What your dad did not understand was the torque vs RPM curve would force engine rpm in first gear to get the car moving fast enough for putting it in third gear. The engine's corresponding low RPM provides such little torque that pressing the accelerator causes spark so the fuel-air mix preignites, carbonizes and burns the valves and compression rings, and produces very hard stresses on the engine parts.
Doubtless his engine tuneup and repairs cost far more than any possible reduction in mileage when operating it according to design. He probably had to slip the clutch at low speed in third gear to keep the engine from stalling until the speed was fast enough to fully engage the top gear, thus causing extra wear of the clutch plate.
Using the machinery this way is abusive and causes early failure of critical parts, if not for him, then for the next owner.
It's not something to brag about, IMHO.
“It’s not something to brag about,”
Oh, believe me I was not bragging. I tried to explain to him what was happening but he said that I was a stupid teenager and didn’t know **** from Shinola about mechanics. ;-)