This item relates to the speed recorded on my Tom Tom SatNav versus the Speedo on my S Type Jag.
When the Jag sits at 70MPH the Sat Nav speed indicator is only showing 67mph. Is the SatNav the correct one?

I have yet to make this comparison between the Sat Nav and the MOG (but I will).

Both Cars have their original wheels and tyres.

Can of worms that one Frank.
The speed reading on your SatNav will also depend on terrain. The only time you can really compare it against a car speedo is if you are driving at a constant speed, in a straight line, on a flat (horizontal) road.
Regards,

influence correctness of the satnav: nr of sattelites & refreshing rate of the signal

roadster: perfect speed-measuring
maxie: ouch!!!!

Will the M25 do for that test?

IIRC there's some EU directive which says car speedos are allowed to over read by about 10% but are not permitted to under read at all. In older cars, I've been told that speedos may also start to get more and more inaccurate over time - although in more modern cars (which use electronic wizardry) this shouldn't happen. No idea if that's true or not.

Doesn't really answer your question though. On my Fiat, I'm pretty sure the speedo over-reads by about 5%.

Pretty much the same for my Audi. I think its probably the same for all cars, a built in safety margin, otherwise you could potentially go after the manufacturer if you got nicked for speeding at 33 when your speedo read 30!

I tried the same comparison on my previuos MOG (2001 4/4), and it was WAY out, one occasion I looked down and the speedo read 120! I know for sure it wasn't!
The dealer recalibrated it as its electronic I believe.
Kevin

For your info, the electronic speedometers on the Morgans are easy to calibrate. I have a deviation of 20 meters on 10 km. When it came out of the factory it had a deviation of 20% !!! (driving on the motorway at 160 and everyone passing by didn't look normal)

Midas,
Care to share how this calibration is done???

Guess you'll be doing that one in the middle of the night when it's quiet - if you want to get a constant speed run??

As others say, car speedos are allowed to read up to 10% fast - this allows for tyre wear and not under-reading with new tyres. The only way to be sure is proper calibration on a rolling road.

My old 4/4 actually under-reads, but I know by how much (I suspect it's due to age and a lack of use for a few years before I bought it). The +4 seems reasonably accurate. The MX-5 reads high, but within 10%, as for the 90, I haven't bothered to check it.

Cheers,

Triumph motor cycles in the 1960's and 70's were always reckoned to have the most accurate speedos,typically reading less than 1 mph slow at 60 mph, probably due to Triumph supplying the Saint [Stops Anything In No Time] 650 to police forces wordwide, if you rode a Guzzi Le Mans [I wished I did] at an idicated 80 mph you wouldn't qualify for a speeding ticket in a 60 zone!