Ford Focus 99.9mpg at 80mph, Is the computer correct?

[MrWardle]

Was dirving my 2007 Focus Zetec 1.8 yesterday along a motorway and was reporting 80-99.9mpg on the Instant Fuel trip computer at most points. I know these things are accurate to some degree but can these figures really be believed. Understand average mpg will be lower.

Also found that when traveling 80mpg (legally) in 5th it also reported 99.9mpg, which sounds very strange since apparetly going over 60mph causes you to use more fuel.

[SD1992]

Have you worked out the fuel consumption yourself? I highly doubt the trip computer is anywhere near accurate if it is reading that.

[robsa]

yes it will report that on the 'current mpg' meter when your foot is off the throttle or you are travelling down an incline. Because you won't be using any fuel momentarily. If it is showing this for longer than a few seconds (unless you are going down a big hill) I would imagine it is wrong?

[DanielC4GP]

[Deanno1dad]

A lot of cars show this..mine does the 99.9 mpg when you coast along with no revs.

[Picard]

MrWardle said:

Was dirving my 2007 Focus Zetec 1.8 yesterday along a motorway and was reporting 80-99.9mpg on the Instant Fuel trip computer at most points. I know these things are accurate to some degree but can these figures really be believed. Understand average mpg will be lower.

Also found that when traveling 80mpg (legally) in 5th it also reported 99.9mpg, which sounds very strange since apparetly going over 60mph causes you to use more fuel.

I've had this at 70mph when I take my foot of the accelerator (new shape fiesta)...No car off the top of my head that is in production, would achieve those MPG figures sorry to disappoint!

[sospan]

I have same engine.
Mine will show 99.9 at times - when foot is off the pedal such as running downhill. I have coasted to see what happens and it goes to 99.9 then.
I am intrigued to know how it works out the instantaneous mpg.....
It must be mapped from sensor readings such as throttle pedal sensor/engine temp etc as I can't see any flowmeter being that accurate! I suspect its a calculated figure based on periodic sensor readings using a formula.

[MrWardle]

Deffnetly have to work it out manually, got a long trip coming up so perfect opportunity.

Noticed the downhill and gas off but in some cases neither condition existed. See if I can replicate it again 8) Tanks for the pointers.

[slinky]

With very careful pedal pressure, down hill, I can generate an instantaneous reading of 199.9mpg in the Fabia..

Instantaneous being the key word

[backpack]

What happens generally is that people have time to glance at the fuel consumption read outs when they are not driving "enthusiastically" or hard out. They are taking it easy and coasting and glance at the fuel consumption to "pass time". So we only generally have time to look when we are getting a higher mpg than the actual average for that given speed.

Subconsciously as well we might "help" the figure by backing off a bit too.

I only use the " whole trip average" as semiauthentic.

Incidentally on my 'modern' car a 1999 Citroen XM 3 litre V6 24 valve auto you can get 28.2 mpg on trip averages.
(Journey 112 miles on mixture of A roads, B roads and dual carriageways and a mixture of driving styles/speeds).

[FloppyRaccoon]

I've had mine showing 999.9. If only I could go everywhere downhill.