Do you not check the gas station pump's gauge when you fill it? That will be what you need to look at, how much you're putting in, not how much it actually holds. Do NOT calculate mileage based on the tank's capacity. Here's a tip for calculating MPG: At a complete fill-up, reset the trip meter or write down the total mileage. Next fill-up, divide the miles driven since the prior fill by the amount of gas it took to refill the tank completely. (for instance: 280 miles driven /
14 gallons at fill-up = 20mpg) The formula is not MPT (miles per tank) it's MPG- miles per gallon. :)
I have a '96 4cyl and I typically drive around 250-312 miles between fill-ups. I estimate there's around 2 or 3 gallons leftover once the light comes on (and I do not wait for the light to tell me I'm low.) I have NEVER run out of gas, ever. My MPG is typically 19-22 depending on whether I'm in town or on the highway more. I've been keeping a detailed mileage and maintenance log since we bought it in 99 so I figure my data is as good as it'll get. ;) A typical 'fill up' for us is anywhere from 12-15 gallons.
The common gas gauge is a bit inaccurate in that it isn't telling us exactly what's left, it's an estimate. I've never seen a vehicle that didn't have some quirks in the gas gauge- either it 'uses gas' more above the half-tank line or uses it quicker once it gets below that, or the light comes on at a quarter-tank, etc. It is not a scientific instrument.
Other info on my 4Runner: Original O2 sensors, K&N air filter (used correctly for the past 5 years,) BFG TA/KO's on alloy wheels, 5spd manual transmission, 2.7L engine, Bilstein shocks, 2 year old spark plugs. No other performance mods or upgrades, everything is stock including the brakes (yikes!) I use Chevron 87 octane gas religiously. (All hail the mighty Techron!)