Less milage than clock says

[kingkhung]

Hello T25 lovers,
I was driving along the other day looking at my speedometer and my satnav and the speedometer was around 5-10% slower than the satnav. This got me thinking about how accurate my odometer was, so when I got home I looked up one of the journeys I do regularly on google maps and it said it was 18.4 miles. Now i've done this trip a few times everytime I have done it clocks up 20 miles on the clock.
My van has done just over 100k now (or has it) if it has been like this all the time shes only done just over 92k miles.
Does anyone have any idea about who I should try and claim these 8k miles back from???
I'm thinking about trying the trip a couple of times in a car with a more reliable odometer just to check what that says. Has anybody else had any experience of something similar?
I'd be interested in anybody elses thoughts or experience
Cheers
Leo

[Mocki]

with standard factory fit wheels and correct size tyres these vans are normally the oposite to yours

mostly seem to read 3-8mph high over satnav speed.
(Ofcourse, satnav speed and distance is only accurate on the level, as the speed is incorrect assending or decending hills due to the altitude difference.)

[kingkhung]

Sorry yes i meant over the speed of the satnav, sorry for the confusion. Im running on standard rims with standard tyres

[BOXY]

I'm not sure if the distances quoted by Google take into account changes in elevation? If you plot a track up and over a mountain does Google just measure the distance as a flat A-B, or does it factor in the says 800mtrs up and 800mtrs down the other side? I know if I use Memory Map to plot routes the total distance is always less than the total recorded by a hand-held GPS.

[kingkhung]

I cant find anything that specifies whether goggle take elevation into account, but given how detailed there breakdown of a journey in miles and times it must be somewhere near accurate. I'm starting to get a bit of an unhealthy obsession with this now.

[BOXY]

If you want to really check out the actual distance you could try fitting a cycle computer to one of the front wheels. As long as you get the wheel circumference right that should be as accurate as you'll get unless you spool out a ball of string behind you

[kingkhung]

Ive found someone on yahoo answers that says they've done a trip of under 100km (they dont specify exactly)and google directions was spot on. I'm going to have to try the trip in a different car. I dont know why this is bothering me so much, its not like me, but I really need to know now. If results are different I might have to get myself a big old ball of string.

[CovKid]

Highly unlikely the odometer will be that accurate on one that uses plastic cogs and worn ones at that.

[dugcati]

Satnav's never read your speed accurately - least not in any combination of vehicle and satnav I've every used......

[Mocki]

you mean the vehicle never agrees with the satnav?
it wont, too many variables, tyre size ( worn tyres differ plenty from new ones) tyre inflation , and thats without the age and wear on the mechanical moving parts of the speedometer.

i would think the satnav ( well my satnav) is going to be more accurate than a pushbike speedo computery thingy.