If your mechanic said you only need 2 tires, I would tend
to respect his opinion. He has nothing to gain by his
recommendation. He's simply looking at the depth of the
tread and saying you need new front tires for adequate
Since the Honda Accord is a front-wheel drive, it's not
unlikely that the front tires show more wear than the
rear ones. When you go to replace only 2 tires, the tire
company will tell you to put them on the back, for fear
that the older back tires will slip during a sharp turn,
but, realistically, you want the newest tires on the
wheels that grip the road during acceleration, especially
in a rainstorm.
If you can easily afford to replace all four tires, of
course this is preferable, so that they will all match
in traction and wear. However, if it would be easier on
you financially to replace only two at this time, do so,
and keep an eye on the tread on the back tires as time
goes by, replacing them, as well, when the tread seems
This article on FineLiving.com discusses the measurement
of tire tread depth. You can always use a penny as a
rough measurement tool:
"If part of Lincoln's head is covered, you have some
minimal amount of tread left. If all of Lincoln's head
is exposed, the tire is completely worn out and it's
time to replace it."
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Searches done, via Google:
minimal replace "tire tread"