GPS Speed disparity | SatNav Discussion forum (Page 2)

r.meeder3

GPS: Nüvi 765T

speedo

My car is about 8% off above 100km/h.
Allmost all cars from the VW group (Audi, Seat, Skoda, VW, etc) have deviations of about 10% above 100 km/h.

They create this deviation on purpose to prevent the speedo to give you a speed that is to low which (here in the Netherlands) is not allowed.

When I hook up my OBD-II diagnosis unit to my car the speed indication is accurate to about 2 km/h from the GPS unit which shows that the deviation is added deliberately by the manufacturer. Your speedo allso changes with changes in tyre pressure, tyre wear and the use of a wrong tyre size.

Red

Guest

Car speedo isn't supposed to be exact, GPS is

The speedometer in a car is designed to give you a good idea of your speed--not the exact speed. In fact, it can't tell you the exact speed because that depends on the diameter of your tires, and that changes as the tire temperature changes, the air pressure changes, the outside temperature changes, and the tire wears over its useful life.

So if your car speedometer was calibrated--and there are speedometer shops that routinely calibrate police speedometers and can calibrate yours too--the calibration would still change a bit as temperatures changed and your tires wore down. Or you put on new tires that weren't the exact same size. (Like changing from snow tires to regulars or vice versa.)

On top of that, cars are built to a price, they don't waste a lot of money on speedometers. They are meant to be accurate to something like 5% plus-or-minus and because car makers don't want you to sue them ("But Officer I wasn't trying to speed, the speedometer was wrong!") they usually set the speedometer to read slightly faster than the real speed. Cops know that, in a 65mph zone you usually can drive 69, tops 70, and they'll let it go because they know the speedometer just isn't that accurate.

I think Honda actually got into trouble over this last year, because when you set the speedometer to read a little high that way, it also means the odometer is counting miles before they really have been driven. And folks who were denied warranty coverage on repairs because they were just over the mileage limit? Got together and sued the car maker, saying that since the speedometer and odometer are built to read high--they were entitled to warranty coverage beyond the actual number shown on the odometer. They won.

Bottom line? The number on the GPS is the accurate number. GPS is built to show accurate speed, cars are not.

pablo_gsxr750

GPS: Garmin Nuvi 1390T

GPS Speed disparity

As a qualified engineer the word for you all to understand is…

'Tolerance'

or to be more exact…

'Tolerance Measurement Allowance'

Therefore, if in any doubt motorists should have their Speedo's calibrated to ensure they are as accurate as is reasonably possible (Every Police vehicle has to have its Speedo calibrated before it is permitted onto the road).

Your GPS is by default more accurate unless you've had the Speedo calibrated but, not infallible!

Cheers P

Smile

johmazz2006

GPS: Garmin Nuvi 765, Garmin 3950LMT, Garmin 2455LMT, Navman40, Garmin StreetPilot 3

gps speed

Just bought the latest Suzuki sx4 S for the wife and the speedo shows 100kph and actual speed is 95 kph. Evil or Very Mad

Being the 'S' model with larger wheels, my bet is the speedo is calculated to suit the standard model with 16" wheels.
Not a good look with todays technology. Standards should be 99% accurate at purchase.

Optimize

Guest

Confirmed

My Garmin nuvi 360 shows a consistent 3mph less speed than my BMW speedomenter. I can confim that my cards dial is 100% accurate as when I got a ticket from a cop, he measured the speed with his radar gun same as my car, while the Garmin max speed in the trip log was 3 mph leass than what I was ticketed for. I wish garmin reimburses me the ticket cost Laughing

hfa8592

Guest

SPEED CALIBRATION

There is no doubt that your sat unit is more accurate than your spedometer. As one user commented, the circumference of your tires make a difference in the read-out of your speedometer, but there is also the wear factor. Even original tires can be off due to wear, and mechanical equipment gets old when there are moving parts involved.

That said, the thoughs about the sat unit being off when climbing or descending hills is also flawed -- most modern sat units know your elevation relative to sea level. Therefore it stands to reason that your changes in elevation are taken into account when the unit is calculating your speed.

Bottom line -- your speedometer is wrong, your sat unit is right!

Good luck,

hfa8592

johmazz2006

GPS: Garmin Nuvi 765, Garmin 3950LMT, Garmin 2455LMT, Navman40, Garmin StreetPilot 3

Re: Confirmed

Optimize wrote:

If your bmw speedo is 100%, why look at GPS speed ?? Good luck with expecting someone else to pay your fines for you. Laughing

brendan.hobbart

GPS: Garmin Nuvi 3790T

Re: speedo

r.meeder3 wrote:

Not so sure about this when it comes to all cars from the VW group. Here is a list of the VW/Audis I have had over the past few years and the associated speedo diffs vs GPS :

2006 VW Passat Wagon : about 2-3mph faster on speedo at 70mph
2007 Audi A3 : about 1mph faster on speedo at 70mph
2008 VW Jetta : about 1mph SLOWER on speedo at 70mph
2010 Audi A4 : less than 1mph faster on speedo at 70mph

Basically, various vehicles even within the same manufacturer will show different results for all the reasons already mentioned in previous posts. I trust the GPS when it comes to avoiding tickets......

ross.bowden3

GPS: Nuvi 760

Car Speedo Vs GPS

Hi,
My understanding is that a car speedo must be set by the manufacturer to be 2% - 4% in error. Eg if your speedo shows 100km/h your actual speed would be 96-98km/h.
The reasons are: slight differences in wheel diameter by using lower or higher profile tyres would cause a degree of error, so the manufacturers allow for this degree of error. It also means you won't get a ticket if your are doing the signed limit, according to your speedo.
Police will check your speed with the car manufacturers settings taken into consideration with their radar units.
I have heard of drivers being booked for exceeding the speed limit and using their GPS as a defence, which unfortunately did not hold up in court.
Trust this helps.
Cheers.

harry3333

GPS: Garmin nuvi 765

Re: Car Speedo Vs GPS

ross.bowden3 wrote:

Hi,
Police will check your speed with the car manufacturers settings taken into consideration with their radar units.
I have heard of drivers being booked for exceeding the speed limit and using their GPS as a defence, which unfortunately did not hold up in court.
Trust this helps.
Cheers.

Does the motorist need to provide evidence on their GPS to prove their innocence? If so, how to do it? A couple of days ago, I was travelling about 10kph below the speed limit and was shown on my gps, but I saw the police shooting his radar gun at me. Am I going to be booked? How long does speed ticket usually takes to reach the motorist?

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