You're right you know.
According to Yahoo the thermal coefficient of expansion for diesel is 0.00082/C, so on a 60l tank of fuel, assuming a 10 deg C temperature difference and a price of £1.35/l, you can save about 66 pence on a tank. This is about 1 percent, or a THIRD of the topically discussed hike in fuel duty coming up.
(.00082 * 10 * 1.35 * 60 = 0.6642)
For petroleum it's even a little better!
I'm out of work and I miss science btw.

Edit: Just seen the above post, so I'm obviously assuming uninsulated tanks.
( P-MONKE We're all fucked, Wed 16 Nov 2011, 16:42, 1 reply)

I don't think insulation is really gonna make much difference in your scenario.
Once you get more than a couple of meters below ground the temperature is pretty stable year-round.
You might save more by filling up shortly after a delivery in the winter (before the delivered fuel has warmed up to 8-10c), or a day or so after delivery in the summer (to allow the delivered fuel to cool down).
I dunno, I'm freestyling here...
( blaireau69 , the Cumbrian Travis Bickle., Wed 16 Nov 2011, 18:07, closed)

It does make a difference with LPG
Autogas is almost always kept in above-ground tanks, and there can therefore be substantial temperature variations. So if you fill the car's tank to the limit (there's a cutoff) first thing on a cool morning and then leave it parked in hot sunshine, you're quite likely to lose gas as the safety valve vents it. Which is fine as long as there are no smokers or smouldering dog-ends around ...
(ubergeekian, Wed 16 Nov 2011, 22:12, closed)