So go to Walmart buy Autolite plugs size 25, just like there little computer tells me. get home take my old plugs out and they are the same plugs just size 26. does anyone no the correct size spark plugs for a 1989/302 (25 or 26)
The number describes the heat range, not necessarily the size. 26's are one heat range warmer than 25's... Depending on the year, 302/5.0 engines did use different heat ranges... For '87-'95 Mustangs, 25's are correct.
If you have a power adder. even then 1 step and maybe 2 steps. If you are changing your heat ranges and on a N/A engine you should be well versed in reading spark plugs and figuring out how to correct the problem, ie air/fuel. there are different reasons for changing the ranges and I don't know them all but anything from carbon foul to detonation problems.
you generally want a colder spark plug for an increased compression ratio or adding a power adder, so just increased compression in general. Might want to if your cam changes your cylinder compression enough.
I would use whatever spark plug the cylinder head manufacturer recommends. It should be listed in the instructions somewhere, and will likely not be autolite 24-26.