Trip computer basis for average

[gks]

This is admittedly a completely idle, immaterial question... but I thought someone might have info! I use the trip computer (2003 v70) to check on my gas mileage average and instantaneous — just seeing a number there makes me a more efficient driver. I have seen people on here say that the trip computer may not be accurate, but for me it has tracked closely to gas mileage calculated from gallons filled/odometer.

The owner's manual says that the average MPG is calculated "since the last reset," but I am certain this is not true. I never reset my trip computer (I think it's been at least 15k mi), and the average changes much too fast to be calculated over such a long time.
For example, after a highway trip, the average might be at 27 mpg, but after a few trips in city traffic (<10 mi total), it will be down to 26 mpg.

I am assuming that it is a running average of something like the last thousand miles, the last hundred gallons of fuel, etc. Does anyone know? Of course it would be possible to figure out the answer by running a controlled experiment...

[jonesg]

I never look at those things, my eyesight washes out from bright sunlight and I can barely see anything unless at night.

[Blacklab467]

Mine appears to be very accurate as I reset it at every fill. As Canada has adopted the metric system in 1981, it displays in litres/100km. I am usually around 10.6 l/100km with mixed city and freeway driving. I usually look for a gas station at around 600-650 kms on the odometer and fill with about 63-67 litres, in that regard it is accurate within 5%.
I can't comment on the accuracy of average without resetting the trip and mileage, I'd be surprised if Volvo doesn't just run a similar algorithm based on a longer trip but I can't say for sure.

[scot850]

I recommend not running the fuel tanks lower than 1/4 full to save your fuel pump. The fuel helps cool the pump.

Neil.

[Blacklab467]

Good point, Thanks.

[abscate]

I recommend not running the fuel tanks lower than 1/4 full to save your fuel pump. The fuel helps cool the pump.

Neil.

That’s the rule in our house. Definitely no Idiot Man Lights, which looks like gas pump

[gks]

I understand the car to have an 80L (21 gal) fuel tank—but a significant part of this is "reserve" not indicated on the gauge...
I usually drive until about the point the gas gauge needle starts to dip into the red ("1/8") and will pump about 14 gal to fill it back up to MAX—usually right around 400 miles if I've been on the highway.
This would suggest that 1/8 tank according to the gauge is closer to 1/3 of a tank in reality. The red light will come on around the time the tank is actually 1/4 full (~5 gal), so perhaps for the health of the fuel pump, the tank need not stay >"1/4" full by the gauge?

I recommend not running the fuel tanks lower than 1/4 full to save your fuel pump. The fuel helps cool the pump.

Neil.

[abscate]

2 gallons in the tank will keep the pump cool. We keep 4 in the tank in winter for storm safety reasons