Discrepancy between odometer and sat nav

clummzie · Post by **clummzie** » Tue Sep 28, 2010 10:13 am

Hi,

Just got back from 3 weeks in Spain and having driven quite some distance we've noticed that the odometer is claiming we've travelled further than the sat nav.

It's work out at about an extra mile for every 40.

Should I get this looked at/have the odometer recalibrated or ignore it?!

Cheers...

scanner · Post by **scanner** » Tue Sep 28, 2010 10:28 am

clummzie wrote:Hi,

Just got back from 3 weeks in Spain and having driven quite some distance we've noticed that the odometer is claiming we've travelled further than the sat nav.

It's work out at about an extra mile for every 40.

Should I get this looked at/have the odometer recalibrated or ignore it?!

Cheers...

The satnav assumes you've been travelling on a flat road with no hills so it is likely to always be behind the odometer.

If the satnav is looking vertically down at you travelling up a 1 in 1 hill (unlikely I know but it helps the arithmetic) it sees you travel half a mile, but looking sideways at the road you will see that it is actually a mile long and that is the distance the odometer measures.

briwy · Post by **briwy** » Tue Sep 28, 2010 10:34 am

I think thats fairly accurate actually.

Our old Freda's speedo (so presumably the odometer) was about 5/6% fast according to the GPS.

Remember too that the GPS reading is a flat linear reading, ie it does not take into account the extra mileage up and down hills, could account for quite a bit if you were in the hilly bits.

clummzie · Post by **clummzie** » Tue Sep 28, 2010 10:37 am

Of course, that makes perfect sense!
Thanks for the replies

widdowson2008 · Post by **widdowson2008** » Tue Sep 28, 2010 11:39 am

scanner wrote:
The satnav assumes you've been travelling on a flat road with no hills so it is likely to always be behind the odometer.
If the satnav is looking vertically down at you travelling up a 1 in 1 hill (unlikely I know but it helps the arithmetic) it sees you travel half a mile, but looking sideways at the road you will see that it is actually a mile long and that is the distance the odometer measures.

Apologise for this scanner but 'numbers' are my thing.

Theory is spot on scanner but the maths aint - sorry.
The theoretical 1:1 hill (1 horizontal and 1 vertical) gives a gradient length of 1.414....
.....and so, to use your phrasing, looking straight down, the satelite sees 1 mile, but looking sideways the hill is actually 1.414... miles long, and as you say, the odometer will read 1.414..

Coat on - I'm off........

Post by **Alison01326** » Tue Sep 28, 2010 11:45 am

Scanner, why didn't you tell us this before?!

There's been talk on here about checking speedos against sat navs, and odometers likewise, and although I'm reasonably happy that mine's not too far out, from what you are telling us there is little point in checking my odomoter against the sat nav round here! I doubt there's a level mile within 10 miles of here

Cambridgeshire, on the other hand, would be a much better place to try it

Seriously, good explanation, and thank you

Post by **francophile1947** » Tue Sep 28, 2010 12:23 pm

That explains why my satnav and speedo discrepancy varies - never thought of hills

Post by **Alison01326** » Tue Sep 28, 2010 12:30 pm

francophile1947 wrote:That explains why my satnav and speedo discrepancy varies - never thought of hills

Do you know what they are?

Post by **francophile1947** » Tue Sep 28, 2010 12:43 pm

Alison01326 wrote:
francophile1947 wrote:That explains why my satnav and speedo discrepancy varies - never thought of hills
Do you know what they are?

Of course

I live on one

Norfolk is only really flat when you get to the fenland areas, which are shared with Cambridgeshire and Lincolnshire, although the Broads aren't very hilly

scanner · Post by **scanner** » Tue Sep 28, 2010 12:50 pm

widdowson2008 wrote:
scanner wrote:
The satnav assumes you've been travelling on a flat road with no hills so it is likely to always be behind the odometer.
If the satnav is looking vertically down at you travelling up a 1 in 1 hill (unlikely I know but it helps the arithmetic) it sees you travel half a mile, but looking sideways at the road you will see that it is actually a mile long and that is the distance the odometer measures.
Apologise for this scanner but 'numbers' are my thing.
Theory is spot on scanner but the maths aint - sorry.
The theoretical 1:1 hill (1 horizontal and 1 vertical) gives a gradient length of 1.414....
.....and so, to use your phrasing, looking straight down, the satelite sees 1 mile, but looking sideways the hill is actually 1.414... miles long, and as you say, the odometer will read 1.414..

