Bigger and heavier takes more energy. That's straight out physics. Both of the cars mentioned are big and heavy and non-aerodynamic. (I'd even call the RX300 '

Bigger and heavier takes more energy. That's straight out physics. Both of the cars mentioned are big and heavy and non-aerodynamic. (I'd even call the RX300 'boxy'.) If you want something with efficient MPG, you want something small, light and aerodynamic. You don't mention the transmission, so I assume they're both automatics. If you want good MPG you throw away the automatic and use a manual. As regards the RX300 being not so great, it's old, you've worn down the engine and transmission and wheel-bearings, and the electronics are not as advanced as today's. I'm too lazy to look, but what are the figures for fuel-octane rating and compression-ratios? Would those also have a bearing on the question?