While it is useful to have a historical data as a baseline, accurate data is often difficult to obtain Without a record of the vehicle's fuel consumption history, it is difficult to have a good idea what its typical fuel consumption should be. US EPA fuel mileage ratings started in 1972 and were based on FTP-72 (FTP - Federal Test Procedure) FTP-72 was originally modeled on rush-hour driving in Los Angeles and did not often correspond well with real-world fuel economy.
Keeping modern fuel consumption records is a good way to see trends in your vehicle's fuel economy. Spreadsheets are readily available and are an excellent means of tracking fuel economy, especially when their built-in charting capabilities clearly show trends. Use a GPS with a built-in trip odometer to calibrate the vehicle's odometer because modern metric tires may not have the same dynamic (rolling) diameter as OEM tires.
Many people often recall that big cars with V8 engines would often have better fuel economy than compact cars with inline 6-cylinder engines It is difficult for larger cars to have better fuel economy for 2 reasons: frontal area (physical size) and drag coefficient (how streamlined). For the same vehicle and transmission (eg, automatic vs manual), a smaller engine should theoretically have better fuel efficiency because of reduced internal friction (mechanical - piston rings & bearings, pumping loss).
Pumping loss is the work the engine must do to draw a fuel mixture across a nearly closed throttle valve and is reflected by high manifold vacuum Reducing manifold vacuum (ie, having it closer to atmospheric pressure) is commonly done by using highway gears (lower numerically) in the axle and overdrive transmissions Newer, emission-controlled vehicles also use EGR (Exhaust Gas Recirculation) to reduce pumping loss in addition to NOx control.