Speedo Accuracy

jake000 · 9 Dec 2006 at 21:24

I was driving in my 1.0L 1996 micra the other day and at about 61mph. Someone went past me and I noticed that their speedo was a digital one and said 64mph on it. It seemed to be doing around 10mph more than me as it wouldn't have got past me as quick as it did only doing 3mph more than me.

How far is my speedo out and what is the average micra's are out by, if it works like that?

Thank you

Hawk Tuah · 9 Dec 2006 at 21:34

Without using GPS's you've never know for sure.

My Fiat is dead on what my GPS reads. However my girlfriends Rover 218 is well out. 90mph on the speed is about 82 on GPS.

Scuzi · 9 Dec 2006 at 21:44

Off the top of my head, there is a tolerance to overread by about 8-10% at 70mph although most cars will indicate about 73-75mph at an actual 70mph. Your speedo shouldn't under read your actual speed.

jake000 · 9 Dec 2006 at 21:46

Maybe a friend has one and I can test it, thanks for the information

Do you think it would be out at 30mph?

jake000 · 9 Dec 2006 at 21:50

Scuzi said:
Off the top of my head, there is a tolerance to overread by about 8-10% at 70mph although most cars will indicate about 73-75mph at an actual 70mph. Your speedo shouldn't under read your actual speed.

My speedo has had problems in the past. When the car is left in direct sunlight, the speedo gets stuck. I have had it looked at but everytime I take it to the garage, it works fine lol.

Thanks for the help guys

Hawk Tuah · 9 Dec 2006 at 21:52

jake000 said:
Maybe a friend has one and I can test it, thanks for the information

Do you think it would be out at 30mph?

If it was it would be negligible. 28 maybe. Typically, the faster you go the faster it will be out.

chrislusty · 9 Dec 2006 at 22:03

Mt speedo is out 10% from 30mph onwards, its quite odd when you think your doing 70mph but only doing 63mph!

Stonedofmoo · 9 Dec 2006 at 22:15

So driving along at 61mph you were actually looking at another cars speedo rather than the road ahead :eek:

Seriously mate, concentrate!

Jotun · 9 Dec 2006 at 22:29

In a work car I've drove before, it indicated ~115-120mph but gps said 97. So thats about 20% out :eek:

When I was going 30 indicated it really was crawling

jamoor · 9 Dec 2006 at 22:51

mine seems to be 10% out
any way of reconfiguring it as its annoying.
my mondeo was negligible (2%)

Dreadi · 9 Dec 2006 at 23:04

when put against a satnav, mine was almost 10mph out at around 80 indicated

Guigsy · 9 Dec 2006 at 23:07

I think the only rule is that it can't under-read and it has to be within a certain accuracy at 30mph (can't remember what the figure it, sorry). Basically it can be miles out, as long as it's not to bad at 30mph.

Jotun · 9 Dec 2006 at 23:11

Guigsy said:
I think the only rule is that it can't under-read and it has to be within a certain accuracy at 30mph (can't remember what the figure it, sorry). Basically it can be miles out, as long as it's not to bad at 30mph.

I thought that it could be +/- 10%, so it could under read it by a certain amount, but *most* manufacturers tend to read less than the actual speed.

touch · 9 Dec 2006 at 23:15

Changing your wheels really messes up your speedo. Larger/smaller raidus has the biggest effect, where weight has a much smaller effect

Acolyte · 9 Dec 2006 at 23:41

touch said:
Changing your wheels really messes up your speedo. Larger/smaller raidus has the biggest effect, where weight has a much smaller effect

Weight has no affect at all on the speedo reading, only the rolling radius.

Evil-Penguin said:
I thought that it could be +/- 10%, so it could under read it by a certain amount, but *most* manufacturers tend to read less than the actual speed.

Has to something like +10% It cants by law under read.

Clarkey · 10 Dec 2006 at 00:09

I would imagine most if not all speedo's have some degree of adjustment in them. I know that some nissan ones do, a little potentiometer on the pcb, you can adjust it while driving to calibrate it with your GPS or whatever quite easily.

Paulj · 10 Dec 2006 at 00:23

Acolyte said:
only the rolling radius.

Huh?

Type_R · 10 Dec 2006 at 00:29

My Golf GTI (MK5) over-reads by 10% exactly throughout the speed range when compared to GPS.

The rolling radius = wheel +tyre and not just wheel size, as on bigger wheels you can stick on lower profile tyres to match the rolling radius

Acolyte · 10 Dec 2006 at 00:33

Paulj said:
Huh?

The rolling radius of the wheel dictates how far the car travles.

14 inch wheel with 1 nch tyre = 15 inch rolling radius
16 inch wheel with 2 inch tyre = 18 inch rolling radius

the larger the rolling radius the more the car will under read as it is travveling further for each wheel revolution.

So if you have tiny wheels on your car chance are its over reading quite a bit.

PMKeates · 10 Dec 2006 at 02:15

At 130 indicated GPS is reading 127, so about 2%. Not bad really, and I generally look at is as "actual", as for pretty much normal intents and purposes it's never going to be more than 1 or 2 mph out.