Computer Average MPG indicator Accuracy

[Daleman]

Have any forum members done a comparison test brim full to brim full miles per gallon as against the computer miles per gallon .How accurate have found the computer average miles per gallon indicator?

[Manic-me]

Last time I did it the calculated MPG based on a full tank was ~1.5mpg less that the displayed. I've done it a couple of times and it does not seem to stray much from that figure on averages between 35mpg and 45mpg.

Coming back through France on a mix of RN and motorway driving (so between 60 & 80 MPH - about 370 miles) the calculated brim full gave 49.7 mpg :-) Unfortuneately I had not reset the trip comp .... :-(

[dodgyblue]

today the computer MPG in my XC60 gave me distance to empty of 1090km .. that's after I travelled 130km after I filled up the tank to the full.

[nosmoking]

Originally Posted by Manic-me

Last time I did it the calculated MPG based on a full tank was ~1.5mpg less that the displayed

I think the actual MPG will always be less than the displayed MPG. When the car is stationary, the instantaneous display shows a dashed line and I believe the average display remains fixed at the last calculated value. Since fuel is still consumed by the idling engine, the actual value will deviate from the displayed. The longer the car is stationery (traffic lights etc.) the more the two values will deviate. Perhaps someone could confirm or refute this logic.

[wimorrison]

Spookily I have just done exactly this over the weekend!

My brim to brim comparison gives me 32mpg and the trip showed 32.4 mpg which means that the trip was reading 1.25% high for 784 mls.

I regard this as well within acceptable error as the car does not know the tyre inflation pressure, hence actual rolling circumference - perhaps if I used the 'economy' 39PSI rather than 36PSI it would have been spot on

I did the test because of the 40mpg claims that have been aired for the d5 205 AWD - I am now satisfied that the trip is sufficiently accurate for my needs and will trust it

I am also happy that my car is getting around the 'average' for this engine - as shown in the other thread it appears that 30 - 33 is the 'normal' figure that most people achieve.

[Clan]

Originally Posted by nosmoking

I think the actual MPG will always be less than the displayed MPG. When the car is stationary, the instantaneous display shows a dashed line and I believe the average display remains fixed at the last calculated value. Since fuel is still consumed by the idling engine, the actual value will deviate from the displayed. The longer the car is stationery (traffic lights etc.) the more the two values will deviate. Perhaps someone could confirm or refute this logic.

I think you will find the average fuel consumption will take into account fuel used when stopped , the engine computer carries on calculating fuel flow.
Try re-setting the average fuel consumption , with display on average fuel consumption , drive a mile down the road , at this stage the average readout will be at its most sensitive .. then stop with the engine running and watch the display , you should see it start dropping .... This is certainly the case with older model displays .

[Clan]

Originally Posted by wimorrison

Spookily I have just done exactly this over the weekend!

My brim to brim comaprison gives me 32mpg and the trip showed 32.4 mpg which means that the trip was reading 1.25% high for 784 mls.

I regard this as well within acceptable error as the car does not know the tyre inflation pressure, hence actual rolling circumference - perhaps if I used the 'economy' 39PSI rather than 36PSI it would have been spot on

I did the te4st because of the 40mpg claims that have been aired for the d5 205 AWD - I am now satisfied that the trip is sufficinetly accurate for my needs and will trust it

The rolling circumference is actualy programmed into the instrument panel there are different ones for different tyre sizes of course , this has always been the case going right back to 1986 with the 480 ... i dont think a difference of 3 psi will make any measurable difference to this figure ...

[wimorrison]

Originally Posted by Clan

The rolling circumference is actualy programmed into the instrument panel there are different ones for different tyre sizes of course , this has always been the case going right back to 1986 with the 480 ... i dont think a difference of 3 psi will make any measurable difference to this figure ...

two points - how does the car know if 17", 18" 19" or 20" wheels are fitted - it won't unless it can be changed by the dealer and the car is taken for this change to be inputted into the system.

The second is that 5% difference is sufficient to affect the tyre contact area when doing braking and steering tests and a 3 PSI difference is 8% - I maintain that this will affect the rolling circumference.

The tyre pressure variation is also a reason why you are allowed a variation in actual speed from indicated speed

I guess that is 3 points