Cash for Clunkers ???

Ok, the government wants you to get rid your old car and wants you to get a new more fuel efficient new car. Ok, skipping the fuel costs in building and transporting a new car, how much is saved.

Say you have two cars, one getting 18 mpg and the other 12 mpg. And you buy a new car with 22 mpg.

At 4 mpg savings, the new car will use 101.01 gallons less per 10,000 miles. At 10 mpg savings, the new car will use 379.78 gallons less per 10,000 miles. If the old car would have lasted 10K miles, it would have cost the taxpayers, $34.65 per gallon saved at 4 mpg, or $11.88 at 10 mpg more efficient. If the old car had stay on the road for 100K miles, at 4 mpg less efficient it would have used 1010 more gallons of gas or $3.465 cost per gallon for the taxpayers, or 3787 gallons saved at a cost of $1.188 per gallon saved at 10 mpg efficiency.

Best case: Currently getting 10 mpg, buy a car with 45 mpg. That would by save 777.77 gallons by 10K miles. Or costing the taxpayer $5.78 per gallon at 10K, or $0.58 per gallon for 100K miles.

If it is a clunker, how many more miles would you get? How much is this worth?