Toyota Prius gas cap
CrasherCarl: EDGR Engineering, Design and Government Regulations
Why are some fuel fillers on the opposite side from the driver?
Maybe the first question is: Where is the gas tank? Years ago, about the time cars became smooth-as-soap streamliners without separate fenders, there was one solution in America. The fuel tank was inside the left rear fender, separated from the trunk or station wagon box. In those days the filler cap was simply sticking out. Usually it had a simple screw-on cap.
Don't remember those days? You're lucky. First of all, despite the 25 cents or less price of gasoline, people could easily siphon gas from the wide open fuel tube. Worse, yet, there were numerous car fires resulting from being struck on the left rear corner. That often popped the seam on the tank, unless the filler was sheared off first.
Ah ha! Now we are getting close to the answer to the first question. It should make sense to put the gas tank, filler tube and cap on the passenger side, away from passing traffic. Next, designers realized that the spare tire could be shifted from under the trunk floor into the space inside either rear fender. And finally, it made sense to put the tank under the floor, with a long two-piece tube running up over the frame rail (those were the days) to the side of either fender.
Sounds good, right? Well, then the bean counters got involved. First of all, that long twisted two-piece filler tube had to be formed, then connected to the side of the gas tank with a short rubber pipe, then connected to the upper tube with another rubber pipe, and fitted to the exposed filler on the fender. Second, all the connection had to be done while the assembly guys were under the rear end, strapping the tank to the bottom of the trunk, or station wagon cargo hold under the floor. Next one or more people had to make the alignments of the filler tube and clamp them together.
Compared to the process of slapping the tank into the rear fender and covering it with the cardboard trim of the trunk, that was labor intensive (read COSTLY).
Now the bean counters were able to overrule the safety conscious engineers like me. All of the big-three Detroit makers were persuaded to weld a short stubby filler directly to the rear end of the tank. Now one guy could strap the tank under the floor, with the filler positioned so you could reach the gas cap when you folded the license plate down at the top.
Seems pretty dangerous to have the filler so close to any impact on the rear of the car doesn't it? Well, no, we were told. The tank and its filler neck were strongly welded and ductile, so that they would deform like a grape when crushed, but not spring a leak.
Baloney. That might be true when the tank was tested in the laboratory. The real world was tougher. Rear end collisions include more than just a push straight ahead. Some crashes pushed the bumper (and tank) down, up or even sideways in shear.
About that time real engineers at Volvo and Mercedes Benz concluded that the safest place for the gas tank was just above the rear axle, behind the rear seatback. Yes, the filler moved back to the fender. Now however, all the work of installation could be done inside the car body shell before the trim covered it all.
But wait, were not done. About that time, the whole industry, imported and domestic, discovered that front wheel drive systems had become reliable, thanks to greatly improved universal joints, and offered other safety benefits. There was no need for a solid rear axle. Cheap independent rear suspensions and the absence of a front-to-rear drive shaft left room UNDER the rear seat cushion. That tank is between the body frame rails so it is very well protected from moderate impact from any direction.