Effect of the wrong spark plugs?
 

I just swapped out my plugs and realized the old ones installed by previous owner were not correct - NGK TR5GP was printed on them.

The electrode looked completely different! Theyve been in there since I bought the car - buy i never realized they were "wrong"

I havent had a chance to drive the car yet - but has anyone seen these plugs in a miata ?

I just checked the NGK site to compare specs on those plugs. Two differences, A) not a big deal but the ones that came out were 1mm shorter reach (18mm vs 19mm stock), B) the possibly bigger issue, the ones that came out were tapered seat plugs. The miata head uses gasket seat (crush washer) on the flat head surface.

If your lucky, having the tapered plugs in the head didn't deform the seating surface of the head enough to cause any issues with the new proper plugs to seal. Best way to check if they are all seated OK would probably be to rotate the engine by hand and listen for and air leaking out from the plug wells in the head under the compression stroke of all 4 cylinders. Good luck man.

I will double check and report back - but I think I am lucky. The car seems to drivectge same as before ( if not better) over these past few days

What did you end up replacing them with?

The number in the middle (such as BR8ES) designates the heat range. If I'm not mistaken, the higher the number, the hotter the plug.
I've never noticed a major difference in cars when running different plugs, but I can tell you this, I bought a yz 250 2 stroke dirt bike a few years ago. The thing wouldn't idle for anything. If you didn't constantly blip the throttle it would stall, so I went over the thing and started by pulling the plug. It was wrong, and the gap was way off. I put in the recommended plug and set the gap according to spec and have yet to have idle issues or foul a plug.

Higher the number, the colder the plug. (NKG)

I replaced them with some NGK BKR5E-11.

I still haven't checked the air but I'm fairly confident I made it out fine - the car runs as well as before (if not better!)

Thanks for the correction.