NGK Iridium 7s and 8s ???

For the past two months my gas mileage has been slowly decreasing from 22mpg city to 19, then 17mpg city. I replaced the spark plugs about 30K miles ago but I had the notorious LIM gasket coolant leak about 20K miles ago and I am beginning to wonder if one of them is fouled. I was going to buy the NGK Iridium 6's from ZZP again but I saw the new 7s and 8s and wondered if anyone has any knowledge or experience with them over the 6s.

Are you stock or modded - mainly supercharger pulley size? The NGK TR55IX is the stock heat range and the 6, 7, 8 are colder heat ranges for mods to reduce knock retard (KR).

I'd like to add that since you have a GTP, its unlikely that coolant fouled out your plugs.

However putting in a set that is too cold of a heat range will foul out from just regular driving.

I have a 1997 GTP with 154,000 miles. Mods included:

CAI,
throttle body spacer,
ported lower intake manifold,
front ported exhaust manifold,
ZZP 1.9 high ratio rockers,
ceramic coated exhaust crossover,
3in ZZP stainless downpipe,
180 thermostat,
ZZP PCM,
fuel pump rewire kit,
and alternator booster kit.

I also have a Thrasher MPS that I have yet to put on with a 3.4 pulley. I am currently running the NGK 6's.

You should have no issues with running the NGK 6 (one range colder) or 7 (two range colder) with the 3.4" SC pulley. But be sure run a check of knock retard (KR) with a scan tool that has that capability. You do not want to go too cold or you may find the 6 are fine so need to waste money.