I've noticed that my speedo is showing a considerable difference to my GPS reading. So much so that at 70mph on the speedo shows as 64mph on the GPS. This seems quite a large error for an electronic speed measuring system.

Also the average speed indicator for the car is always 38mph regardless of the trip.

I'm starting to think all is not good here.

 

Over reading by 9.38%.. can over read but not under read....up to 10%...I think that's right...someone put me straight?

Does seem like a lot for modern speedo's though.... Allan

 

Are the tires standard sizes?

 

She is running on 225/55 R16 tyres

 

seems ok to me
i use a skoda octavia for work (taxi) and mine shows from 4 to 5 mph difference on a 2011 model, sat nav is far more closer to correct speed than a car dashboard.

 

Can live with the speedo error, even more so since it over reads, but why is my average speed always reported as 38mph?

 

Have you tried resetting the counter? Once you start averaging over hundreds or thousands of miles it is not going to change much.

 

starting from zero to seventy will over a hour show avg of 35,also depends on distance and time you are doing high speeds, my day varies from 100 to 300 miles a day,some days more on motorways give a avg of 40,round town gives avg of 18,as for mpg dont go there with a jag 3.2 v8 jag does 18 to 23 mpg, taxi does 55 to 72 mpg on a 1.6 tdi 5 speed manual.

 

So this is an accumulated average until the computer is reset? Not an average over a single trip?

 

Yes...accumulated until you reset it. For example you can use the A counter for a single trip by resetting it at the start of the trip and the B counter for long term.

 

Ah thanks

 

By regulation, speedos must never under indicate- they must be dead on or over indicate. It would be unaffordable to have a speedo that's 'dead on' at all possible vehicle speeds, so OEMs take the option of having them over indicate. It's always been this way, we just didn't always have GPS units to tell us how much the error is.

Your tires may be the correct size but there's variation from one brand to another in diameter and of course they decrease in diameter as they wear which increases the error factor.

I have two sets of wheels for my car, one with summer tires the other with winters. The summer tires give an 8% error, the winters about 1%.