Heres one to chew over, how accurate is a satnav. I checked my speed today, 60 on the speedo, 55 on the Tomtom. 50 on the speedo, 45 on the TomTom. I'm inclined to believe the satnav. as its accuracy as far as positioning goes is spot on. Therefore the speed it shows must also be accurate?. I'm running a 300TDI Disco on part worn 215/80 R16 tyres, bit of an odd size I know but they were on it when I bought it. Could this be an accurate way to check our speedo's.

 

the wrong size tires will throw off your speedo!

 

I have heard of this before. And it was exactly the same as what you described. Do you have a DI or DII? In the other case it was a DI and its speedo tended to be 5 MPH more "optimistic" than the satnav.

 

we've definetly had this discussion before. wasn't it depending on the age and mileage, the speedo cable has a tendency to stretch and become slightly inaccurate? i think the wrong tire size is a culprit as well!

 

I think I'd rely on the satnav more than the speedo. But I guess that would mean that your M.P.G is even worse than you thought. Yuk!!! that sucks. Unless your odometer is right

 

dizzy-I also have a TomTom and the speedo on my '97 is the same way. The odometer is spot on but the speedo is fast by 5 everytime.
When my son took his drivers test he hated it because he had to go by the speedo, he was being passed by everyone, but in order to pass his test...
Also, is you go to TomTom's web site you can download (free) a Land Rover startup screen, it's pretty cool, you turn the unit on and it shows the LR emblem across the whole screen.
My wife says that I have a problem.

 

I deal with GPS a lot for my work (hydrographic survey) and there are some inaccuracies with GPS speed as you are actually navigating on an 'imaginary' surface. The speed going up or down a hill will not be as accurate as on a flat surface, and you will also need to maintain speed to get an accurate reading as the speed is calculated from taking a position fix every second.

That said, I would imagine that even with these snags the GPS would be more accurate. From personal experience I generally find in whatever vehicle the speedo reads higher than a GPS.

 

tires... even tire wear can effect this, an 8th inch of wear means about 3/4s a inch difference per revolution static tire movement, now with about a 28 in tire, youre looking at over 700 wheel rotations per mile, with that kind of tire wear youre looking at about a 4 percent loss in velocity per rotation, which means a higher speed displayed on your dash. so at 60 mph, youd be seeing about 63 on the dash. this is the reason that the newer rovers with factory nav actually calibrate their speedos for tire wear (based on gps, by the way) so even if you put 24s on a 06 range, your speedo will be correct. anyway, go with the gps to be exact, assuming a good unit

 

Interesting response, By the way Cameron its a 96 D1. I thought tyres would have a lot to do with it, I am going for a new set soon so it will be interesting to see the difference. seems a good way to calibrate your speedo though. bearing in mind what Britishdisco says I did check it over a fixed speed/time on the motorway. Makes sense why I'm always getting tailgated though.

 

Interesting, so far everyone who has mentioned this problem has a DI. It could be a mix of the older speedo Vs. the newer technology along with the tires, ect. throwing it off. And yeah, that would explain why people are tailgating you, unless you have a really small bumper sticker that they just HAVE to read.

Anyone have a DII with this problem?