38. FUEL CONSUMPTION: How do I work out my car's fuel consumption reasonably accurately?

Fill to the brim and write the mileage on the receipt. Then do at least 250 - 300 miles, fill up again to the brim and note the mileage on the receipt. Subtract mileage 1 from mileage 2. Then multiply the litres of fuel on the 2nd receipt by 0.22 and divide the answer into your mileage. Continue brim-fill after brim-fill. Though this depends on odometer accuracy, it will give you a better idea than the car's fuel computer which is a notoriously inaccurate accessory on any car. You can check you speedometer and odometer accuracy by using a good multi-satellite GPS device, then correct recorded distances on that basis.