Speedometer Accuracy??

[Brian WH]

So how accurate is your Speedometer, and how do you tell?

Against GPS Satellite reading from my "Indic8tor speed management solution equipment ( Speed Trap Detector ), my speedometer is 5% fast at all speeds.

ie. at 31.5 mph speedo - GPS Satellite reading is 30mph
at 84 mph speedo - GPS Satellite reading is 80 mph.

Some people claim their Speedometers are accurate, others claim within 10%, so which is right.

By the way GPS readings can only be truly accurate in a straight line.

 

[imadoofus]

So how accurate is your Speedometer, and how do you tell?

Drive past a speed camera at what you think is the speed limit, and if you crash into the car in front, or get rear-ended, your speedo is inaccurate.

The system I have used a couple of times is to follow a friend's car. When he gets to 20mph, he flashes his hazards briefly. You then check your speedo, and flash your lights to tell him to speed up to 30 and do the same again, all the way to 50, or 60, or 70 if you want.

Yes, the other car's speedo may be inaccurate, but it's not likely to be out by exactly the same as yours.

 

[BTB 500]

It varies from car to car and also with speed (i.e. it's not necessarily x% out all the time). As you say the only way to check with GPS is on a straight & level road, preferably using cruise control. Our Audi A4 over-reads significantly at motorway speeds, my VW was almost spot on, IIRC the SL was too, not checked the Vito.

 

[BTB 500]

Drive past a speed camera at what you think is the speed limit, and if you crash into the car in front, or get rear-ended, your speedo is inaccurate.

The flaw in that cunning plan is that most morons slow down to way below the speed limit for cameras (and they do the same for each SPECS gantry too)

 

[Brian WH]

The flaw in that cunning plan is that most morons slow down to way below the speed limit for cameras (and they do the same for each SPECS gantry too)

But seem to forget that they don't work like that.

 

[BTB 500]

Yup. From what I see on the motorways, at least 50% of drivers don't understand the concept of average speed.

 

[srtaylo0]

think they do understand the concept of average speed... people just want to play it safe with fine/points combo risks...
drove through the m1 roadworks at the weekend - didn't see anyone speeding up then slowing down before cameras - everyone I saw was keeping a constant 48 ish

 

[Fady]

...a couple of interesting points on this topic on the BBC website that are worth a read.

 

[BTB 500]

think they do understand the concept of average speed...

Not the ones who treat the gantries like GATSOs, driving at 70+ between them ...

 

[Ade B]

My speedo is about 10% over, I get an indicated 80 and a GPS of 73 which is my default motorway cruising speed.


Ade

 

[kikkthecat]

Mine always reads 3 Mph above the sat nav reading at all speeds. In that respect I suppose it's accurate as the reading is always a fixed value of true speed and not a percentage.

Yup. From what I see on the motorways, at least 50% of drivers don't understand the concept of average speed.

Only on motorways eh ?
2 days ago I got flustered which is something that doesn't happen often. I was following a woman on Huddersfield road in a 40 limit and she was doing 30. Nothing wrong with that and it wouldn't normally bother me but she braked to less than 20 Mph for 4 speed cameras.

 

[Brian WH]

Poll added, please complete.

 

[chriswt]

Poll added, please complete.

A little difficult for me to complete because my speedo also reads about 3mph over my GPS speed so it could be 5% or 10% depending on speed.

 

[imadoofus]

The flaw in that cunning plan is that most morons slow down to way below the speed limit for cameras (and they do the same for each SPECS gantry too)

Yes, but you live in the South. Nobody up here has even heard of SPECS. Up here, if the camera is in a 30 limit, then the traffic does 29.9mph.

 

[Brian WH]

A little difficult for me to complete because my speedo also reads about 3mph over my GPS speed so it could be 5% or 10% depending on speed.

You can now mate.

 

[Bobby Dazzler]

If you don't have sat nav but do have a trip computer...

Set cruise control so your cruising at a known speed on your speedometer, eg indicator 60mph. Then reset your trip computer, and cover a fixed distance without deviating from that speed, eg 6 miles. Monitor the trip computer noting the average speed at the moment you've covered the set distance. The average speed is a close approximation of the actual road speed.

If you don't have cruise control repeat the above but using your foot rather than a computer to maintain constant speed!!


If you don't have a trip computer...

Find out the gearing in top gear, eg 20mph per 1000rpm. Multiply it up to a suitable speed, eg 60mph would equate to 60mph. Then cruise at that speed in top gear, and note the indicated speed on the speedometer.

 

[BTB 500]

If you don't have sat nav but do have a trip computer...

Set cruise control so your cruising at a known speed on your speedometer, eg indicator 60mph. Then reset your trip computer, and cover a fixed distance without deviating from that speed, eg 6 miles. Monitor the trip computer noting the average speed at the moment you've covered the set distance. The average speed is a close approximation of the actual road speed.

I suspect the trip computer and speedo would always agree. Must try it sometime!

 

[glojo]

Don't forget that all local authorities will have a calibrated measured mile on a straight stretch of road. This measured mile is there to enable speedometers to be calibrated.

Regards
John

 

[st4]

My speedo reads 30mph and the car is doing 27mph. This widens by 4mph by 70mph on the speedo. Cars will always under read.

Interestingly my 4.4i X5 under read by 1mph, the 3 lite under read to the same extent as my E class.

It made me think and I will pose the question, do cars with higher top speeds have more accurate speedo's (or ones that under read less) to enable a reasonable accuracy @ speeds a less powerful car won't be doing. I imgine my old V8 X5 would probably reach 150mph and its speedo would be on 153mph. In the E class it would be on 155+.

 

[crockers]

When driving on a motorway - look at the mileage markers on the side.... drive at 60 mph and time how long it takes you to cover one mile (it should be a minute) extrapolate from the reading...

Best to have 2 in the car whilst doing this....one to look at the posts and click the stop watch..

I have no idea how accurate mine is .. the law states that it can't read under the speed I am doing, but can be over by a margin up to 10%....

Just think -- your 50,000 mile car is infact around 45,000..... hmmmm...me sees a conspiracy here...but it makes you feel better thinking your driving a faster car and getting better mpg...