> Why levy high taxes on low mpg vehicles? Wouldn’t it make more sense to increase fuel taxes?
It makes sense to do both, because it's necessary to catch the average driver without pinching either end too hard.
It doesn't take much to convince the heaviest drivers to buy more efficient vehicles -- it may not even take anything, since the fuel savings is already thousands of dollars for them regardless of any taxes.
But they're still paying the fuel taxes even with the more efficient vehicle. Meanwhile the average driver would need a much higher fuel cost to convince them to buy the more efficient vehicle -- high enough that it would hammer the innocent heavy drivers who have already bought more efficient vehicles.
On the other hand, a combination of fuel tax and incentives for purchasing more efficient vehicles will convince the average driver to switch without putting too severe a burden on heavy drivers who have already done the right thing.
It's also generally not good to discourage productive usage. Better to get consumption from 100 gallons to 60 by going from 30MPG to 50 than by only being able to drive 1800 miles instead of 3000 and losing whatever value the extra 1200 miles of travel was worth.
And of course it would be the opposite if you tried to do it entirely through a fee on inefficient vehicles -- the amount required to convince the median driver on its own without a fuel tax would be too much burden for vehicles with light usage. Which is why you want a balance of both.