My husband Jeff works for a man named Terry. Terry believes that Jeff is using company vehicle for personal use and not putting gas in it. Another words Terry is accusing Jeff or steeling gas.

Jeff states that he always put gas in the van even if it's just five dollars. Jeff believes that if only five or ten dollars is put in the tank that it gets less mileage than if the tank was full.
His boss Terry is challening him.

So the question I have is do you get let gas mileage if you only put five or ten dollars in virus fulling it up?

Please help.

No gas milage is gas milage and the tank being full or empty has little to do with it.

One could show ( although it would be very minor) that in fact the additional weight of the extra gas may actually give lower gas milage but it would be so minor I doubt it could be measured by standard means.

Jeff needs to keep better logs of his busienss and personal use. Which is he requried to do so by IRS rules. Since any and all personal use of the vechile would be taxable.

In theory you'd actually get better mileage by keeping the tank near empty, not worse milaeage. This is because the net weight of the vehicle is less, so less energy is needed to accelerate the vehicle.

But in practice - the difference in gas mileage is way too small to measure accurately. The weight of, say, 10 gallons of gas is about 83 pounds. If the van with its passengers and cargo has a nominal weight of 4000 pounds, this 83 pound saving in eight by keping the tank near empty amounts to a 2% weight reduction overall. This reduction in weight helps save gas when accelertating (and saves on brake wear when decelerating) but the majority of the vehicle's energy actually goes toward overcoming aerodynamic friction and the internal friction of the engine and drive train, powering the A/C, etc. Saving 83 pounds of weight does nothing to help these factors. So your actual mileage improvement would be much less than 2% - probably around 1/2% or so. Bottom line - putting in gas in small amounts and running the van near empty has no appreciable effect on gas mileage. Much more significant is if Jeff drives with a heavy foot, tends to drive at high speed, or drives more city than highway - all these factors can significantly affect gas mileage.

As already answered by others : the percentage of petrol in the tank has very little influence on the mileage.
Mileage depends on total weight and on driving habits. Putting your foot down on the accelerator only once for 10 seconds has much more influence on a bad mileage.