Modern Vespa

I noticed a dip in MPG during the switch. I think they have to sell all the "summer gas" and it may be old. Fresh is better, right?

This is a theory of mine. Educated minds please chime in.

Cold weather also reduces MPG. Takes longer to warm up the engine to proper operating temperature.

It was too big of a swing to be that.

To clarify, I see a ~10% drop in MPG during cold weather. 2 and 4 wheels about same change. Ethanol (car) and non-ethanol (scoot) show similar results.

Al

It is very hard to find fuel at a reputable petrol station that is "old" enough to affect MPG. Typically, it is the distributors that "switch" on a given date, adding the seasonal blend into retailers' existing stores in the ground. Thus, the pumping of "winter blend" into your vehicle is somewhat "phased in". This mixing of the two blends is of no consequence, other than the partial elimination of the vapor point suppression additives used in summer until the supply is 100% "winter blend".

There are a couple factors that impact on MPG in winter, and the winter formulation is just one of them. Ambient temperature also has an impact, due to the lower pressure/density altitude resulting from lower temps. A fuel injected engine will introduce more fuel into to mix to accommodate the denser air, and a carburetor engine will run less efficiently, if the jetting is set for higher ambient temp.

Also, as ambient temps drop, engines take longer to warm up to normal operating temp, and the automatic choke function gives you a longer period of time at a richer fuel mixture.

Theoretically, "winter blends" contain about 1.7% less energy than "summer blends". Even if the "theory" is off by a factor of 3, that only means about 5% less energy, so any drop in MPG beyond that is most probably due to other factors.