I've done a ton of research on this topic and many have found that there is an error of +/- 1 to 3 MPG with the dash reading based on different claims around various Jeep forums. Which, doesn't really surprise me.
The most accurate way to record this is via spreadsheet, and hand calculation
Above is a quick mock MPG log I made. Basically, you just need to log your previous and current mileage... and literally fill your vehicle up to the brim. You can drive it cautiously, and test it. You can gun it around town all day and test it. The whole point is that you're cross testing what you're normal habits are and seeing what a ball park really is. Then, compare that to your dash and maybe you'll surprise your self and you'll get more (or less) than it claims!
I LOVE this kind of testing, for some reason. On long road trips in college, my room mates would make fun of me for doing this. But I legitimately enjoy looking at data like this :2thumbsup:
You don't even really need to do a long trip either. Pick a 50 mile round trip you'd normally do around town. Or pick a day you're driving to work and fill your Jeep literally to the brim... fuel should be coming out of the filler. This way you know you've filled it with (around) 22.5 gallons. Then run your desired trip. After your trip, fill it back up as close to full as you can again - this will help you know exactly how much fuel was used in the trip.
Then take how many ever gallons you put in the tank and divide that by the trip mileage and you've got your MPG!
