Neither version asks the proper question:
1) Would you prefer to pay $30,000 out of your $60,000 take home pay for your new Toyota AND PAY YOUR CURRENT MARGINAL FEDERAL TAX RATE OF APX. 13% (ed note: my guess)? 2%
or
2) Would you prefer to pay $39,000 out of your new $90,000 take home pay for your new Toyota AND PAY NO FEDERAL INCOME TAX? 98%
My $60,000 number and $90,000 figures were intended to show the difference in take home pay -- the former including income and payroll taxes, and the latter no income or payroll taxes.
In other words, starting with a $90,000 gross income in both cases, under the current system a single person might take home $60,000 and then have to go buy the Toyota. So your taking away an extra 13% is superfluous.
If arguing for the FairTax, inflating the numbers causes skeptics to assume all your numbers are wrong. So it is best to keep numbers accurate and conservative.