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!

That's one I'd not heard before!
As for Guzzi speedos, have they improved?

Cheers,

Hi Robbie, the calibration mode is selected by switching on the ignition while simultaneously holding the "trip counter button". After +3 seconds release the button and the speedometer digits will flash. For safety, write down the number you see on the display.
First test the accuracy of the meter, noting the difference between 10 miles on your display and 10 "real" miles on the motorway mileposts. Or time your speed over a set distance and a fixed speed. Then I used the "rule of 3" calculation to get to the correct pulses/mile (might take a couple of changes till you get to the best one, but you can try unlimitedly).
By momentarily pressing and releasing the reset button each number (starting from the left) can be incremented to your calculated value. When the desired value is selected wait a couple of seconds till the next digit will flash, increase this number, etc to the full pulse setting. At the end of the reset, wait another +3 sec for the speedometer to return to normal operation.
Well, this is how it worked for me anyway. Up to you...

Suitcases in the boot of my Aeromax renders my SatNav blind, let alone unreliable!

I’m having an auxiliary receiver added to allow the system to have line of sight to the sky; this should cure my problem.

In a previous car I once had a portable SatNav set up, a Micro Road Pilot hardwired in and, of course, the speedo running. They were all telling me I was going a different speed.

My understanding – and perhaps incorrectly – was that part of the reason the ‘reasonable’ police forces gave motorists the 10% plus 2-3mph leeway when doing speed traps, was to allow for some variance in the calibration of the speedo in the car?

I was told that the law allows for mechanical variance of the speedo of -3% and +10% so permitting your speedo to show (67)70(77).

In my experience pretty much every car I have had shows in the region of +5-10% with some Ford/Vauxhall being even worse. You could also loose a little rolling diameter from under-inflated tyres. My Phaeton shows about 2mph more than the TomTom at 70mph. My Aero shows about 6mph more than my TomTom at 70mph. Ironically I am far more likely to speed in the Phaeton due to the silence and massive torque/autobox/turbos.

In my experience the boys in blue will not look to upset your journey until you are a further 5mph over the real limit or more on the motorway. If you are showing bad road craft or near a known problem area like a school they tend to allow no variance.

I always find it fun if you look at the speed people do through roadworks or as they are passing a police car. Most police sit at 67mph on the motorway to try and reduce bunching so allowing people to make fair progress. You get piles of cars tailing the policeman and then they look at you like you are mad as you gently pass them and make off into the distance.

So if you add all of that up you could see 82 on the clock and still be safe. Then again you could be looking at a ban !