car speedo and sat nav discrepancy

usignuolo

Original tyre specs as fitted by manufacturer

OddballJamie

You could always get on a motorway with your partner in the other car and both drive with your speedo reading 70.

Alternatively where I live they have the digital readouts on the road showing your speed, any of them by you?

adouglasmhor

ILW wrote: »

You should not be in the middle lane unless overtaking.

I drive in it when not overtaking if the nearside lane is rutted and potholed by lorries. Are you suggesting I should bounce along over ruts and potholes at 70MPH?

Lum

The actual maximum allowed overread for a speedo is 10%+6.25mph. This means that someone doing an indicated 70mph may in reality only be doing 59mph, likely while cursing everybody overtaking them as nutters and maniacs.

Satnav will generally only give you an accurate speed reading if you're going at a constant speed in a straight line with a clear view of the sky. In those situations it is worth comparing speedo to satnav and seeing how much faster you can get away with if you're that way inclined.

In answer to your question "is it safe to go at an indicated 75mph", the answer is maybe. If you'd asked "is it legal to go at an indicated 75mph", the answer is yes, if you have confirmed that an indicated 75 = an actual 70.

Don't forget to recheck your speedo calibration from time to time. As your front tyres wear out your speedo will start to overread slightly more and then when you replace them the overread will reduce again.

Also don't assume that because it's +5 at 70mph that it's also +5 at 30mph, it may only be +1 at 30mph so doing an 35mph there would see you doing an actual 34mph which is right on the edge of the threshold for setting off a speed camera.

If you want to stay 100% legal, this method is best combined with cruise control, but even then, be careful when going downhill.

Roland_Sausage

I drive a transit van which is speed limited to 62mph. When I am driving it flat out my Sat nav shows it as 62mph. But the speedo shows it round about 68mph.

I always assumed the sat nav was the accurate one.

Richard53

I trust my satnav for speed - as long as it is on a longish stretch to allow for sampling errors. I had an ex-police Honda a couple of years back, with a proper calibrated speedo, and that agreed 100% with my satnav (agreed to 1 mph at 100 mph), so I take this as confirmation that both were pretty accurate. It was very noticeable when riding how traffic around me was always travelling a bit slower than the limit (clear road etc), which I put down to everyone's speedo being a bit optimistic.

In answer to the legality question, the speed limit is for the actual speed the vehicle is travelling at, not the reading on the speedometer. I have passed police cars on the motorway at a GPS speed of exactly 70 mph (speedo reading nearer 80) and have never been pulled over, so I assume the police only bother about people actually exceeding the limit.

Car speedometers are built to a price, and accuracy costs money. They cannot under-read, but are allowed to over-read by 10% + whatever (correct details in posts above), so most manufacturers set them to over-read in the middle of this range, about 5-6%, which should mean (allowing for manufacturing tolerances) that all their units fall within the legal range. Most cars travelling at 70 mph will read about 66 mph.

It is possible to have a precision instrument which could register the vehicle speed to +/- 1% or less, but the car makers assume no-one would be willing to pay the considerable extra cost. (Same applies to fuel gauges.)

bigjl

diesel_dog wrote: »

Most car speedo's under read.

Actually they usually overread, I think you will find they are not allowed to under read.

GPS are more accurate.

The only time you will find a car with a really accurate speedo is if it is an ex Police vehicle with a properly calibrated speedo and even they need to be checked regularly.

Some manufacturers speedos are worse than others.

When driving in motorways with average cameras I use the GPS all the time as a quick glance gives an almost 100% accurate speed.

iltisman

I agree that most speedos over read but I have noticed in the USA they are very accurate usually within 1 or 2 mph .

vikingaero

I use my satnav to drive through road works at a true 50mph - the speedo reads about 54-55mph.

Most latest satnavs are pretty accurate on long straight stretches of road without large gradient changes.

Hintza

vikingaero wrote: »

I use my satnav to drive through road works at a true 50mph - the speedo reads about 54-55mph.

Most latest satnavs are pretty accurate on long straight stretches of road without large gradient changes.

I use my satnav to drive through cameras at 75-76 on the speedo. I trust my sat nav to be pretty accurate and certainly well within any tolerance plod might have.

I don't care on sampling rates, uphill, downhill and if isn't reading then I know what the speedo reads at a true 70mph.