Every time I log onto fuelly.com, I have to click the UK button at the top left of the screen to get correct figures. Why does fuelly not remember this between sessions? I have cookies enabled for the site and it remembers all my other details; so why not this preference?
When you click UK in the upper left corner of the page, Fuelly sets a cookie on your machine that never expires. That cookie tells Fuelly which units you prefer.
Since it's not sticking for you, I'm guessing that the cookie isn't being set. The first thing to try is using another browser. That way you can at least narrow things down to a problem with one. If you try another browser and still can't get the preference to stay, start looking at the network level. For example, do you have any anti-virus, firewall, or security software that might block certain cookies from being set? Try disabling it, choosing UK, and see if it works.
If you can narrow things down to one particular browser, you might start looking at browser extensions. Some extensions such as Greasemonkey and NoScript can interfere with cookies being set. So you might disable those and try again.
It is the same for me, last time I fulled up great, not done anything to the computer and now it lets me enter the data in UK units but when I go to the vehicle page it is back to US, time to find a new site I suppose, I have bee recomended this one http://www.torquecars.com/tools/uk-mpg-calculator.php