Coat on - I'm off........

I've just propped a 1ft/30cm ruler at a 45degree angle (1 in 1) on top of my calibrated paper trimmer and yes it measures nearer 8"/20cm than 6"/15cm from lower end to vertical line down from upper end.

BUT it certainly doesn't measure 1ft/30cm - how could it?

By your reckoning if I stand it at 90degrees it should still appear to be 1ft/30cm long when viewed from above and it doesn't - it vanishes.

widdowson2008 · Post by **widdowson2008** » Tue Sep 28, 2010 12:59 pm

scanner wrote:
widdowson2008 wrote:
scanner wrote:
The satnav assumes you've been travelling on a flat road with no hills so it is likely to always be behind the odometer.
If the satnav is looking vertically down at you travelling up a 1 in 1 hill (unlikely I know but it helps the arithmetic) it sees you travel half a mile, but looking sideways at the road you will see that it is actually a mile long and that is the distance the odometer measures.
Apologise for this scanner but 'numbers' are my thing.
Theory is spot on scanner but the maths aint - sorry.
The theoretical 1:1 hill (1 horizontal and 1 vertical) gives a gradient length of 1.414....
.....and so, to use your phrasing, looking straight down, the satelite sees 1 mile, but looking sideways the hill is actually 1.414... miles long, and as you say, the odometer will read 1.414..

Coat on - I'm off........

I've just propped a 1ft/30cm ruler at a 45degree angle (1 in 1) on top of my calibrated paper trimmer and yes it measures nearer 8"/20cm than 6"/15cm from lower end to vertical line down from upper end.

BUT it certainly doesn't measure 1ft/30cm - how could it?

By your reckoning if I stand it at 90degrees it should still appear to be 1ft/30cm long when viewed from above and it doesn't - it vanishes.

Not too good at words - I'll post a diagram in 2 mins

2sticks · Post by **2sticks** » Tue Sep 28, 2010 1:08 pm

scanner wrote:
widdowson2008 wrote:
scanner wrote:
The satnav assumes you've been travelling on a flat road with no hills so it is likely to always be behind the odometer.
If the satnav is looking vertically down at you travelling up a 1 in 1 hill (unlikely I know but it helps the arithmetic) it sees you travel half a mile, but looking sideways at the road you will see that it is actually a mile long and that is the distance the odometer measures.
Apologise for this scanner but 'numbers' are my thing.
Theory is spot on scanner but the maths aint - sorry.
The theoretical 1:1 hill (1 horizontal and 1 vertical) gives a gradient length of 1.414....
.....and so, to use your phrasing, looking straight down, the satelite sees 1 mile, but looking sideways the hill is actually 1.414... miles long, and as you say, the odometer will read 1.414..

Coat on - I'm off........

I've just propped a 1ft/30cm ruler at a 45degree angle (1 in 1) on top of my calibrated paper trimmer and yes it measures nearer 8"/20cm than 6"/15cm from lower end to vertical line down from upper end.

BUT it certainly doesn't measure 1ft/30cm - how could it?

By your reckoning if I stand it at 90degrees it should still appear to be 1ft/30cm long when viewed from above and it doesn't - it vanishes.

Are you disagreeing with Pythagoras Scanner ?

Post by **francophile1947** » Tue Sep 28, 2010 1:09 pm

Just ask Pythagorus

I don't think scanner was trying to be spot on, just demonstrating the principal involved

widdowson2008 · Post by **widdowson2008** » Tue Sep 28, 2010 1:21 pm

Here we go.
To my understanding, hill gradients are quoted in the format 1:100 which translated into English means for every 100 feet travelled forward (horizontal), you will actually climb 1 foot.

In your previous example of a bloody steep hill 1:1, this means that for every 1 foot you travel forward, you will climb 1 foot, as shown in the diagram, but the actual distance you travel will be 1.414...
ie: satelite sees 1 foot whilst distance travelled is 1.414
Hope this explains what I was getting at.

scanner · Post by **scanner** » Tue Sep 28, 2010 1:45 pm

Well there is where you are going wrong ..........

I said a mile TRAVELLED uphill, so now redo your diagram with the slope of the "hill" as the 1 not the horizontal and vertical distances.

Perhaps I should have said "it sees you travel 0.8(ish) of a mile whereas the side view and the odometer shows you have actually travelled 1 mile".

Where did you get the idea I had driven 1.414miles up the hill? I didn't say that.

I'm not disagreeing with Pythagorus

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