if you seal a can well enough you can't smell it, wouldn't it be dangerous during temp changes? i mean, wouldn't you be pressurizing the gas inside as your car warms up? even if the lid seals completely, i'd think you could still smell gas from the vent
As Andy, Greg, and Road noted above, the correct fill level is key. We used the steel NATO cans in Dubai where the temp changes were pretty extreme. The fuel would go from a nice cool underground tank to hanging off the back of a Jeep for a day or two in 120+ degree temps. We never had a leak, but we definitely pressurized the cans. No vents on these, so no smell, but overfilling would result in fuel geysers when you opened the can. Even when filled properly, we tried to refuel early in the morning before the temps got up.
I mentioned carrying the can inside as an example of how well the can works, not as a recommendation. I've done it, but I would approach it with caution. It worked out pretty well in the desert for the folks that carried the sealed can inside their air-conditioned cab. As long as you clean any spills off the outside before you lash it down, you didn't smell anything, and you were a lot less likely to get a gasoline shower when you opened it. We were just dealing with expansion from heat on those trips, expansion from altitude would obviously give a different result. And it's always a question of how you balance the risks.
There are obvious safety concerns when carrying a fuel can inside, and there's no doubt that everything mechanical can fail. When a can does inevitably leak, I'd much rather it be strapped to the outside. Crashes and rollovers offer different considerations. I don't get a warm fuzzy from having the fuel can in the cab with me, but I like that idea a lot better than fuel spilling on the ground around me before I can exit the vehicle. The can is a lot better protected and less likely to leak lashed down in the cab than if it's the point of impact in a crash or under the truck in a rollover. But you pay your money and you take your chance. Fuel is nasty stuff, and there are advantages and disadvantages to any way you carry it.
The sweet spot for me, and first mod I made to my current truck, is a long range tank, but if I have to carry more, then it's steel cans, outside the vehicle (off the rear, avoid the roof if possible), and a super siphon.