Speedometers

[Wicked_and_Crazy]

why is it that all uk cars I've tried the speedo reading at about 77mph is 70 mph according to a sat nav. Yet in the US a speedo reading of 70mph is also 70mph according to a sat nav.

Why is there a difference in speedo accuracies?

Is it a manual vs auto gearbox issue?

[Xaccers]

It's to ensure you're driving under the speed limit given the inacuracies of speedos.
No normal speedo is 100% accurate, they must over read and never under read, so you can't be doing 85mph while your speedo shows 70mph.

[Graham M]

I've always taken it as a given that there is a +/- 10% difference

[keithwalton]

Its a safety and bragging rights thing, as said above it makes it seem as if you're going faster than you really are. So it also thinks you are accelerating to 60 faster, and on that drive to the pub you were doing x miles an hour etc. Ford are notoriously worst for over reading the most.

I also wouldn't 100% trust sat nav certainly not in this country and not until Galileo is up and running

[Graham M]

Its a safety and bragging rights thing, as said above it makes it seem as if you're going faster than you really are. So it also thinks you are accelerating to 60 faster, and on that drive to the pub you were doing x miles an hour etc. Ford are notoriously worst for over reading the most.

I also wouldn't 100% trust sat nav certainly not in this country and not until Galileo is up and running

v. true, the resolution just isn't as good, I often notice some weird speeds our taxi drivers are doing at work but I tend to ignore it as they are taxi drivers :p

[Wicked_and_Crazy]

Its a safety and bragging rights thing, as said above it makes it seem as if you're going faster than you really are. So it also thinks you are accelerating to 60 faster, and on that drive to the pub you were doing x miles an hour etc. Ford are notoriously worst for over reading the most.

I also wouldn't 100% trust sat nav certainly not in this country and not until Galileo is up and running

I can't believe its a safety inaccuracy aat all. That sounds so stupid, the UK needs an overstating speedo for safety but the US doesn't. I would expect its more likely to be a cost issue.

Why don't you think sat navs are accurate? you only need 3 sattelites to be able to triangulae you current position. given that ,most times you connect to 6-8 this is going to be pretty accurate and given that each sat has an atomic clock your position and speed ought to be quite accurate.

[Wicked_and_Crazy]

v. true, the resolution just isn't as good

Resolution of what? They can pin point within a few feet can't they?

[pop80_uk]

Speedo's have always been out, but this is because they are manufactured within a >10% tollerance range, depends on manufacturer but you will find that all sorts of things affect the reading, size of alloys, tyres, what type of pseed sensor the car has, as soon as manufactures start altering the dynamics of their cars then this results in speedo readings being wrong.

Sat navs also have a tollerance and are not totally right when displaying speed. This is because they monitor your position in relation to either a distace ahead or behind you, its never a pin point display of your position, instaed you travel in somewhat a bubble of coverage.

[Taf]

The speedo doesn't measure speed, it measures revolutions... so deflate your tyres and you will travel slower, pump them up too hard and you travel faster.... then try to explain that to a traffic cop or speed camera as a defence....

[handyman]

What would low profile tyre do to speedo accuracy (factory fit).

Our max speed indicated tops at 104mph which seems to put us 9mph over the max top speed.

[pop80_uk]

The speedo doesn't measure speed, it measures revolutions... so deflate your tyres and you will travel slower, pump them up too hard and you travel faster.... then try to explain that to a traffic cop or speed camera as a defence....

yes but bigger alloys / wheels alters this, thus altering the speed shown on the speedo ultimately.

[zovat]

Sat navs are accurate to within a few metres (unless you have WAAS compatability in which case you can get down to with 1-2 metres when static) but having tested the speedo on my car and the satnav (tomtom) against a radar speed check, I can say that my speedo is always 8-10% over and the satnav is always within 2 mph.

[King Of Fools]

Actually, speedos are non-linear and will typically under-read at slower speeds and over-read at higher speeds. For example, if your speedo reads 30 MPH you might actually be travelling at 33 MPH but if your speedo reads 70 MPH you might actually only be travelling at 65 MPH.

yes but bigger alloys / wheels alters this, thus altering the speed shown on the speedo ultimately. The speedo will also change over time as the tyres get worn down!

[Taf]

The important measurement is the height from the ground to the centre of the wheel, as this sets the effective circumference of the wheel.... the greater the height, the greater the circumference, the greater distance travelled in one revolution of the wheel.

Plus a cold tyre has lower pressure than a hot one, so as the tyre heats up the ground/centre distance will increase and so therefore does the speed.

One important factor though is GRIP...... an over-inflated tyre has less surface in contact with the ground, so grip drops right off. Tread at the dead centre of the ground contact is moving slower than the outer edges of the tread contact (front and back). It's this difference in speed, and the rapid deceleration/acceleration that causes grip (rolling resistance).... and wear....

[pop80_uk]

The important measurement is the height from the ground to the centre of the wheel, as this sets the effective circumference of the wheel.... the greater the height, the greater the circumference, the greater distance travelled in one revolution of the wheel.

Plus a cold tyre has lower pressure than a hot one, so as the tyre heats up the ground/centre distance will increase and so therefore does the speed.

One important factor though is GRIP...... an over-inflated tyre has less surface in contact with the ground, so grip drops right off. Tread at the dead centre of the ground contact is moving slower than the outer edges of the tread contact (front and back). It's this difference in speed, and the rapid deceleration/acceleration that causes grip (rolling resistance).... and wear....

.......Which will all ultimately affect how much the speedo is out.

[keithwalton]

Comparing a US to a UK car is very broad and ultimately meaningless not every us car will be the same. Displayed speed will vary between identical cars due to the reasons given above mainly due to the circumference of the tyre varying so distance per revolution varies. When fitting non standard wheels / tyres you should aim to match this circumference as close as you can, its rare that you will get an exact match.

commercial gps coverage in the uk isnt brilliant, at best it will get you within a few m if you are static. If you are moving it'll have a range of about 10-20m on your location. It is better than the restricted 100m system that was in place a few years ago.

The best way to check the accuracy of your speedo etc is to measure the time taken to travel between a fixed distance at a fixed speed.
you will notice that every now and then on dual carrigeways mainly those leading to / from motorways you will see a white circle in the road. Exactly one mile later there will be an identical circle.

This is what the police use to calibrate there equipment, at 60mph you should take exactly 60seconds to go from one marking to the next.