We have just returned from a weekend away, my son was also away with us. Drive a 2014 Superb estate 2.0 CR tdi elegance, my son drives a 2010 Superb twin door 2.0 CR tdi, we litterally travelled the exact same distance from being parked side by side in the Petrol station to parking side by side at home, no missed turns, no difference at all. We intended to fill up, zero the trip meter and refill at home to check our respective mpg and also to compare the mpg. However the mpg check pailed almost into insignificance, when we went to calculate our respective mpg's my trip meter showed a travelled distance of 135 miles my sons showed a travelled distance of 141, a difference of 5 miles travelled. We were never separated by more than a couple of car lengths between us. Has anyone any suggestions as to why this should be, clearly something is wrong.

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taking a wider turning circle could account for some of it but not that much.

Do you have both original rims and correctly sized tyres on it? If fit incirrect sizes (rins or tyres) can cause such differencies.

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Do you have both original rims and correctly sized tyres on it? If fit incirrect sizes (rins or tyres) can cause such differencies.

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Yes both cars are as they left the factory, both are totally standard with factory standard size tyres etc.

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I must check mine, I have a very accurate taximeter set by the council to work against

 

We also have a set mile to check THAT against

 

in fact, if you sweet talk a local taxi driver or your council they may tell you where your nearest is

 

 

certainly messes with your MPG figures if its out that much

Edited 5 July, 201510 yr by lichfielddriver

I must check mine, I have a very accurate taximeter set by the council to work against

We also have a set mile to check THAT against

I'm trying to find a measured mile local to us in sheffield

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BTW, thats 6 miles not 5

 

"my trip meter showed a travelled distance of 135 miles my sons showed a travelled distance of 141, a difference of 5 miles travelled"

I'm trying to find a measured mile local to us in sheffield

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see my edit to the post

And if you check it on googlemaps, how much would be?

If i am correct, abs and wheel speed sensor is defining the distance you drove.

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Is it an easy enough route to put in to a satnav or route planner.... Even if it means putting a few waypoints in to make sure it selects the same route you took.... Then check what the satnav says the distance was to see which car is out or whether they are both slightly out either way etc....

 

 

And did you both definately reset to zero before leaving the petrol station..... Your lad didn't forget did he and reset shortly afterwards?

Is it an easy enough route to put in to a satnav or route planner.... Even if it means putting a few waypoints in to make sure it selects the same route you took.... Then check what the satnav says the distance was to see which car is out or whether they are both slightly out either way etc....

And did you both definately reset to zero before leaving the petrol station..... Your lad didn't forget did he and reset shortly afterwards?

Yes we both set the trip to zero, we had our windows down checking with each other

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I'd try setting the same route in a satnav or route planner then to see what that comes back with so as to ascertain who's mileage is incorrect.... Or if they both are slightly.

New/worn tyres have a lot to do with it. I've gone to the extremes of a new tyre with 8mm tread down to the legal limit of 1.6mm.

 

Figures roughly rounded.

 

If you take a new 235/45 17" with 8mm tread it has a rolling circumference of 2021.25mm (or 79.58").

 

A worn 235/45 17" with 1.6mm tread has a rolling circumference of 1981.0mm (or 77.99").

 

A difference of 40.25mm for one wheel revolution (or 1.585"). Quite a difference.

 

The car has no way of telling if new or worn tyres are fitted (so doesn't know the exact distance travelled for each turn of the wheel) the speedometer and mileage will only ever be right for new tyres, so as the tyres wear discrepancy begins to creep in. The more the wear the greater the difference.

New/worn tyres have a lot to do with it. I've gone to the extremes of a new tyre with 8mm tread down to the legal limit of 1.6mm.

 

 

 

 

Not 6 miles over 140 miles, thats way out, 1 of the cars has an issue, over 140,000 thats 6000 miles out!

