Here we go, another thing involving calculations and guesswork!
Fuel gauge, forget it!. If you rely on this for any estimation then you will end up in the poop. Get a notebook, fill the tank and note the mileage. Front tyre size plays a huge part in the calculations. A 165/50 x 15 as I have on the front means a reduction of about 16% on what is seen at the speedo. (so for 100 miles clocked, call it 84)
The tank holds (I believe) 60 litres which at 4.55 litres per gallon equals roughly 13 gallons. 25 mpg I would expect on a run at least, so 13 times 25 miles = 325 miles. I have calculated 18 mpg when messing about close to home and it not getting warm etc.
This weekend have done a nominal 600 miles of varied driving, primarily cruising, and got 26.4 mpg. Should be and is usually better than that. I expect 27 to 29 from past trips.
As a rule of thumb, you should get close to 300 miles on a tankfull in my opinion even when not being too carefull.
Just factor in the mileage difference when looking at different size tyres. Try hhtp://www.tacomaworld.com.forum.tirecalc.php" onclick="window.open(this.href);return false; for calcs.