Measuring Speed re Speedometer +/-

victor2

You can check your speedo on a motorway by driving at a set indicated speed and getting a passenger to time you between some of the marker posts, which are usually 100 metres apart. Some motorways even have signposts every 500 metres with distances along the motorway on them.
Either way, time your car between a number (the more the better) of posts/signs at a steady indicated speed. Bit of simple arithmetic gets your actual speed.

Outpost

victor2 wrote: »

You can check your speedo on a motorway by driving at a set indicated speed and getting a passenger to time you between some of the marker posts...

I'd rather just play I-spy.

JJ_Egan

I'd rather just play I-spy.

Me too both eyes closed .

jje

JJ_Egan

It won't get worse. It'll not be covered under warranty either.

No surprising that but it will be checked under warranty ,

Driven thousands of brand new cars and never had one so noticeable as this including a few with no speedo .Had a few bounce at 140mph though .
But as the question was an easy method of testing without a rolling road and that's been answered as a no thats all folks must go the nice man with the blue light on his car must be lost and coming to ask me directions .

jje

martinthebandit

Hintza wrote: »

There is nothing wrong with your speedo. Change your tyres that will have more effect than anything else. Every millimeter you scrub off your tyres will affect your speedo reading.

yep, just put some tyres on it with a mm more tread and your speedo will probably be spot on

reeac

martinthebandit wrote: »

yep, just put some tyres on it with a mm more tread and your speedo will probably be spot on

If the speedo is reading 10% too high you'll need a lot of extra tread thickness ... maybe 30 mm. extra! [assuming a 195-15 tyre of aspect ratio 0.65
.

james2012_2

I have a Micra and was always being overtaken on the motorway even although I was hammering along at 75 mph according to my speedo.

Then I got a sat nav and discovered my speedo was reading 10% less than the speed according to my speedo.

So I started to drive at 80 mph on the motorway.

I still get passed.

There is one of these radar signs that say too fast if you are going too fast, that has appeared on a road near me when the speed limit changes from 40 to 30mph. I only set it off if I am going 33mph or faster according to my speedo and my sat nav reads 30mph.

Flyboy152

JJ_Egan wrote: »

A speedometer measures speed does anybody know one if modern service repairs for main dealers in this case Citroen are able to check a speedo without a rolling road .
Reason new car last week Citroen C3 Picaso is giving me distinct impression that speed is out by about 3 to 5 mph . That is reading 30 MPH where true speed is probably nearer 27MPH .
Based on one driving impression that i am driving slower than the speedo reads . GPS is reading roughly 3MPH under speedometer and limited tests with Your speed is type camera reinforces that .

jje

A satellite navigation system is not an accurate enough tool to measure exact speed.

Flyboy152

reeac wrote: »

GPS is, in principle, extremely accurate. The limit is your ability to drive in a straight line at a constant speedo. reading for, let's say, 100-200m.

No it isn't, in fact it is quite the opposite.

Flyboy152

Dave101t wrote: »

my gps says im 4mph slower than reality. means i can do 74mph and know its only 70mph. They are supposed to be like that.

No they are not. I wouldn't recommend you test your theory.