He's making a calculation using hours based on an analysis originally written in micromorts per mile. He has to convert from miles to hours to do that. He uses the estimate you gave him which assumed 57.5 mph to be exact to make that conversion. I used 60 as a rough approximation. To calculate the miles he's using for his analysis, I have to use the same estimate. It's an unstated assumption gwern makes to get to $50k in a lifetime. A lower average speed would make Nick's argument stronger and gwern's cost estimate lower.