Try the one at http://www.euronet.nl/users/grantm/frans/fuel.html

If you want to do it yourself, here are some conversions for various
parts of the process. I found these at

http://en.18dao.net/Measure_Conversion/fuelconsumption

 ------------------
To change         into

miles per gallon (UK)    miles per gallon (US) multiply by 0.833
miles per gallon (UK)    miles per litre multiply by 0.22
miles per litre        miles per gallon (UK) multiply by 4.546
miles per gallon (UK)    kilometres per litre multiply by 0.354

miles per gallon (US)    miles per gallon (UK) multiply by 1.2
miles per gallon (US)    miles per litre multiply by 0.2642
miles per litre        miles per gallon (US) multiply by 3.785
miles per gallon (US)    kilometres per litre multiply by 0.4251

X miles per gallon       gallons per 100 miles: divide 100 by X
            (both gallons must of the same type)

X miles per gallon (UK)  litres per 100 km: divide 282.5 by X
X miles per gallon (US)  litres per 100 km: divide 235.2 by X
X km per litre         litres per 100 km: divide 100  by X
X miles per litre     litres per 100 km: divide 62.14  by X

 -----------------

I wrote up a little spreadsheet using these conversions. I'd email it,
but it's so simple, I'll just write it this way:

Cell    Contents
----    --------------------------------
A1    MPG -> litres/100km
C1    litres/100km (US Gal)
E1    litres/100km (Imp Gal)

A2
C2    =IF(A2;100/(A2*0.4251);0)
E2    =IF(A2;100/(A2*0.354);0)

A4    Litres/100km -> MPG
C4    MPG (US)
E4    MPG (UK)

A5
C5    =IF(A5;100/A5*0.6213712*3.785;0)
E5    =IF(A5;100/A5*0.6213712*4.546;0)
-----------------------------------------

A1, C1, D1, A4, C4, E4 are all strings

Enter MPG in A2

Enter litres/100km into A5

LoL If it isn't then the online calc i'm using doesn't work haha

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My math skills ain't what they used to be, but I don't think 39 MPG
equates to 8.6 l/100km.

I've started to keep track of my milage simply calculating it based on the
amount of gas and distance travelled since the last fill up.

I'm driving a 2002 2.0l Elantra.

I do mostly highway driving around 100-120km/h which is about 60-70mph and
get 39MPG or 8.6 l/100km for those north of the boarder like me ;-)

Those figures are based on 6 fillups mostly from about 1/4 - 1/2 empty
having travelled 2218km.

The only thing i've really gained from this is that i drive a heck of a
lot more than i thought. However, i am getting about what i expected from
the data i've seen on my vehicle.

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Well, the Scangauge I bought is working great, and I thought I'd share the
new tank of gas mileage with the group.

In -pure- city driving, and we're talking not any free speed length of more
than a quarter mile, ninety percent on stop and go with lights and
stopsigns every two or three blocks.  But decent slow driving.

At this point with close to half a tank used, 22.5 mpg.  And still very
slowly dropping.

Frowny face.

Normally it's a mixture of freeway and town, which puts it around 26 to 27,
but this stint of stop and go really takes it's toll.  The good part is
that I know it will get around 40 on the road, which makes it a little
better.

But still impressively low I think.