get a ticket if you deliberately drive 4 - 5 mph under the limit.

Really!? You're one of those annoying §§§§§§s I always seem to get stuck behind going through a camera! Lol.

If you're in a 40mph limit, drive through the camera at 40mph!! It won't activate at 41mph! More like around 46mph. THERE IS NO NEED TO SLOW DOWN to go through a camera if you're already driving at the speed limit!! It just holds people up, and in a line of traffic, causes the domino effect of baulking traffic. By the time you get to the poor people at back, they're anchoring up down to 25mph!!

[JamesT5]

Really!? You're one of those annoying §§§§§§s I always seem to get stuck behind going through a camera! Lol.

If you're in a 40mph limit, drive through the camera at 40mph!! It won't activate at 41mph! More like around 46mph. THERE IS NO NEED TO SLOW DOWN to go through a camera if you're already driving at the speed limit!! It just holds people up, and in a line of traffic, causes the domino effect of baulking traffic. By the time you get to the poor people at back, they're anchoring up down to 25mph!!

This reply made me smile. :D

But I will defend my driving slightly by saying that limits are limits and not targets and the camera has every right to activate and book me for being 1mph over the speed limit, even though as you say, it likely won't.

Although I do slow down for speed cameras, I tend to ease off gradually and only by a couple of mph, just to allow for any inaccuracy in my speedometer or the camera (just in case). Call it 'belts and braces'. :)

Consequently, the traffic behind shouldn't have to brake and there shouldn't be a domino effect, if there is, they are driving too close together. I always leave extra time to allow for people easing off or slowing down in front of me when I'm driving and as a result, I use the brakes a lot less and just 'ease off' until the car/vehicle in front has picked up speed again. I find it saves me fuel too.

Anyway, I'm straying from the topic there a bit but my point really is I never trust anything to be accurate enough so I just drive a bit slower just to make sure. :D

Regards

James

[jardon]

Mine over-reads by ~10% ie: at GPS 60 and 100 my speedo reads 67 and 109 respectively.

At GPS 165 it looks like ~190 ie: beyond 6 o'clock position by a fair margin.

Changing wheels will go nowhere near this level of discrepancy and there is no means of correcting it with VIDA/DiCE on my vehicle - on some you can enter the "wrong" wheel/tyre dimensions to arrive at a more accurate speedo figure. It's odd as the canbus/ECM data is spot on with GPS. I regard it as a positive "feature" with respect to speed cameras as others have mentioned.

[LeeT5]

Checked mine the other day and interstingly my speedo reads exactly the same speed as my RoadAngel gps. When it said 70mph my speedo was bang on 70mph. Same at 30, 40, 50 and 60. Didn't bother going faster cos i think the general consensous is..it will be the same.

[JamesT5]

Checked mine the other day and interstingly my speedo reads exactly the same speed as my RoadAngel gps. When it said 70mph my speedo was bang on 70mph. Same at 30, 40, 50 and 60. Didn't bother going faster cos i think the general consensous is..it will be the same.

Either your speedo has been calibrated at some point or your SatNav is out, an unlikely answer.

99% of cars that are not Police vehicles will have an over-read on their speedometers because they have to by law. They cannot under-read for obvious reasons.

[stephenevans99]

Checked my speedo accuracy today using my Sat Nav and was pleased to discover that my speedo matched the speed displayed on the GPS exactly.

[JamesT5]

Checked my speedo accuracy today using my Sat Nav and was pleased to discover that my speedo matched the speed displayed on the GPS exactly.

My car definatley over reads at higher speeds and it over reads by about 4 - 5 miles per hour at times. It's more accurate at 30mph, being only 1 - 2 mph out.

[T5RPete]

Mine matches my satnav at 60mph and 70mph. i've not checked any other speed.

However the mechanical trip distance on the dash does not match actual distance travelled. I can only assume I have different size tyres to standard? Fronts are 215/40/17.