How does employer handle and what is impact on employee's taxable
income if an employer raises would elect to raise the mileage
reimburse rate to a rate much higher than the current IRS published
rate of $.405 (emplyee need to keep exact recipts and have difference
between higher rate and $.405 reported by employer upon their W-2's as
inputed income)?

==========background info start===========
Federal tax laws allow all employees to write off the business portion
of car expenses, other than your normal commute to and from work. 
Those rules allow you to tally the cost of gas, oil, repairs, tires
and other automobile costs. You then calculate the percentage of
business use and deduct that portion from your taxable income.  "The
IRS rate is not meant to be a good measure of vehicle costs,".  "It's
meant to be a safe harbor tax deduction for people who don't keep
receipts. If you can keep receipts you'll have a higher deduction."
The IRS rate is based on a typical vehicle that's driven 15,000 miles
a year. The rate includes an average for gas, maintenance, repairs and
insurance for cars and trucks, small and large, regardless of where
the owner lives.
====background end.=================