S40: Is Trip Computer Accurate??

[Volvophile]

My driving is mostly urban short journeys, which I know are never good for a car's fuel economy anyhow in the first place. It's just my average economy reading lately has been 22 mpg and I have struggled to better it. I would expect this sort of consumption from the larger and heavier five bangers, but not the 1.8 straight four in a car as light as the old S40.

I have changed the thermostat in the past and maintain the car to a very high standard.

Could there be something wrong?

[Jason989]

My trip computer seems to be fairly accurate. Have you tried working out the mpg yourself? My previous S40, a 1.6 used to struggle to do about 25mpg (over the course of an entire tank full, with mainly urban driving and some motorway driving.)

[ITSv40]

The trip computer on my old phase 1 2.0lt showed 31mpg and 33mph average over the life of my ownership 300,000mls and mixed town and motorway driving. The average never changed much even when I made a conscious effort to remove the lead from my right shoe.

My phase 2 2.0lt shows the same averages over the 10,000mls I have covered.

[happy diver]

I pretend that temp mode is MPG and it helps me forget
the almost fire hose flow of fuel pumping out the exhaust.

[Uncle Tom Cobley]

No.

The cumulative mpg figure shown on the display is certainly not accurate, although it does appear to be consistently inaccurate.

I’ve had two V40’s now. On the first one the mpg figure displayed was optimistic (i.e. the actual figure was lower than shown on the display) by about 10% while my current V40 display is pessimistic by almost the same percentage.

I know this as I keep a note of the quantity and mileage each time I fill the tank (full) and calculate the accurate mpg from these.

Any figures of mpg quoted by people based on the display are almost certainly not likely to be very accurate!

UTC

[bobthecabbage]

Mine has never been more than 2mpg out.

If you reset it each time you fill up it will be more accurate.

[Uncle Tom Cobley]

I do reset it each time I fill the tank.

I’m not suggesting that all displays are inaccurate by 10%, one was optimistic and the other pessimistic, yours appears to be about 5% out. Simply that the display cannot be relied on to give accurate information and I’m quite sure the degree of accuracy varies car by car.

To be confident you need to fill the tank to the brim each time and even then the mpg may vary from tankfill to tankfill depending on the many variables which effect fuel consumption. Any calculation based on less than a tankful is worthless. You really need to take the average over several tankfills.

UTC.

[bobthecabbage]

I don't know how the car measures the fuel used and whether that is the same for petrol and diesel. It could be that one is more accurate than the other?

I know that my sisters petrol fiat is sometimes 100% out on the instantaneous readout which suggests the ECU is counting every signal twice (or the signal quality is poor).

As a rough guide as long as you brim the tank both times for your calculations you should get a good indication of whether the cars readout is miles out or not.

As for what could be wrong, its a very long list!

My V40 is not as economical as some other peoples but I haven't found a reason why yet.

You could start by looking at the codes to see if anything emissions related has been logged.

On petrols the lambda sensors can have a large affect on fuel economy and I suspect the temperature sensor is something else to check.

Other than that binding brakes maybe? Even a slight drag can have a big effect.

On the MOT they are supposed to check for brake binding by applying the brake and releasing it slowly on the roller brake tester without locking the wheels. I think they have to be pretty bad to fail though and I know they don't do this test in my garage anyway.

I noted that one of my rear wheels was harder to turn than the other and that the brake pads were more worn on that side so I replaced the calliper.

Fuel economy seemed to go up briefly but now seems even worse so maybe it wasn't that! My brakes work better though.

If your wheels all spin fairly freely, I'd look at the lambda sensors and temp sensors but after that you'll have to turn to a fellow petrol head as I've not done much with petrols.

[Volvophile]

I noticed a contributory factor to my poor fuel consumption; the tyres were terribly low PSI all round since I last check them.

Either way, there's no getting away from the fact that these cars could be better on fuel given their size and weight!

[tutike]

Appologies for the necro, but just wanted to share my experience with the fuel estimate:

In my tests the displayed value is at least 2x higher than the actual fuel use - it's displaying 35 mpg, but at the pump I've calculated somewhere around 16mpg.
(Using my 1.8 petrol for about 5-10 minutes at a time, almost constantly cold engine, mostly city driving)


Is it normal that the displayed value can vary that much? Sometimes climbing a hill, flooring it in 3rd gear the instant reading shows 30mpg, that can't possibly be right.