Fuel Usage Calculation?

So I've been driving the V8's recently. I have been doing 30 min races which equate to about 24 laps. I put in fuel for 30 laps (6 extra) and I constantly run out at lap 22? The fuel usage is normal. I have to pit but the AI doesn't?

Is it my driving or are the "gallons/laps" that are shown in the garage not accurate?
 
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