Regular readers have probably noticed that when we describe a vehicle that really gets our juices flowing, we tend to hyperbolize about the accuracy and precision with which the steering wheel and pedals communicate exactly what is happening down where the rubber meets the road. It has recently come to our attention, however, that many of the cars we like best are surprisingly inaccurate about reporting the velocity with which the road is passing beneath the tires. Or, to put it another way, speedometers lie.

Yes, ladies and germs, we are scooping 20/20 and 60 Minutes with this scandal: Speedometers Lie! Okay, "exaggerate" may state it more aptly, if less provocatively.

When traveling at a true 70 mph, as indicated by our highly precise Datron optical fifth-wheel equipment, the average speedometer (based on more than 200 road-tested vehicles) reads 71.37 mph. Wait, wait! Before you roll your eyes and turn the page, let us dig just a bit deeper and reveal some dirt.

 

Sorted by price, luxury cars are the least accurate, and cars costing less than $20,000 are the most accurate. By category, sports cars indicate higher speeds than sedans or trucks. Cars built in Europe exaggerate more than Japanese cars, which in turn fib more than North American ones. And by manufacturer, GM's domestic products are the most accurate, and BMW's are the least accurate by far. One other trend: Only 13 of our 200 test speedos registered below true 70 mph, and only three of those were below 69 mph, while 90 vehicles indicated higher than 71 mph. Are our cars trying to keep us out of traffic court?

To understand, let's first study the speedometer. In the good old days, plastic gears in the transmission spun a cable that turned a magnet, which imparted a rotational force to a metal cup attached to the needle. A return spring countered this force. Worn gears, kinked or improperly lubed cables, tired springs, vibrations, and countless other variables could affect these mechanical units.

But today, nearly all speedometers are controlled electronically. Typically, they are driven by either the vehicle's wheel-speed sensors or, more commonly, by a "variable reluctance magnetic sensor" reading the speed of the passing teeth on a gear in the transmission. The sine-wave signal generated is converted to speed by a computer, and a stepper motor moves the needle with digital accuracy.

Variations in tire size and inflation levels are the sources of error these days. Normal wear and underinflation reduce the diameter of the tire, causing it to spin faster and produce an artificially high reading. From full tread depth to baldness, speeds can vary by up to about two percent, or 1.4 mph at 70 mph. Lowering tire pressure 5 psi, or carrying a heavy load on the drive axle, can result in about half that difference. Overinflation or oversize tires slow down the speedometer. All our speed measurements were made on cars with new stock tires correctly inflated, but one might expect a manufacturer to account for wear and to bias the speed a bit low; results suggest that not to be the case.

So we sought out the rule book to find out just how much accuracy is mandated. In the U.S., manufacturers voluntarily follow the standard set by the Society of Automotive Engineers, J1226, which is pretty lax. To begin with, manufacturers are afforded the latitude to aim for within plus-or-minus two percent of absolute accuracy or to introduce bias to read high on a sliding scale of from minus-one to plus-three percent at low speeds to zero to plus-four percent above 55 mph. And those percentages are not of actual speed but rather a percentage of the total speed range indicated on the dial. So the four-percent allowable range on an 85-mph speedometer is 3.4 mph, and the acceptable range on a 150-mph speedometer is 6.0 mph.

 

Variations in tire size and inflation levels are the sources of error these days. Normal wear and underinflation reduce the diameter of the tire, causing it to spin faster and produce an artificially high reading. From full tread depth to baldness, speeds can vary by up to about two percent, or 1.4 mph at 70 mph. Lowering tire pressure 5 psi, or carrying a heavy load on the drive axle, can result in about half that difference. Overinflation or oversize tires slow down the speedometer. All our speed measurements were made on cars with new stock tires correctly inflated, but one might expect a manufacturer to account for wear and to bias the speed a bit low; results suggest that not to be the case.

 

 

Exactly my point.

