Could me many things. it converts over to 16mph. Take a phone with a speed app and check it. It may have had a tire change when it was new to a different tire. Air temp and fuel viscosity have something to do with it. Air cleaner iced up and also cetane of the fuel. Fuel filter, restricted fuel filter. At below 0, it takes h.p. to keep it running. What fuel are you running? winterized blend, #2, #1, kerosene. It is so cold the oil does not thin out. Mine at 30 degrees is running 16.7 for what is worth. I run #2 premium with 3 gal of marvel mystery oil in 250 gal tank. Up in Canada , I would not run #2. I had an Oldsmobile diesel that froze up on winterized blend in Illinois. The towed it into the dealership, changed the filters and put 5 gal. of jet fuel in it and sent me across the street to fill up with kerosene. A very old mechanic told me to always run Marvel in the fuel to lube the upper of the engine. Car ran like it was drunk. Could not hold it under 75. Just a personal experience.
I just think you are a victim of the cold. Put a pint of marvel mystery oil in a full tank and it will lube the upper cylinders and help you get some more rpm.