You really need to look at the margin. If oil at $3/gallon means it costs $400/month to run your Hummer, the discretionary income loss is $400 minus what it would cost w/oil at, say, $1.80/gallon. In other words, not nearly $400 month.
Thus far, there is zero evidence that high oil prices are slowing the economy. The comparison to the 70s doesn't work, IMO. Between Nixon and Carter, we had the worst economic policy in the history of the country during that decade. First wage and price controls under Nixon and then windfall profits tax under Carter. Both served to create artifical shortages, which drove inflationary pressures even higher and made my parents have to wait in line to buy gas.
If those morons had just let the markets clear, there would have been no gas lines and no hit to economic growth. Yes, prices would have been high for awhile, but so what. Like the price of every other commodity that's ever been produced, they would have come down.