Well the only real way to tell what your mileage is accurately is to calculate it out by tracking miles driven since last fill up divided by the number of gallons at the next fill up. While there will be minor inaccuracies in this method due to the fact you can’t fill your tank up to the exact same level everytime, but by doing this method consistently over time, that variance diminishes to insignificance the longer you keep tracking with every fill up. Fuelly.com is an excellent source of the most accurate MPG for any make/model on their website because the data is the result of tracking mpg as I just described.
In every vehicle I have owned that had an onboard mpg calculator (and I’ve had many), the dashboard readout has been wrong - always showing more mpg than what was actually achieved.
What you are seeing is a trip summary, not a tank summary. You may have very well gotten 42 mpg on that trip, but it is impossible to corroborate without actual measured data.
In my limited experience so far, the onboard computer on our 2021 Venza has been optimistic by 1-3mpg. The same is true of our old 2011 Hyundai.
I have used Fuelly to track every fill up on my 2017 Mazda. Gallons pumped divided by miles driven. IMO using that method - and especially over a long period - is the most accurate way to measure your fuel efficiency.
Any car can achieve 40mpg...once. Try doing it consistently over several tanks and get back to us.