A mile?
Not far enough.
Try 100miles or 1,000miles or even 10,000miles.
What's the error then?

How do you measure distance on your TomTom?
Ours only says what the Blue+Me says and that says only what the 500TA says.

Out tomorrow and my Garmin Montana is ready to record the mileages over the next few days. Maybe the whole weekend of driving back and forth.

Regards,
Mick.

Lol! Mick, me thinks you're looking far too deep into this. I suppose I could work it out reasonably accurately myself using my works van, 237 miles a day over 5 days, heck, I could even do it over a month, 4740 miles!.....thank goodness I actually get some holidays occasionally! but I've got far too much to do in a working day.

For the record, our TomTom is a GO 5000 mated (when it's actually used in the TA, it spends most of the time in my works van), to a Fiat dash port. I have no idea about the TomTom blue and me and how it interacts with the 500's systems.

ahmett said:

They can pull you over all they want but I very much doubt they can prosecute you at 78

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Legally they can prosecute you at 71mph if they want to, though this would go against the ACPO recommendation that you should only be stopped or fined if you are going over 79mph.

But there are indications that the rules might be more strictly enforced in future; make of this article what you will.

Fuel consumption accuracy based on gps readouts is also prone to error, especially over hilly terrain.
On the flat it is probably as accurate as you're going to get, but gps essentially measures distance across 'flat' ground. Go up and down a few hills and you might cover, say, ten miles while gps is telling you you've only covered, say, eight.
In hilly country, the car's distance recorder can be more accurate than gps!
You pays your money ......!

Hi guys, results of my experiment.
500TA said 40.4miles when we returned home, having reset the Trip A distance as I started up.
At the same time, I started recording on the Montana GPS. On return, I uploaded the track into my Mac.

The elevation profile takes into account the distance and compensates for the slight increase in distance from the Flat Earth system that GPS may suffer from. Also, the computer takes out any anomalies and glitches.

Difference?
Nowt!
Car = 40.4 miles indicated
GPS = 40.42 miles recorded

I am absolutely gobsmacked. When I've done this with other cars, the errors have been usually a few percent wrong - in that the vehicles always read high. It seems that our 500TA is as accurate as you can get.

Attached is the journey profile.

Regards,
Mick.

Mick F said:

Difference?
Nowt!
Car = 40.4 miles indicated
GPS = 40.42 miles recorded

I am absolutely gobsmacked.

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I've found much the same in my own experiments (but your mileage may vary according to the tyres you have fitted, their state of wear & inflation pressure).

Some folks get confused by the speedo accuracy (they are by design a few percent optimistic) and the odometer accuracy - on modern cars, the odo is often spot on.

Great info, Mick.
It would be interesting to see if that accuracy is maintained over a series of different journeys, or if it varies by much.
As JR said, tyres, pressures and wear factor all alter the results, as does the type of terrain.

Mick F said:

Hi guys, results of my experiment........

Difference?
Nowt!
Car = 40.4 miles indicated
GPS = 40.42 miles recorded

I am absolutely gobsmacked. When I've done this with other cars, the errors have been usually a few percent wrong - in that the vehicles always read high. It seems that our 500TA is as accurate as you can get.......

Click to expand...

Lol! That's what happens when you try the experiment in a Hillman Avenger or Vauxhall Victor!!

You'll find that most modern cars leaving the factory these days, the odometer when it comes to measuring distance, is pretty much spot on. The speedo reading however when indicating 30 mph for instance, the car will actually be travelling at up to 5 mph less than indicated. Police traffic vehicles usually have certified calibrated odometers for evidential reasons. As has already been stated though, actual distance, speed etc, will undoubtedly be affected by tyre inflation, wear etc.

Anyway Mick, I should thank you for leaving me one less thing to think about! Well done for satisfying your curiosity though. Fuelly all the way for me! :devil:

I'm glad you'r happy!
I drove up to Derby and back earlier in the year circa 500miles. I used my Montana GPS for the navigation into town as we didn't have the TomTom then. I never took accurate figures off the 500TA's trip meters. Pity really, because that would have been a good long test.

No plans on long drives in the near future, but if the opportunity arrises, I'll do a test.

Going back to tyres, how do the rolling circumferences vary from size to size?
Some folk have bigger wheels and lower profile tyres than our 185/55/15.

No doubt the rolling circumferences won't be "correct" for the standard odometer calibration. There must be a mean circumference that they use.

Interesting eh?
Mick.

Mick F said:

......Going back to tyres, how do the rolling circumferences vary from size to size?
Some folk have bigger wheels and lower profile tyres than our 185/55/15.

No doubt the rolling circumferences won't be "correct" for the standard odometer calibration. There must be a mean circumference that they use.

Interesting eh?
Mick.

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I'm guessing you're right. All I know is, we have no issues swapping the 15's for the 14's in winter because of the profile of the tyres. Same with my Saab, when I swap from 235/45/17's to 215/55/16's, rolling diameter is the same and therefore the odometer is pretty much spot on between both sizes. As long as you're using homolgated sizes from the handbook, odometer should be ok.

Going back to the mpg thing, we try as much as possible to avoid using the TA for very long journeys. The longest journey so far being, a 400 mile round trip from our place to Pompey. At the end of the day, my Saab 1.9 Tid averages 55 to 58 mpg on motorway runs, is a darn sight more comfortable, goes like dung off a shovel and the sound system is a lot better lol!

Strange, that. I bought our Panda Multi for economy, but whenever we set off on a long jaunt, where the economy would really count, we always use our diesel Merc.
Why not use the Merc for local hops and the ultra-economical Panda for the long runs?
- Because there's more to a journey than just economy of fuel.
I guess it's a preference many drivers would make, given the choice.

