PRO RC-SHOP   (GREAT VALUE ITEM THAT EVERY NITRO RACER SHOULD CARRY)
connect your fuel line to the end of the measure and suck out the rest of the fuel after your run to see what your fuel mileage is. 
 
This is the perfect size tester, hold a maximun of 43ML of fuel & will get you that precise measurement that you are looking for, small in size so it does't take up much room in your pit bag.
 
Sorry no returns or warranty once installed into the vehicle.