"Average MPG"... Or is it?
 

How does your car's computer fair when it comes to calculating the MPG? Of course this only applies to cars fitted with computers and doesn't have to be Volvos.

Our 2005 Nissan Primera 1.8 petrol does quite well on averaging the fuel consumption, although the "distance to empty" is way off, but in fairness it goes by the reading given to the fuel gauge and it itself is too pesimistic, so it is being as correct as it can be.

The last fill up ocurred at 520 miles and took 54 litres of unleaded to fill to the brim again, therefore the average MPG for the tank used was 44MPG - whilst the computer said 41MPG! Not bad at all for a 10 year old petrol engine with almost 200,000 miles covered over its life.

At that point (where being a 62 litre tank it had over 8 litres remaining) the "distance to empty" was at zero and the fuel guage was bang on the empty line. Of course, a further 40-odd miles could easily have been covered, but that's the way with most fuel gauges, gotta prevent the stupid people from running out I guess!

So, from my experiment with the Nissan it attained 44MPG whilst the computer said it was averaging 41. Not bad really, if a little negative! Better that way though as I've heard of some cars claiming in excess of 50MPG whilst they were guzzling it back at a rate of 35!

I gotta say though, I'm loving the 40+ MPG again! The V40 was a great car but with an average of 31-33MPG I was not impressed considering it was only a 1.8 and with the auto transmission it had the same amount of power as a 1.2 manual.

On my 2009 V50 DRIVe, the true fuel consumption ranged between 0.93 to 0.95 of the indicated over my 134k miles of ownership from new. This is based upon brim-to-brim filling. So, the recorded mpg was always optimistic, but fairly consistent.
My XC60 is showing something roughly similar, but it is early days yet, just 4k miles done to date.

My S60 is a pathological liar. By some distance the most optimistic car I've ever encountered.

My Vectra used to proclaim 340 miles when I filled it up, I never got more than 220 on a full tank, it once ran out with '19 miles to go' and another day ran out 32 miles after '0 miles to go' ( I had a can of fuel as I was testing it)
I don't take any notice of the one on the V70.

When I was working, I had to keep accurate records of fuel purchased/used & mileage, and on my manual D5 XC70, I regularly got 50+ mpg, compared to Volvo's official average of around 38. Figures from keeping accurate records.

XC90 D5 geartronic me. 42 mpg. From accurate records. Volvo 34.1 ave. Working

XC60 D5 geartronic me 32 mpg. From accurate records .Volvo. 44.2 ave. Working & retired

V60. D2 Powershift me 43 mph on board computer only. Volvo. 67.2 ave. Retired

My current XC60 is optimistic by 1-2mpg. My previous V60 was optimistic by 3-4mpg so the XC60 is an improvement at least.

Hi All,

My current 2009 Model V70 2.4D Geartronic:

Volvo's Manual says 38.2 MPG.

Average MPG over 1 year of accurate records (approx 18,000 miles) = 36.9 MPG

Volvo computer says 35.8 MPG averaged over the same period as above.

In terms of mftr average vs. reality, i use fuelly and currently just over the combined average despite it being quite old! Average over 20k is about 48mpg but that count started back when there was numerous running issues.

It just did a 54.5mpg actual with the computer getting 54.9mpg over 400 ish miles, not bad for a 17 year old V70 diesel

Whenever I brim mine I reset the average, which is nearly always pessimistic - I actually get better economy that it says! By around 5% on my usual brim-full at 470-500 miles, less so if I run longer with interim partial fills before brimming it. It currently shows 44.8MPG - not bad in itself for a D5 in a heavy car. I'll post back with what I calculate when I brim it tonight.

On my regular routes, where I know I can, I deliberately spend a lot of time off-throttle on the over-run for economy. I have a theory that the under-reading of the average MPG might be because it uses the 99.9MPG instantaneous value that it shows to recalculate the average as it goes along, whereas off-throttle it's probably doing 999.9MPG or more.

340 miles? Jesus, that's pathetic - even if it were accurate!

The Primera read 490 miles when I filled it to the brim tonight and I know it will easily exceed that. I can go 500 miles without getting worried and could push 520 if absolutely necessary.