Milometer Accuracy.

[Andrew T]

A couple of weeks after we got our Galaxy we visited friends in Cornwall, and I noticed that the Journey was 25miles nearer than it used to be in our Citroen. I've finally managed to run some tests using a borrowed GPS and sure enough the Milometer under reads by 6.3%. The speedometer over reads by just over 1mph at 30,40,50,60 and 70 MPH. The fuel consumption feature on the trip computer always seemed to over read by by 5% or more compared to a manual check, but may really be accurate based on the true milage covered. That should delight the V6 owners.

Any guesses why this should be ? Tyre wear means that such a device cannot be completely accurate, but according to my CSE grade 5 maths (equivalent, I think to the modern A*) the difference in rolling radius between a new tyre with 7mm of tread and one with 2mm is less than 2%, my tyres are at 4mm.

Has anybody else checked their accuracy ?

[seatkid]

What makes you think that distance covered as reported by the GPS is accurate?

GPS positional accuracy and speed are fairly accurate only with good satellite view. Distance covered is a different kettle of fish.

 

Likely both methods of measurement are out.

[Andrew T]

Fair comment, although I think the GPS gives a warning if it cannot acquire sufficient sattelites. I measured a few trips each of around 20 miles and 6.3% was the average. Accuracy varied between 5.63% and 6.58%, I put the variation down to the milometer only registering to the nearest 10th of a mile.

[seatkid]

Have you the same original spec tyres fitted? (was original 215 60 R 15? or 205 60 R 15?) Clock changed?

Thinks maybe the clock can be programmed for different tyre size as different variants came with different tyres..

 

Size - Wall Height - Diameter - Circumference (inches) nominal 8mm tread

 

195/65/R15 - 4.99 - 24.98 - 78.48

205/60/R15 - 4.84 - 24.69 - 77.55

215/60/R15 - 5.08 - 25.16 - 79.03

215/50/R15 - 4.23 - 23.46 - 73.72

[Andrew T]

That's interesting, its fitted with standard spec Ghia 16" alloys with 215/55 x 16R Dunlop SP2020s. I make that 4.66" sidewall 25.31" dia 79.52" circumferance. That means it is7% bigger than the smallest size you qoute. Do we get a "one size fits all" milometer calibration from Ford ?

If correct, my 99500 miles is really 106K. :D

[Guest Crimz00]

If you use maths you can show that the difference between a new set of tyres and a "just legal" set can be quite a bit

 

We all know that the circumference of a wheel is 2PIEr (cant do the pie sysmbol on this forum).

 

So a new tyre say a 195 65 15 is about 615 mm in diameter lets for argument say that its:

620mm new = the circimference is 1947

614 mm fully worn = the circumference is 1928

 

Thats a difference of approx 1%.

 

Think on how much difference it will make on how much presure is in the tyre (and how laden the vehicle) as this will alter the effective radius of the wheel, as distance from the ground to center of wheel is changed.

 

I have a PPC GPS System and my Galaxys spedo shows as follows:

 

@30 Mph on the spedo the GPS shows 26-27Mph

@50 Mph on the spedo the GPS Shows 45-46 Mph

@70 Mph on the spedo the GPS Shows 64-65 Mph

@90 Mph on the spedo the GPS Shows 83-84 Mph.

 

on my mondeo 2.0 Zetec. its worse

@100 Mph on the spedo the GPS Shows only 90-91 Mph

 

I prefer the GPS as accurate and quite happily cruise past cop cars doing 65 on the motorway when no one will dare go past them.