It would take a leap of faith to assume of the 2 cars in the OP that 1 had brand new tyres and one had virtual slicks fitted, but ill stand being corrected

 

mines done 9000 on factory originals, i will check 1st thing out of curiosity, but seeing as my taximeter and odometer share a drive will they show any difference?

Edited 5 July, 201510 yr by lichfielddriver

i will check 1st thing out of curiosity, but seeing as my taximeter and odometer share a drive will they show any difference?

As they share a drive probably no difference but it will be interesting to know.

 

I have a Quartix system fitted to my Superb. I checked my last journey, the Quartix registered 48.8 miles, the car showed 54 miles.

I had the cruise control set to exactly 70mph on the car (I have the digital cruise setting activated in mine), the Quartix showed this to be 67.7mph consistently over several miles so not too bad, but quite different on the distance. 

My journey to work was exactly 100kms door to door in my last Superb (on factory 17" wheels). Exact same journey reads 98kms on my current car (on factory 18"s). Same inflation on both but different profiles. Not certain how much is down to tyre size though. Not as much as the OP but still a difference on what should essentially be similar cars.

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My journey to work was exactly 100kms door to door in my last Superb (on factory 17" wheels). Exact same journey reads 98kms on my current car (on factory 18"s). Same inflation on both but different profiles. Not certain how much is down to tyre size though. Not as much as the OP but still a difference on what should essentially be similar cars.

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That seems pretty accurate to what we saw (me being 'the son')! Mine is on factory 17's and my dad's is on factory 18's. Seems coincidental that you've seen the same sort of descrepency... :wonder: 

 

I checked on Google Maps last night, and point to point with the same route showed the distance to be 140miles. As my dad said, my trip read 141 and his read 135. His tyres are relatively new, whereas mine have done a bit (i'd say approx 5mm tread remaining).

 

The reason we initiated the 'test' was my mpg was reading 10mpg higher consistently on the journey down and local running than his. Actual difference was approx. 3mpg, mine reading 0.5mpg low - 71mpg Glouchester to Sheffield!!

Right, think on this....

 

My taximeter went in when the car (and therefore its tyres) were new.

 

The meter may pickup info off the wheels via the same source as my odometer but the installers set the meter according to a set mile and they have been doing the job since Noah built the Ark

 

Today i set the meter before moving and also zeroed the main trip (i.e. not the Maxidot)

 

Taximeter total miles at end of day 177.2 Odometer miles 179.2

 

at 100 miles it read 101 miles

 

Now if the discrepancy was due to my 10,000 miles of tyre wear both would be out wouldnt they?

 

Over to the clever ones here cos that aint me

DOES the info source for MPH relate to the diameter of the tyres or to something else, like ABS sensor ring rotations? 

DOES the info source for MPH relate to the diameter of the tyres or to something else, like ABS sensor ring rotations? 

Even if its ABS sensor rings or anything else like that one revolution of the wheel is one revolution regardless. The only difference here is the circumference of the tyre, which varies.

Now if the discrepancy was due to my 10,000 miles of tyre wear both would be out wouldnt they?

They should still both be out by the same amount, whether on new or worn tyres, the relationship is still the same.

 

In other words the error should remain constant, although the distance travelled may vary.

 

I wonder how your meter picks up the distance and how they calibrate it. Maybe someone here knows.

They should still both be out by the same amount, whether on new or worn tyres, the relationship is still the same.

 

In other words the error should remain constant, although the distance travelled may vary.

 

I wonder how your meter picks up the distance and how they calibrate it. Maybe someone here knows.

 

id imagine it was set to start and end over the measured mile and knows how many turns of "whatever" that was, now if the "whatever" was the outer edge of a tyre that could alter with wear, maybe if something else it would be constant, im not an engineer so no point me speculating

 

HOWEVER, as taxi fares are a matter of legislation and control by a council and possibly trading standards as opposed to the "simple" odometer one does wonder....

 

I will email my meter fitter