I am having trouble relying on the gas gauge.

here's the scenario, Anyone have the same.

Got my oil changed 15miles away ( went well)

On my way back the gas gauge was approaching E so I stopped at the nearest gas dock. Put 10 gallons in ($4.85 gal) Just to get me Home (nervous about the E reading) now I have slightly above half tank.

Went to the gas dock near our cottage ($3.99 special) 17 gallons to fill up.(now 40 gallons total)

So my conclusion is that 20 gallons is a half tank and E respresented about 13 gallons. Is this consistent that there is a buffer. My Chevy Tahoe when on E is empty( proceed to gas station immediately) Any Thoughts, Reading boats gauges on gas is always hit or miss.