car speedo and sat nav discrepancy

Lagoon

My speedometer is 4mph out - SatNav systems all say this, as do those police speed boards that check your speed as you're driving past.

I see this as a bonus! Keeps me in safer limits even if I'm aiming for the speed limit but accidentally slip over by a few mph.

diesel_dog

diesel_dog wrote: »

Most car speedo's under read.

I meant to have put under read compared to sat nav units, whoops! :beer:

George_Michael

usignuolo wrote: »

He kept telling me to speed up when I was doing 70mph on the motorways until we realised the car speedo was showing about 70mph while the GPS on the dash, which he was looking at, was reading around 64mph at the same time.

This sort of discrepancy is will within the permitted tolerance of the speedo.
If it is showing 70mph, you could legally be going as slow as 58mph. The instrument can't under read at all, but it can over read by as much as 10% + 6.25mph.
http://www.legislation.gov.uk/uksi/1996/3013/schedule/2/made#text%3Dspeed%20lower (Paragraph 19)

so if you were in reality driving at 58, your speedo can over read by 10% (5.8mph) + 6.25mph giving an indicated speed of
70.05mph

usignuolo

Well the sat nav we use is within 1 mile ph same as speedo on OH's current car, at 70mph and higher (we tested it on sunday) and it was also that accurate on his previous car (changed his car last year). I would assume sat nav is more accurate than car speedo as it is measuring speed relative to the orbiting satellites. So I think my car speedo must be wrong.

My problem is if I am driving in middle lane at what I think is 70mph but is actually 64ish, I may be holding up cars wanting to drive at a true 70mph. And does it mean I am safe to over take at 75 mph as my true speed is in fact under 70mph?

Would changing tyres make that much difference?

StrongWork

usignuolo wrote: »

My problem is if I am driving in middle lane at what I think is 70mph but is actually 64ish, I may be holding up cars wanting to drive at a true 70mph. And does it mean I am safe to over take at 75 mph as my true speed is in fact under 70mph?

Given what you've written here, an over-reading speedo is the least of your worries.

shaun_from_Africa

StrongWork wrote: »

Given what you've written here, an over-reading speedo is the least of your worries.

I was thinking exactly the same thing myself.

My problem is if I am driving in middle lane at what I think is 70mph but is actually 64ish, I may be holding up cars wanting to drive at a true 70mph

It doesn't matter what speed you are doing or what your speedo is reading. If you are driving in the middle lane (which you shouldn't be doing anyway as this is for overtaking) and holding up other cars, you are not driving correctly.

gazzak_2

The last 10 cars I've driven ALL showed 5-10% under using satnav against speedo. It's normal.

There is no middle lane on the motorway btw, it's an overtaking lane.

kaya

strange, my last sat nav read spot on in my merc but was about 10% off in my shogun , both had the correct tyre sizes on

shaun_from_Africa

gazzak wrote: »

There is no middle lane on the motorway btw, it's an overtaking lane.

If there are 3 lanes, then there must by definition be one in the middle, hence it's the middle lane.
I know that this isn't the official designation but that doesn't mean that the term "middle lane" can't or shouldn't be used for general discussion.

There must be some term to identify the lanes, and what is wrong for inner, middle and outer lanes or nearside, middle and offside lanes?

mcfisco

There must be some term to identify the lanes

• Trucks and caravans
• Most people
• Mercedes lane