At 100 km/hr on the car's speedo the 1390t is showing 95 km/hr. Several times when this is happening I have checked the satelite screen to see the accuracy which has always been at 3 metres. By my reckoning, a 3 metre error in a km (1000 m) is 0.3% which should amount to a 'error' of 0.3 km/hr at 100 km/hr, effectively zero.
I can't believe my car's speedo is 5 km/hr per out. (However I will check it next chance I get.)
You know your car speedo could be showing more than the real amount.. I know thats the case with my car - every time I pass one of those cameras that tell you the speed ur going (they dont take picture, just warn you), it shows my speed 5-10km/h lower than what my speedometer shows.. how old is ur car?
Not sure where you are, but in Australia the national standards provide for a tolerance of 5% on car speedometers. If you can find a review on the NRMA (RACQ, etc), they usually tell you what the error is for that car.
My 2006 Mitsubishi 380 is doing 95 when 100 is displayed on the speedo. This is EXACTLY what both my CoPilot and Garmin GPS units indicate.
Checked my odometer on the highway with a series of thoses "check your speedo" signs. The odometer was 2% out measuring 5.1 km over an actual distance of 5km.
Does this necessarily mean the speedo is out by 2% as well?? Dunno. The speedo is analogue while the odo is digital. The speedo seems to be out by around 5% to the naked eye. In an case it sounds like this is all well within the Australian standards so I guess I will probably be more likely to trust the GPS from now on...
Just sold this vehicle, always displayed higher speed on speedometer than GPS (Nuvi 265W). Took a risk and tried past local speed cameras, no ticket received. My theory is that a margin is built in to the speedo to avoid litigation.
For my money, trust the gps. Speed variation was not constant, around 5kmh @80KMH and 8KMH at 100 kmh.
Great idea - I'm passing on a Nuvi 255w to a friend tonight - didn't fancy the £100 map upgrades for 'life of machine' however long or short that may be when a new 1340 was only £4 more from online sellers. This'll give me a chance to check both against the analogue speedo in my current car (SEAT Leon). Incidentally, some car speedos are un-nervingly accurate. My previous Citroen C4 with massive central digital display which annoyingly could be read by the speed-cop behind, only disagreed with my sat-nav by 0.5 mph.
To the poster who reckoned his odometer reading was only still 2% out compared to mile posts, this could be down to tyre wear altering the circumference of the wheels, which is easily overlooked. As they wear they go round more often for any given speed.
Re: my previous post. Put both the Nuvi 255W and the new 1340 side by side in car, and despite the fact that they took differing amounts of time to come up to the actual speed, once cruising, they were identical. Ok, they're both the same make, but it tends to bear out what I've thought all along, that sat-navs make much better speedos than the one in the car. I've certainly had no trouble using the sat-nav's version of 50mph in those motorway roadwork sections with the av speed monitoring, even if I do slowly cruise past most other people.
Curiously, their compass bearings didn't always agree entirely, although they were sited at different angles on the glass.
I have the same speed discrepancy between my in-car speedometer and the GPS. My GPS reads a bit higher.
Here is the explanation: The car's speedometer is geared from the car's road wheels and most people don't realize what this means. The speedometer installed in your car is designed for the specific tires that the vehicle is originally equipped with. The circumference of the tires affect the speedometer calibration. If you replace/change the tires for a different brand and/or size - I wanted a wider tire for better wet and winter traction - the ratio of road distance travelled to wheel rotation may change slightly. When I wanted to put a better tire on my mini-van, I sat down with my tire dealer and the tire manufacturer's catalogue/specifications and compared the tire diameters. Of the tire series I wanted to put on, there were none whose diameter EXACTLY matched the diameter of the original equipment tires. I replaced P215/65R17 tires with P225/60R17 and ended up with a tire diameter 0.4 inches (1 cm) smaller. This results in a speedometer reading 1.5% lower.
The GPS, since its not based on mechanical wheel rotation, may be more accurate than the speedometer - I certainly treat it as such.
It looks like we're all coming to the conclusion that a sat-nav is the more dependable speed measurement. However I think I've discovered an occasion when it might not be.
If we accept the premise that a sat-nav works by measuring your progress around the Earth's circumference, then steep hills might be a problem.
Take the ridiculous example of a 45 degree slope for illustration purposes only. According to my 'school geometry', a slope like can be represented by a right-angled triangle of sides in the ratio of 1:1:1.414, where the latter is the hypotenuse (i.e sqr root of 2).
The hypotenuse of this triangle would represent the road surface on the slope, so therefore for every 1 meter travelled across the surface of the Earth (the base of the triangle), 1.414 meters have actually been travelled by the vehicle on a sloping road. Thus if travelling at about 70 mph (or kph), the sat-nav may well be telling you that you're only 'doing 50' as this reflects your horizontal speed.
OK, I know we don't travel up or down 45 degree slopes for very long if at all, and most highways with speed cameras are usually pretty well smoothed-out, but a steep slope could make your sat-nav give you false confidence if only by being a % or two under the speed that you really are achieving.
Think of it like this - if you skydive and check your sat-nav as you plummet vertically to the ground, does it reckon you are travelling at any speed at all? I'm guessing not even though the noise in your ears tells you otherwise!