My Navara records it and keeps it so if you want me to prove it I can, had it for 12 months and it says the overall average for that period has been 38.6 mpg. knock 10% off and it is still circa 35mpg total average for 12 months driving which includes towing 14' triaxle.
It will be accurate pump to pump, as it uses the same odo that you do. You likely covered around 10% less miles than the odo claimed. GPS Speedo is a good way to check, compare the GPS Speed to the dash speed.I regularly get mid 30’s and I don’t hang about. I did once see high 40’s on a run, but I was trying to squeeze everything out of it at the time and was driving like a grandmother on the way to Sunday service. I have found the “dream o meter” on navara to be reasonably accurate when I tested it pump to pump.