Mick F said:

Whatever the error, it is constant and additive.

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Not quite.

The error will vary continuously as tyres wear and tyre pressures change.

Even the extra weight of a passenger in the car will have an effect on the rolling diameter.

Just because it's spot on today doesn't necessarily mean it will be spot on tomorrow.

jrkitching said:

Even the extra weight of a passenger in the car will have an effect on the rolling diameter.
.

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You could also say that air and road temperatures will have an effect, because they will alter the tyre pressures (ever kicked a tyre on a hot day after a fast run? Bruisemaking) - and that will increase the rolling diameters. But that's all getting beyond the realm of everyday practicality!

Just drive the thing and enjoy it, huh?

Hi guys.
Been into this conversation a few times on the Garmin forums.

With a bicycle using a Garmin Edge GPS unit, you can buy a device that counts your rear wheel revs. The GPS unit can be set to calculate your wheel circumference or you can tell it what it is. It's just preference, but I always had mine set to "auto".

The GPS tracked your progress but your speed and distance was taken from your rear wheel. The unit re-calulated the rolling circumference periodically and adjusted the actual rolling circumference to correct the errors. At any time, you could look at one of the data fields and see your circumference. For argument's sake, let's say it was 2100mm.

When struggling up a hill, the circ would be less by a few mm's and on the flat or downhill a few mm's more. Also, as the tyre pressure varied with general leakage or through temp changes, the circ would vary again.

This never made much difference at any one time, but over (say) a 100mile ride, it could make a difference of a mile or two had you not set it to Auto. Some folk reckoned that it was more accurate to use it set to "manual", but it was a moot point and open to debate.

Of course you could not bother with the wheel system and use the GPS as a stand alone unit, but sometimes you go under heavy foliage or through a tunnel or in a city with tall buildings, and they can effect the accuracy of GPS. By using the wheel system, the speed and distance was far more accurate whether you used auto or manual. As I say, mine was always set to auto.

At one time, I cycled up to the A30 dual carriageway near Okehampton and headed west downhill for miles. Before I set off, I stopped at a marker post and reset my device, then off I went for 12Km counting the markers as I went past. At the 12Km point, my Garmin said 12Km, so it was spot on.

Now, when it comes to cars, the distance measuring system is rather unsophisticated. All it is is a detector of some sort on a drive shaft somewhere that is picked up electrically and makes pulses for each revolution. The distance readout is designed to be as accurate as they can make it, but if things conspire against the accuracy, the system cannot know.

In the old days, there was a bowden cable rotating away from the output shaft of the gearbox to the rear of the speedometer. Alter your final drive in the gearbox or diff and you needed to change the worm drive at the gearbox end to match so you could keep your speedo as near correct as possible.

These days, we have computers, so everything is perfect ........... isn't it?

Regards,
Mick.

It's all a bit OTT.
Computers may be precise, but us mere humans ain't.
Interesting thread though, i'll think of it whenever I kick my tyres.

UFI said:

The on board MPG readout is accurate so long as you're using between 2 and 25l/100km. In real life, I find it all but useless, as I spent a lot of time below that 2l/100km threshold, on my last tank it was a full 10mpg out (50 on the MFD, 60 in real life).

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Resurrecting an old thread, interested about the under 2l/100km bit, How do you know this. I find that if I pop it out of eco, on a known downward slope, floor it for about 2/3 seconds ( up to about 75+) then knock it out of gear, whilst tapping it back into eco,it does roll an increadably long way, infact one stretch, last night in pouring rain, I did this, (as I always do in any vehicle) and was astonished to see that I travelled way past my usual point for re-engaging 5th, way past, onto to the slip road, and up to the roundabout, and then slipped it into 3rd and away. Never achieved this before in another vehicle I have owned.
But my point is that in our old Yaris, doing the above, even with x,000milkes on the trip meter, I would still see gains on the readout. But in this little puppy whilst foot off throttle in gear coasting, and neutral coasting the readout doesn't seem to respond, and there is only 140 miles on it in total. Still reading 56.1mpg, but like you, when its fill up time, I would expect more than the readout shows. How many miles to a tank? its lost 1 little segment, with about 140miles on it

Mick F said:

I've found that the speedometer reads 32mph at 30mph, and 75mph at 70mph. I've not checked the distance measurement but if ours is the same as other cars I've checked, they consistently read a few percent high. Remind me to check ours asap!

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Your ODO will be correct if the wheels haven't changed, displayed speed is displayed over your correct speed with a software induced increase, if you drive by the speed direct from the ABS system and OBD port it'll show as being spot on

ah, I'm not saying the odo is wrong, but when coasting, either in gear or not, the mpg readout doesent gain numbers. Its as if...unless its using fuel, its doesn't want to acknowledge the fact it isn't. Hence UFI's comment of using less than 2l/100km...ie coasting.

yostumpy said:

but when coasting, either in gear or not, the mpg readout doesent gain numbers.

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Well, you don't coast long enough then! Try this: Reset the trip computer. Accelerate to 60 mph. Coast in gear with downshifts at 1500 rpm.

Most Easterly Pandas said:

Your ODO will be correct if the wheels haven't changed, displayed speed is displayed over your correct speed with a software induced increase, if you drive by the speed direct from the ABS system and OBD port it'll show as being spot on

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Now that sounds like an experiment!

What I need to do, is employ a suitable person to read the speedometer and also read the PC at the same time. Somehow I doubt I can get Mrs Mick F to co-operate.

As a separate issue, did you know that the speedo reads (positive) when in reverse?

Regards,
Mick.