Having just filled up with diesel - the first brim to brim fill up on my new Scout 2.0 Tdi it was time to check the accuracy of the trip computer and the speedometer.

The trip computer merrily quoted 45.3 mpg over 427 miles of running-in but the brim to brim calculation showed 41.04 mpg. So the first thing was to check the odometer. Yesterday I drove an indicated 192 miles but measuring the distance on my OS maps showed 196 miles. Being as maps don't allow for gradients the odometer under-reads by more than the 2% this suggests. So that adds 0.82 mpg. But that still means the displayed consumption is 8% optimistic. My previous experiences have been 5% over-reading on the Mk 1 Octy and 10% on a Toyota Yaris.

When I filled up for the first time the low fuel light had come on but only 51 litres went in the tank which is supposed to hold 60 litres. This time the gauge was at the top of the red area and the trip computer said 105 miles left. At 41 mpg this would be 11.5 litres, but only 47 litres would go in. So how much fuel is left when the trip computer says 0 miles left? Obviously there is some as I have test-driven cars in this state.

Now to the speedometer. Cruise control is very handy for keeping a constant speed and taking the time on a stopwatch for 1 km on the motorway shows that at a steady indicated 50, 60 and 70 mph the speedometer is 5% over-reading on average (it worked out at 47.45, 56.70 and 66.53 mph respectively). Not bad as my Mk 1 over-read by 7%.

My long term mpg calc, brim to brim, shows roughly the same difference you have found over the trip - trip being about 7% optomistic.

My GPS MPH is always slower than the speedo but varies with speed, a steady 30 is really 26.5 and 70 is really 66 by GPS.

My Octy tends to read 3mph higher than GPS speed

Find that in my 1.9 TDI Octy 11 2500 RPM in 5th is exactly GPS 70MPH

60mph on my hand held garmin gps would be reading 65mph on my mk1 octavia

My Navman generally shows speed at approx. 5-6mph less than the speedo at around 70mph so the speedo shows 70 and the navman shows 64-65mph

Speedo's are always calibrated high so there will always be a speed and distance discrepancy.

And FYI, the fuel tank is 55 litres, 50 litres + 5 reserve. I've only ever managed to get 55. something litres in it when running on fumes and including the filler neck.

When the range has been 0 miles I have been able to get at least 20 more miles out of a TDI

speedo's are calibrated to show a faster speed to take into account of tyre wear, incorrect pressure etc. their is a rumour that speedos show higher speeds as a safety factor.

Very interesting. I have wondered long and hard if the rev counter is the best speedo.

Surely the revs would be an accurate indicator of road speed and would even compensate for tyre wear and pressure would they not?

2500 for 70 mph is very useful. Thanks.

2500 for 70 mph is very useful. Thanks.

That's in my car - double check it for your own.

Essentially the final drive is made up of the gearbox gears, gearing of the diff (if any) and the size of the wheels, so it will change as the tyres wear.

The handbook states the final drive mph for 5th in the 1.9 TDI as 28mph per 1000rpm

When I checked the calibration of my speedo with GPS this maths held true.

WRT '0 miles left'

I drove from St. Neots to Bedford showing 0miles left the whole way - when I did fill up it took 53.81l which isn't bad for a 55l tank

Speedo's are always calibrated high so there will always be a speed and distance discrepancy.

And FYI, the fuel tank is 55 litres, 50 litres + 5 reserve. I've only ever managed to get 55. something litres in it when running on fumes and including the filler neck.

When the range has been 0 miles I have been able to get at least 20 more miles out of a TDI

Sorry to disagree but the Scout is a 4X4 and the handbook says these have "about" 60 litres capacity not "about" 55 litres. The brochure says 55/7 litres for all models which presumably means 7 litres reserve. My Mk 1 had 55 litres nominal capacity but if you depressed the relief valve with the fuel dispenser you could get 64 litres in up to the top of the filler neck. The Mk 2 has no such relief valve.

Now I've looked in the handbook it says that there are 8 litres left when the low fuel light comes on.

The fuel tank level can be calibrated with VAG-COM

I took the 55/7 to mean

48 l before light with 7l left - indeed I've only ever been able to fit 53l at most in...even after driving with the (fuel gauge) needle at the stop for several tens of miles :-)

I've never got more than 55 litres in mine with the needle at the bottom of the red and the zero range 20 miles prior to filling up. The reserve must be within the the stated 55 litres. From that I presume the fuel light comes on at or about the 48 litres used mark.

Normal Octavia = 55 Litres

Octavia Scout (and presumably 4x4?) = 60 Litres.

I presumed the Scout would have the same tank.

Obviously not

I have put in over 57ltrs only yesterday so the tank certainly hold more than 55ltrs. With maxidot the low fuel warning comes on at a 75 mile range distance and knowing that there is still around a gallon left when it hits zero I never bother filling up till it hits 0.

Maybe my range was not calibrated properly, the only way I would have got 57 litres in it is a fill up and a plastic fuel can in the boot

I have put in over 57ltrs only yesterday so the tank certainly hold more than 55ltrs. With maxidot the low fuel warning comes on at a 75 mile range distance and knowing that there is still around a gallon left when it hits zero I never bother filling up till it hits 0.

Of course, we are making the 'fatal' error of assuming the fuel pumps are actually accurate!!

My fuel light comes on at 75/80 left on the 'fuel to empty' display. However I normally wait until it is convenient to fill up. that's an ambiente 1.9 tdi

Now to the speedometer. Cruise control is very handy for keeping a constant speed and taking the time on a stopwatch for 1 km on the motorway shows that at a steady indicated 50, 60 and 70 mph the speedometer is 5% over-reading on average (it worked out at 47.45, 56.70 and 66.53 mph respectively). Not bad as my Mk 1 over-read by 7%.

If you have climate control there is an option on there to see the speed read by the ABS sensors, which is as accurate as using the gps (the speed difference is built into the speedo although you could argue that it helps that if you are speeding you are not going as fast as you think you are).

to see this press the top right button in the middle cluster of 4 and the ECON button at the same time. Turn the passenger side temp dial until you see the number 19 (with a little 0 next to it) on the left hand LCD display. The right hand LCD display will show your true speed in KM/H. To see MPH turn the fan speed dial one click so that the little 0 in the left hand display becomes a 1. You will see that 40 mph true is about 44-45 mph on the speedo (useful for distance cameras I think you will agree