75mph equals 72gps?

[PAULSPEED]

Hi Guys,

I put new batteries in my hand help GPS. I was driving at 75mph and the gps said I was doing 72.1mph. What gives? Do the tires cause this. What way does these cars measure speed? Has anyone else noticed this?

Paul

[White987S]

This is very-very common. My car is off by 2 MPH too. My Porsche and my Ford Explorer.

[Westcoaster]

For what it is worth, the last time I checked it, my UMPC running Streets and Trips, showed the ground speed as the same as that of the digital speedometer.

[benaslan]

God, I've been meaning to ask this too. Mine is WAY off. I just installed the Kenwood DNX7100 and it says I'm going like 5 MPG less than I'm going. Can that be fixed? I mean hell, maybe it's good as it'll keep me from speeding a little, but over time, that's quite a bit of mileage that you "haven't" driven.

[Popolou]

Its all by design. The true speed is as recorded by the GPS. The indicated speed on the speedometer includes a margin for error that all manufacturers include now. I believe its between a 5% - 10% variation.

This is assuming, of course that the car is standard and the diff and wheels are all OEM.

Pop

[1schoir]

God, I've been meaning to ask this too. Mine is WAY off. I just installed the Kenwood DNX7100 and it says I'm going like 5 MPG less than I'm going. Can that be fixed? I mean hell, maybe it's good as it'll keep me from speeding a little, but over time, that's quite a bit of mileage that you "haven't" driven.

This will keep you from "actually" speeding a little, and it's a commonly discussed trait of Porsche speedometers.

What has not been determined is whether or not the odometers have a concurrent error. If so, it could affect whether a car is covered under warranty, and whether someone has to pay excess mileage charges on a leased car.

Regards, Maurice.

[White987S]

This will keep you from "actually" speeding a little, and it's a commonly discussed trait of Porsche speedometers.

What has not been determined is whether or not the odometers have a concurrent error. If so, it could affect whether a car is covered under warranty, and whether someone has to pay excess mileage charges on a leased car.

Regards, Maurice.

I have also used my GPS to track the accuracy of the odometer, and it too was off. About 1% on a +300 mile trip.

[PAULSPEED]

Everything else works great. It is hard for me to belive this.

Paul

[plima]

I think that all car mfgs bias the error in the speedometer to never show a speed below the true speed. This means that all speedos will show "optimistic" speed. The range is typically 0-5%, meaning that a speedo showing 60 mph means that the car is actually going somewhere between 57 mph and 60 mph. The variation on tire size alone can account for the 5% error band. The GPS will give you very accurate info on speed, assuming you drive a constant speed for several samples from the GPS (usually 1 speed update per second).

[tomnash]

The odometer bias is favorable to the car owner in a warranty dispute -- if the odometer says 50,001, you probably are at least 1000 miles over that. That avoids arguments and could be a key reason manufacturers do this.

What I wonder is why Porsche does not display the GPS velocity on the status page of the navigation system. I am sure this is available in the software and is just not being displayed for some legal reason (?). It would also be easy to give the driver the option of displaying the GPS determined velocity on the dashboard instead of the wheel rotation based velocity. That would be really nice.

It would also be nice to have a record of your GPS velocity over the last few minutes stored in the navi module in case of a traffic stop, but that probably never would be in my favor ...

[PQR5CHE]

My 997C2 was about 5mph positive on the speed when i checked it with the gps. What is funny though, is that my wife's new VW Jetta is dead on with the indicated speed... Too bad Porsche couldn't do that.

[ahklein]

My 997C2 was about 5mph positive on the speed when i checked it with the gps. What is funny though, is that my wife's new VW Jetta is dead on with the indicated speed... Too bad Porsche couldn't do that.

[ahklein]

This is my third Porsche and all have been 2-5% high in speedo calibration. My Mercedes' have all been right on. I don't know why they can't be continuously calibrated using the GPS in the car. The technology exists, but I guess Porsche wants its owners to feel they are: going faster, further and getting better mileage than they are???

[mobytoby]

I just ran the Big Bend Open Road Race on Saturday April 26th. My 2006 997 Carrera S has the Sport Chrono Plus option to monitor time and mileage but the odometer and speedometer are off so it doesn't help any. The race has a laser triggered speed measurement, not radar which can be affected by sideswipe etc. My speeds were all off, speedometer showed 72 MPH and laser measurement was 70. Speedometer reading of 117 was laser measured at 114. Length of the course is GPS measured at exactly 59.0 miles, my odometer showed 58.6 miles. The purpose of the open road race is to finish closest to an exact speed. My target was 110 MPH and I finished at 110.198 MPH. The winner, in a Corvette, finished at 110.008 MPH. A Porsche should be exactly measuring speed and distance. Why have it measure inaccurately? That seems ridiculous for such a precision manufactured machine. My PCM display showed an average speed of 107 MPH at the finish compared to the 110.198 MPH actual. The race measurements are accurate and verified by dozens of other cars with much closer times. 80% of the 152 entries were corvettes, only 6 were Porsches. We need to get the factory people involved in improving accuracy. The tolerance between a piston and the cylinder wall is measured to the thousandths, why not have the speedometer accurate too!

[tomnash]

The race measurements are accurate and verified by dozens of other cars with much closer times. 80% of the 152 entries were corvettes, only 6 were Porsches. We need to get the factory people involved in improving accuracy. The tolerance between a piston and the cylinder wall is measured to the thousandths, why not have the speedometer accurate too!

It would be very difficult to get a speedometer/odometer based on wheel rotations to be as precise as you want -- tire circumference variability and tire slip cause too much uncertainty. And the manufacturers build in a bias addressing some maybe legal liability concern.

However, what you want and what would give us real precision would be to use the GPS built into the navi system to optionally provide speed and mileage information. The usually quoted error on GPS speed (see Garmin site) is a tenth of a MPH. That would translate into road mileage errors of a tenth of a per cent at 100 MPH. Straight line distances from stop to stop could be measured with precision at the less than a meter level easily. Why this option is not available, at least on the Navi status display, never mind on the dashboard readout, is beyond me.

Edited May 2, 2008 by tomnash

[savowood]

On my 2000 Boxsera, I have the ability to turn on the diagnostics through the AC controls. I get things like RPM, various temps, and of course the speed. When I pull the speed up, although it doesn't update more than once per second, it's very accurate. I checked it against my GPS and accelerometer. My digital speedo is 2.6 mph high and the analog one is 1.5 mph high. The one in the diagnostics is dead on.

It appears most of the car's functions use the diagnostics speed to do things like the MPG and average speed calculations, as well as the odometer. Maybe things have changed in the cars since 2000, but the accuracy is there, just not in the "safety margin" in the digital and analog speedos.

-Michael

[renzop]

Think about the law suits if the speedometer was in error the other way. If I think I am driving 45 around a corner marked 45 but I am really doing 50 and I get into an accident, some lawyer could make a great case against the car manufacturer that their speedometer error caused the accident and they are liable for all damages.

Also, their are some countries (Germany and Japan, I think) where the manufacturer cannot sell a car model without verifying the speedo and it must indicate at or above the real speed.