I bought a set of euromiester fuchs fro the rear of my fiberglass flared 914. im running 245/40/17s tires. how much spacer do i need?

what is the offset ??

Fiberglass flares may give you more room than steel. It might be best to remove springs while car is in the air. Slide washers on the wheel studs until the tire sits just right in the fender when the control arm is jacked up to your desired ride height. Do it on both sides in case they are different. Measure the washer stack and buy a set of spacers equal or closest to the small side.

i think its +15

then they are like 9x16s and will fit fine. as stated above you can space them per your personal taste to define the look your after