1966 vs 1967 GTO fender
 

alright all I cant remember witch is witch 66 vs 67 front fender what one has the two rounded pieces out of the wheel lip I thought 67 but I cant remember? thanks for you help.

1966 vs 1967 GTO fender
 

Original 66 fenders did not have the 2 oval cut outs for tire clearance when turning hard left and right, replacement 66 fenders from GM sometime in the later years came with them, I have 1 one my car, 67 cars had the ovals, my opinion good deal to have the cut out for tire clearance. jd rim 66 gto

They sort of resemble scallops

66 has a different filler for the header panel than 67. That's the easiest way to tell.

66 first photo 67 second....

Resurrecting this thread. Just bought a used fender and want to confirm it will fit my '67 LeMans.

Are there any bolt hole, trim hole or mounting differences that make the 66-'67 change easy to spot? Is the only real difference the header filler?

Basically it's just the Header extension, Very easy to spot once you look close.
Below is '67 on the left & '66 on the right.
Notice now much Larger the top portion of the grill header on the '67 is compared to the '66.

Read post 1-2-3 above.

It’s not just the header extension, but also the top inner lip.

They are talking about the noticeable factory cut outs at the top inner part of fender (you can feel these at top side behind the wheel lip moulding).

1967 fenders had these, 66 did not.

Chris.

Later "Replacement" '66 had them so again the header detail is your best bet.