Only 47mpg on the computer but 52 at the pump?

Is it possible to have such a large discrepancy between the computer displayed MPG and the actual MPG as calculated at the pump?

Ok it might be a bit premature to ask this question because I haven't actually filled up yet to calculate it for sure, so maybe it's just wishful thinking but here goes...

Since the last fill the computer display has shown an average fuel economy of only 47 MPG. However I seem to have travelled way too many miles for that to be possible. It's currently at 535 miles with 3 pips still left on the gauge! Last time I filled up the gauge was similar and it only took ten gallons. I wont know for sure until I fill up tomorrow, but I really don't think it's going to take more than 10.5 Gallons maximum (unless the gauge is really truly bad). So I'm definitely starting to think that the actual MPG is going to be more like 51 or 52 rather than the 47 that's currently showing.

Has anyone else ever had such a big discrepancy like this?

 

Yes I know what you're saying because I had been thinking the same thing. But now the miles are just so high that it's starting to look more and more unlikely that the computer could possibly be right. The tank is only 12 gallons and at the rate I'm going I should have used 11.4 already. So how could three pips on the gauge be only just over half a gallon, it seems impossible.

Anyway one thing for sure is that something doesn't tally, so I better fill up tomorrow before I run out. Either way I'll know for sure after the fill up and I'll post back here with the results.

 

That's ok Tom, it would be pretty easy to miss since I was using miles gallons and "gas" (we call it "petrol" here). Anyway I had all may data in Litres and Km but then I decided to convert everything to miles and gallons so most readers would follow it easier.

BTW it's nice to know we don't have the rubber bladder thingy here, it sounds like it might be trouble for not much benefit.

 

Yeah I figured that's what it would be for. I thought that most conventional tanks had pretty good vapour recovery systems these days anyway.

 

Ok I've filled up and the numbers pretty much blow away the prevailing argument that this discrepancy is all due to inconsistency of the fill volume.

Now to be totally sure I drove it until the gauge was down to just one pip (but still a solid pip - not yet flashing). The distance was an amazing 1011km (628.2 miles).

Remember that the MFD displayed fuel consumption was 4.9L per 100k (48MPG) so that means it should have used almost 50 Litres (13.1 Gallons). Absolutely impossible right, since that's over a gallon more than the tank even holds. So I was right all along, this is a very genuine case of a large discrepancy and not just a case of error on my part.

So I filled up at one solid pip on the gauge and I filled in nice and slow to be certain that the pump didn't cut out too early. I filled it as much as possible (remember I don’t have the bladder) and all I could get in was 41.9 Litres (11.1 gallons). So amazingly while the MFD was showing a calculated consumption of 4.9L/100k (48 MPG) it turns out that my actual consumption was no worse than 41.9/10.11 which is 4.15 L/100k (almost 57 MPG). Wow this is much more like what I was aiming for. Though I'm much happier with this figure I'm still a little miffed as to why the MFD calculated value is so far out.

 

Good questions about the odo accuracy and rim/tire diameters. I don't have a GPS but I was going to compare the odo reading against another car this weekend. I'll post back with the results.

I just bought the car second-hand so I don't really know the full history, but the tyres are correct as listed in the specification and the rims look like standard.

Over the last few hundred km I've been checking th MFD distance display against that on the odometer and they're identical (as expected of course - it wouldn't make any sense for them to use two seperate distance measurements ). So I don't really see how the odo reading can be a factor here (even if it is slightly off) because I'm using the very same figures to calculate my fuel consumption.

It seems to me that the only variable remaining is whatever measures the fuel usage. Perhaps the sensor that meters the flow is out of calibration and the computer thinks that it's squirting a bit more fuel than it actually is?