I'm not an automotive engineer, but neither are the folks that dream up these regulations. Many folks only think about regulations only in terms of MPG ratings and tailpipe emissions, but the regulators did not stop there. It’s far more complicated.
FUEL TANK
Compliance applies to the entire fuel system—tank design, vapor control, refueling systems, and safety integrity. Toyota had to comply with EPA evaporative emissions standards (40 CFR Part 86, Subpart B), Onboard Refueling Vapor Recovery (ORVR) requirements, and , plus even stricter CARB evaporative standards. Larger or dual tanks generally increase vapor permeation surface area, change how ORVR refueling hardware works. Meeting those requirements for a relatively low-volume model like the Land Cruiser probably didn’t make sense for Toyota. Maybe designing a larger volume tank to meet those requirements would have reduced ground clearance or literally not physically be possible.
CRASH TESTING
Under FMVSS 301 (Fuel System Integrity), the U.S. rules are very strict about fuel leakage after a crash:
During the impact: fuel spillage can’t exceed 1 ounce per minute. After the impact: total spillage can’t exceed 142 grams (about 5 ounces) over the next 5 minutes, and then it must drop to no more than 1 ounce per minute for the following 25 minutes. In addition, the fuel tank, lines, and fittings must remain attached and the system has to show no continuous leaking. That’s why auxiliary tanks can problem every seam, hose, and valve is another chance to fail these limits. Basically you are doubling your potential failure points going into the most expensive and complicated crash test regimens in the world.
MPG
Most people assume the EPA’s MPG numbers come from real world driving a car and measuring fuel burned. In reality, it’s far more convoluted and grossly misinform buyers. EPA fuel economy testing is done on a chassis dynamometer in a lab, where the vehicle “drives” through scripted cycles that claim to simulate city and highway conditions. Vehicles aren’t tested at their exact curb weight—instead, they’re assigned to a test weight class, which includes curb weight plus a 300-lb allowance. The problem is that those classes are stepped in 125 pound increments. Add enough weight—say 75 pounds of extra fuel from a larger tank—and you can push a vehicle into the next higher test weight class adding. That bump matters because the dyno is then set with higher inertia and rolling resistance to simulate the vehicle actually being heavier than it actually is! One other wrinkle is that the EPA “highway” test cycle doesn’t actually look anything like real real world highway driving. People assume it’s a steady 70mph mph cruise, but it’s not. The EPA’s Highway Fuel Economy Test (HWFET) runs on a dyno over a scripted cycle that lasts about 12.5 minutes and covers 10.26 miles. Average speed is only 48 mph, the top speed is only 60 mph. There are multiple periods of braking, stopping, coasting, and acceleration. Does that sound like highway driving to you? Too bad, EPA don't care! Even though 75 pounds doesn’t sound like much and its not in the real world, in EPA math world where weight class and inertia is so important it can drag efficiency numbers down enough to hurt the official MPG label. And since every Land Cruiser sold in the U.S. counts against Toyota’s CAFE compliance, even a 1 mpg hit carries corporate fleet wide cost implications.
The small tank wasn’t Toyota’s preference—it was a direct result of regulation-driven design constraints unique to the U.S. market.
