Have you ever checked your speedo against your GPS?

[onesickpuppy]

I have just driven back from Perth to Aberdeen and I had used my Garmin Nuvi to find an address in Perth. I set the cruise control to 73 mph as I assumed that all speedos had a built in 'over-read' (and I like living on the edge ). Was interested to note that the GPS speed exactly matched set speed in the cruise contol and the speedo reading. I also checked at 70 mph, 50 mph and 40 mph. An exact match every time.

So, is this (non) over-read an urban myth, me being stupid or me having an unusually well calibrated speedo?

Anyone else checked this and if so, what were your results?

[Captain Muppet]

My MX5, fitted with the wrong diff and an iffy speedo face to convert it to MPH from KPH, is within 1mph of the gps speed.

Blind luck.

[jagracer]

Speedo in my Landcruiser over reads by 10%, the one in my Corsa over reads by about 2mph at 50, my 1969 MGB is exact when comparing it with my Tom Tom.

[Allyc85]

After around 50mph my Focus over reads by a constant 5mph compared to my TomTom satnav.

[Dogwatch]

Speedo always 2-3 mph over that indicated on GPS systems (one satnav and a one a speed/scamera display - they both agree).

[big_boz]

2002 Audi A4 was doing 66 GPS at 70 Dash
2012 Kia Rio is doing 69 GPS at 70 Dash
1972 Beetle is doing 30 GPS at 35 Dash

Cant remember any more actual numbers but i have never had a car that agreed to GPS

[Gooly]

Mine over-reads quite a bit (mk5 fiesta zetec), its 28mph at an indicated 30 and about 94mph ish at an indicated 100mph iirc. I think mines particularly bad though because the default spec tyres on the phase 2 which I have are a little lower in profile and I dont think ford adjusted the speedo to suit.

[AllNines]

GPS? Blooming new-fangled technology. That's what the tachy scale on a watch is for! Results showed speedo to be pretty accurate.

[Willy Nilly]

I would be surprised if an off the shelf, retail, GPS unit was particularly accurate. I'd be even more surprised if it was as accurate as a calibrated speedo.

We have 2 guidance systems at work, admittedly they will do more than just steer the machine, but they are 16 grand a piece + £900 a year a piece to rent the differential correction. How much was your Tom Tom?

[big_boz]

Willy Nilly said:

I would be surprised if an off the shelf, retail, GPS unit was particularly accurate. I'd be even more surprised if it was as accurate as a calibrated speedo.

Ah, probably correct, Manufacturers don't calibrate the speedo for all wheel options do they, a small change in wheel diameter will throw the speedo out instantly.

[Willy Nilly]

big_boz said:

Ah, probably correct, Manufacturers don't calibrate the speedo for all wheel options do they, a small change in wheel diameter will throw the speedo out instantly.

Honda had a ad on the telly with a civic rolling off the assembly line. They said they calibrated the speedo for the wheels and tyres it had on it so it was correct. Make of that what you will.

[kambites]

The Elise is miles off - around 10% over at all speeds. The Octavia is surprisingly accurate, about 2% over.

Oddly, taking the figure straight from the ECU in the Elise gives an extremely accurate figure (within 1%), so it's just the analogue display which is rubbish.

[sparks_E39]

At 80 mph indicated I'm actually doing 73-74 according to the sat nav and the electronic speed reader.

[Big News]

The 850 reads about 6mph over at average motorway speeds and about 10mph over at...uh...high motorway speeds.

[martin84]

I have a Navman which is clearly optimistic. According to it I was behind a caravan the other week which was doing 70.

[dcb]

onesickpuppy said:

So, is this (non) over-read an urban myth, me being stupid or me having an unusually well calibrated speedo?

Anyone else checked this and if so, what were your results?

UK speedos are legally required to be 0% to a few per cent over.

For extreme example, I was recently on the Autobahn 8, heading
into Munich, and the speedo was showing 160 mph.

Shirley the Sat Nav, said I was only doing 155 mph ;-|

Neither Sat Nav nor car are perfect, but I'd trust the
numbers from the Sat Nav more.

Not far out, only a 3.2% overread.
Assuming pro-rata in the UK, only 3 or fewer mph over.

[ben5732]

Willy Nilly said:

I would be surprised if an off the shelf, retail, GPS unit was particularly accurate. I'd be even more surprised if it was as accurate as a calibrated speedo.

We have 2 guidance systems at work, admittedly they will do more than just steer the machine, but they are 16 grand a piece + £900 a year a piece to rent the differential correction. How much was your Tom Tom?

Got a calibrated Speedo and GPS is spot on with it.

[v8250]

jagracer said:

Speedo in my Landcruiser over reads by 10%, the one in my Corsa over reads by about 2mph at 50, my 1969 MGB is exact when comparing it with my Tom Tom.

Jagracer, that's very interesting. The VW reads 8-9% over, the Subaru reads 7-10% over depending upon speed, however, my MGB is absolutely on the nail with the Garmin Nuvi 265WT every time.

[g3org3y]

Yes. Last week on the Autobahn the 328i's speedo was showing 140mph, the satnav was reading 220mh/h (137.5mph). More accurate than I would have expected tbh.

[Fantuzzi]

65 is 61-2 in my mr2.