The invention includes temperature controlled fuel systems which may or may not have measuring devices therein. Systems of the invention may be used with engines which return unburned fuel, and treatment apparatus may be included in such systems for cooling the fuel, removing bubbles or the like. In one system having the treating apparatus, a fuel line conducts fuel from the fuel supply to the engine in bypassing relation to the treatment apparatus. One concept uses a single measuring device and another concept includes two measuring devices which give a differential output of fuel measurement. Indicating or recording devices or both are responsive to single or dual measuring devices to indicate fuel flow. Also, in the fuel system which is used with engines that return unburned fuel and which uses a single measuring device, such measuring device is connected prior to the combining of the return fuel and the fuel supply for the engine. Compensating structure to correct the indications of the indicator or recorder due to variations in the physical characteristics of the fuel may also be employed in the system.