On-board computers are notoriously inaccurate - particularly Hyundai in my experience.

Use brim-tobrim and a calculator - but expect significant variations from one tank to the next if usage pattern varies - my Santa Fe varied from 25 in short urban runs to 35 on long motorway runs

Thats very interesting Edlithgow, so the OBC is wrong from the factory and should be completely ignored unless known as updated to UK gallons.

A likely culprit being rear brakes binding enough to make a major effect on fuel consumption, i would have thought would be making enough heat and smell to be easily noticed by an owner.

Something i do regularly both at work on my lorries and in my own cars, is during a long motorway run where no braking has been required i'll stop at a convenient service area by gear downchanging without using brakes at all where possible except to bring the vehicle to a final stop (if you stop in a quiet part by using full lock no braking at all should be needed), then immediately go round all the wheels and hubs and have a feel for excess heat from any one of them, anything from a binding brake to a bearing getting past its best will show up by being hotter than the rest which should all be mildly warm.

Edited by gordonbennet on 17/05/2016 at 10:20