[JFP in PA]

Mike-

All EPA CAFE ratings include cold starts, and according to the CAFE rules, the OEM has to outline the methodology used to obtain the claimed mileage for the owner in the manual. Starting the car and letting it warm up obviously produces zero miles of travel for several min. of engine running time, which over the life of the car will lower its actual achieved miles per gallon of fuel consumed. Simple arithmetic.

Over the life of my business, I have had multiple opportunities to rebuild engines for customers that I subsequently had to look into years later. Interestingly, engines that are always warmed before driving showed less bearing scuffing and tended to have better looking cylinder walls after nearly equivalent miles.

As you know, I have also both owned and crewed on various race cars over the years, and we never allowed the engines to be pushed without a full warm up. Several cars, in fact, used outside systems to circulate warmed coolant before the engines were even fired to reduce cylinder wall and bearing scuffing. Many years ago, a then prominent engine builder (now deceased) ran cell dyno runs on engines that were always kept warm vs. units that were cold started and then warmed up before making power runs. The engines that were perpetually kept warm consistently produced more power for longer periods than the ones that were cold started and warmed. When torn down, the always warm engines again showed less component wear as well. He attributed the differences to the dimensional stability of the engine's that were kept warm, and the additional wear on the cold start engines to dimensional shift processes that take place during the warm up cycles.