Carl F. Thelin

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 connections 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 tubes 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, we’re 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.

But wait; we’re still not done. There arose a demand for four wheel drive cars and SUVs. No problem. That under-seat tank could be shaped like water wings, leaving plenty of room below or above for the prop shaft; especially since many such cars has independent rear suspension. In that case, the prop shaft ran back from the engine compartment to the frame-mounted rear differential without needing to move up and down with the suspension.      

Oh-oh – now we are back to the problem with the long convoluted filler neck. Finally the bean counters had to back down, given their fiasco with the gas tank / filler located just inches ahead of the back bumper. So, we ate the increased cost in favor of genuine crash-fire safety improvements. With a generous length of piping, the filler can be placed high enough on most fenders that it is less likely to be sheared off in a side-swipe collision. An, of course, competition required that even low cost cars have the filler behind a door, and be fitted with anti-siphon hardware.

Ah! Now we are left with the initial question.

I admit that I do not know the answer. One reason would be that many cars are designed by engineers who live where the driver sits on the right hand side. Well, not really. My Toyotas were designed by people who live on Japanese islands where drivers driving down the left side of the road never have to worry about meeting many drivers coming at them head on. Maybe the non-exported Toyota does have the fuel filler on the right side fender.

To make this more confusing, some GM, Ford and Chrysler models have the filler on the left or right side depending upon some whim that I do not know of.

This story has run too long already. I encourage you to do your own research. Make a list of all the cars you see and note where the filler is located.

Keep up the good work. Maybe some day we will know.