In theory as tyres wear, if that means more revolutions per mile, then my meter will increase the shown charge per actual mile

Tyre pressure also affects rolling radius. If one tyre is low or high, it triggers the tyre pressure warning as it can see its out. But if all four are high or low, the difference in rolling radius will be invisible to the TPMS but still cause a difference.

In theory as tyres wear, if that means more revolutions per mile, then my meter will increase the shown charge per actual mile

The charge per mile rate will remain constant, but the odometer reading will vary slightly between new and worn tyres over the same journey. That means the rate your customers pay per mile is constant, but on new tyres the same distance should be very slightly cheaper than on worn tyres, as the worn tyres have to go more revolutions to cover the same distance, the meter thinks you've travelled further. This is likely to be around 2% so little difference in reality and you may never notice it - unless you do very long distance trips.

 

The tyres probably don't explain the whole original issue, but they certainly play a good part. It is the only variable, everything else remains constant, bar faulty or poorly calibrated sensors.

 

Be interesting if you can find out exactly what your meter is taking a reading from, and what the tolerance band is.

http://kouki.co.uk/utilities/visual-tyre-size-calculator

over (say) 100 miles the meter reads LESS than the odometer by 2 miles so wont overcharge but in reality no taxi would do 100 miles on the meter it would be by prior quote (usually) and would also be well out of the vehicles licensing area so in fact the meter rate wouldnt even apply

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

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But have both vehicles the same brand and size tyres with the same amount of tread remaining and the same pressure?

 

I know on my Octavia they came ex-factory with 3 different diameter tyres and everything else was the same.  The odometer & speedometer accuracy varied greatly.  On my car (the most innacurate model), when it was due for the 15,000km service it had only travelled 14250km.

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

40.25mm of 2012.25mm is 2% isn't it?

 

OP is out 4.25%.  

 

As you said, it's unlikely one car had new tyres & the other had baldies but if there was a combination of factors such as:

• one vehicle on 7mm & the other on 4mm = 1%
• one vehicle on 38psi & the other on 32psi = 1% (just guessing)
• different brands of the same size can be significantly different in diameter (1-2%)
• Different rim diameters (225/45r17 is 1% smaller than a 225/40r18)

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:

 

 

Sorry, I hadn't seen this before.

 

So you aren't rolling on the same diameter tyres & quite likely the pressures (and brands of tyre & amount of tread wear) are different too.

 

For example a 225/45r17 (what you probably have) Michelin PS3 is 25" diameter.

 

Your father probably has 225/40r18.  In the Michelin the diameter is 25.1"  Without all the other variables, that's 1% right there.

Sorry, I hadn't seen this before.

 

So you aren't rolling on the same diameter tyres & quite likely the pressures (and brands of tyre & amount of tread wear) are different too.

 

For example a 225/45r17 (what you probably have) Michelin PS3 is 25" diameter.

 

Your father probably has 225/40r18.  In the Michelin the diameter is 25.1"  Without all the other variables, that's 1% right there.

 

 

BUT.....if original sizes (whatever that was/is) wouldnt the speedo be calibrated from the factory to be accurate for that particular size?

BUT.....if original sizes (whatever that was/is) wouldnt the speedo be calibrated from the factory to be accurate for that particular size?

you would think so but from my experience the calibration is the same for 205/55r16 (24.9" diameter), 225/45r17 (25") and 225/40r18 (25.1").  I assume this is because all three are listed under the fuel flap as "optional tyres".

 

They calibrate to the largest diameter with around a 4kph buffer between actual & indicated speed (maybe the odo is accurate for the 25.1" tyre) and the buffer gets bigger as the diameter gets smaller.  

 

On my Octy with 16" the speedometer was 10% out.  110kph indicated was only 100kph actual.  This seems pretty common with Skoda / VW where I live.  There's always a huge queue of cars behind law abiding citizens driving there Golfs on the motorway.

Errors or inaccuracy ?

 

Car set at 70mph (rare for me !) cruise control,distance covered 12 miles.

 

Car says 70 mph.

Dash cam (roadhawk hd) says 68  mph.checked at every mile interval.

Tracker says 66 mph.  checked at every mile.

 

Guess which one of the above I would use in defence of a speeding ticket !.

Our MKII is on its second set of tyres, which were @ circa 95% when we bought the car.

 

The milometer says its done 28,868 and the VCDS scan states 28,682, so a tiny differential. I'd be interested to know where the VCDS measurement is taken?  

 

The milometer says its done 28,868 and the VCDS scan states 28,682, so a tiny differential. I'd be interested to know where the VCDS measurement is taken?  

 

186 miles is a good walk tho!

186 miles is a good walk tho!

 

Yup, but its 0.07%...

Brings the service date round faster i suppose

Brings the service date round faster i suppose

Those cunning Czechs! LOL!

seriously, think about it

 

2 million skodas, all doing 98% of the scheduled mileage between services, that soon adds up

seriously, think about it

 

2 million skodas, all doing 98% of the scheduled mileage between services, that soon adds up

It's less than 1%.....

It's less than 1%.....

 

how many extra services though?

how many extra services though?

700 miles over 100,000....

mine was 5,000km in 100,000km which I thought was a bit rich.

mine was 5,000km in 100,000km which I thought was a bit rich.

Yes, 5% IS steep....