Good point. In fact, money market fund rates are currently @ 4% and going up every month. In addition, anyone who has $1M in the bank probably has implemented effective tax avoidance schemes and does not pay anywhere near 40% tax (and may live in a no income tax state like NV, TX or FL). Not to mention that someone with $1M lying around probably knows that he can earn an average of 8% per year in the stock market, and would likely pay a capital gains tax rate of 15% when he cashes in.

The point is that the real after-tax cost per mile is much much higher for the savvy investor. But for someone who wants to keep a few thousand in the bank and doesn't need branches, they can decide whether the $ per mile rate is worth the tradeoff.