Here's a back of the envelope calculation...
4 million people are unemployed
Assume working and paying 12,500 a year in Fed Income tax
These 4 million people pay 50 billion per year which is .05 trillion per year or .1 trillion every 2 years.
Pay off 1 trillion in 20 years.
Assume a 1 dollar per gallon gas tax
we consume 378 million gallons of gas per day
this is 137,970 million per year
137.97 billion per year
.14 trillion per year
Pay off 1 trillion in 7.14 years
If we combine a dollar a gallon gas tax and assume 4 million unemployed become employed we have an increased revenue of .2 trillion per year. A trillion dollars is paid off in 5 years.