Thread: Website for fuel usage obsessives

I just had a quick look and came upon this tip, displayed at random from a selection of available tips, I presume.


Check speedo against GPS
If you are tracking your mileage using the trip meter in your car, you may find you are being short changed, on average by about 5%. If you have a GPS, do a quick check yourself, work out the difference by driving at 60mph on your GPS and compare against the speedometer in your car. Typically when doing 60mph on the GPS you will only be doing about 57mph on your speedo. So, if using the speedo to track your fuel usage, you will actually be travelling 5% more than you are tracking, making your fuel economy appear worse than it actually is.
by bearmeister on December 24 2009
this tip works for 45% of voting Fuelly members.


I can see a couple of problems with that "tip".

The first problem is that i was under the impression that almost all speedos were inaccurate in the other direction and that it was illegal for a car to be manufactured with a speedo that read slower than your actual speed.

The second problem is that speedos are no use for measuring fule economy because they measure speed, not distance and in any case, the error in your speedo is not necessarily the same as the error in your odometer.

Maybe that is why the tip works for only 45% of members. Maybe the other 55% realise that it is the error in the odometer that matters and that in the majority of vehicles, the error is in the other direction, making the fuel economy appear better, not worse than it actually is.

Yes, quite, and my odometer is 5% over reading as well as my speedo so in fact Ive been getting worse fuel economy than Ive been recording.

I live in such mammoth denial about this fact that I can actually write about it here and yet pretend it doesn't exist.

Originally Posted by JamesH

Yes, quite, and my odometer is 5% over reading as well as my speedo so in fact Ive been getting worse fuel economy than Ive been recording.

I live in such mammoth denial about this fact that I can actually write about it here and yet pretend it doesn't exist.

I actually have a way of justifying the fact that most of the time I am also in denial about my odometer optimism.

I persuade myself that nobody else is correcting for odometer error, so if I am quoting fuel consumption figures, in order to compare them with everyone else's, I need to ignore the same inaccuracy that they are ignoring.

However my odometer error is only about 3.7% while the speedo is 10%.

I've got to use the stuff so I never check fuel consumption other than when filling up to guess how much it'll take to full to the top. I'm usually fairly right at 15/100 if I've been towing the van.
Other than that I don't get too obsessive about something you must have but try not to be lead footed as that consumes more than anything.
AlanH.

Originally Posted by ATH

I've got to use the stuff so I never check fuel consumption other than when filling up to guess how much it'll take to full to the top.
AlanH.

Tracking the consumption can give you an impression of the health of the engine and some information about how the car performs in general. For example, the best consumption I got on this last camping trip was rolling along on a gravel road toward Carnegie Station. The car was badly out of tune (unbeknownst to me, apart from air intake woofing) but at 70kph it didnt really impact. Later on heading home from Kalgoorlie to Perth at 100+kmh it really did impact and had big drink. Similarly I expected terrible economy from one tank heading along the table lands "track" up north. We were crawling in Low 2nd/3rd picking our way for two days but upon re-filling I found out the consumption was normal.

Perhaps it's not so surprising in hindsight but Ive learnt my fuel consumption is affected mostly by pushing the thing through the air, less about, tuning, conditions etc. Ive never done enough beach driving to get an impression on how that affects the figures but I'll go to great lengths to avoid using a roof rack.

That's what I said James, by moderating my right foot and adjusting speed I can keep my consumption figures about the same when towing. On the open road I normally travel at 90 - 95kph which keeps the consumption reasonable and gives enough room beneath the 110 limit for others to overtake comfortably. The competent ones that is.
If my figures changed drasticlly I'd have the thing serviced and checked over.
AlanH.