You can read about it in full on the Toyota iQ blog, but in short:

We set off on a route that aimed to take us through the centre of 18 cities in England and Wales. The car’s official figures are 65.7mpg (imperial) with a 7 gallon (32 litre) tank, so its theoretical range is 462 miles.

Over two days’ driving, we made it to our 18th city – Oxford – took a photograph, and then ran out of petrol two miles further on. We’d covered 504.2 miles on a full tank, despite having driven through bad weather and the traffic of 18 city-centres, so we managed nearly 72mpg (imperial).

That was it, though, we filled up and headed home after that

You’re quite right about the economy gauge. We’ve asked our technical contacts within Toyota why it doesn’t go above 60mpg, and we hope to have an answer soon – please let us know if you hear back before us!