Bought a new 2021 Ram Rebel 2 weeks ago and I've got 2300 miles on it so far. Truck reads 16.3 mpg average but by hand calculations I'm getting 15.5. I've tried 87 octane, 89, and 93 and haven't seen a change in mpg.
That's an important point you made regarding hand calculating your fuel mileage. The MPG calculation function for pretty much any vehicle isn't, in my opinion, more than 85 to 90% accurate at any given time. Fuel mileage is impacted by a lot of things your Rebel isn't measuring like relative humidity, wind velocity and even the average roughness of the roads you frequent impact fuel economy and the sensors in your truck don't measure enough parameters to give you hyper-accurate data.
Good old fashioned arithmetic will get you better results. If you really want to get a pretty damned accurate MPG figure for your rig here's a good way to do it and get damn accurate numbers:
1. Fuel your truck when your fuel indicator read exactly half tank
2. Fill to first trigger pop.
3. Note how many gallons you pumped and your current odometer
4. Drive until you are again at exactly half a tank, then divide the miles by the gallons, record number
5. Repeat this at least 3 times, filling at same station on the same pump with same grade of fuel.
6. Average the 3 results and that'll give you a true real-world MPG average for your vehicle.
Or...you can accept the fact that you bought a gas guzzling V8 5.7L Hemi that sounds bas *** and is fun to drive and not give a crap about MPG because if you can afford to buy a 60k+ truck then why are we having this discussion? Me personally, that's my preferred method!