What is the temperature difference between the Autolite sparkplug 26 and 25?

Temperature difference or "heat range" is that a "hot" spark plug has a long insulator with more surface area exposed in the combustion chamber. This spark

plug will stay hotter, helping to burn off residue and deposits.

A "cold" spark plug has very little exposure of its ceramic insulator to the combustion chamber. This spark plug will run cooler, however the 25 and 26 are not heat ranges or temperature indicators. You can read more in the related link.