.
Okay, are you able to provide a pre-Vatican 2 theologian that gives a lower threshold? I'm not sure why you are saying the opinion of St. Alphonsus is one of the strictest. Normally St. Alphonsus was not on the strict side of most questions, and tried pretty hard to give the most lenient opinion he could.
So the 75 minute threshold is more likely the least strict opinion.
Bishop Sanborn summarizes the theological consensus at time as being an hour ... without any distinction made for, say, doubling it, or something along those lines, because you're in a car. But all these times are generally considered rules of thumb and not absolutes, since other circuмstances can often weigh in. So, for instance, let's say you're travelling, and you make a stop on a Sunday. Yes, there's a Mass 45 minutes away, but it would require you to backtrack (and let's say you couldn't find anywhere else to stay for that evening) ... things like that. Or, yeah, it may be 30 minutes away, but the weather is bad. Is it bad enough to not risk the trip? Well, it might be for someone uncomfortable driving in the weather, or someone who has crappy tires (couldn't afford better), but then might not be for someone with dedicated Winter tires in an all-wheel drive vehicle. 30 minutes might be too long for an elderly person who would be white-knuckling the streering wheel the entire time. Perhaps a single guy might risk the drive (as I often did when I was younger, freezing rain be darned), but I would never drive in those conditions with 6 kids in the car. There are just SO MANY things that could factor into a decision that, as Bishop Sanborn says, you should ask a priest if you can. But in general the Church is very reasonable and always consistent with common sense. If you make a judgement call (can't have 100 people calling the priest every time it snows, based on a weather forecast, weighing in a bunch of factors like your own comfort driving, the state of your vehicle, the temperatues, whether you have vulnerable travellers with you, whether your route takes you into the boonies where if you break down you could be stranded for a while, and you could even make the pre-emptive call because, say, the forecast indicates a chance of freezing rain by the time Mass is over.
In all cases, if you're making a decision based on reasonable considerations, and not because you're lazy or trying to get out of it, just looking for an excuse to skip ... with the litmus test in any toss-up type of situation being that you're upset about having to miss (an indication of not having ulterior motives in making the judgment) ... then the Church would not condemn you or consider you guilty of mortal sin.