Why is my mileage higher on the freeway?

Gas Engines can get less mileage in colder weather, because of warmup times/etc.
In general, the colder, more dense air, results in better "steady state gas mileage" - even with electronic fuel injection.
However, there are many more factors, that often result in lower gas mileage, as temperatures go below ~~60-70F.

Add in the load with lights, heat, defrost, etc, and as temperatures go down, there's often more of a total load on the engine/motor/battery, resulting in a decrease from the Summer.
With Hybrids, and EVs, in colder weather, there is also often the added load to heat the batteries.
Plus, when the batteries are cold, there's a greater chance of the engine coming on for a short time, to aid in more power, heat, etc.

Plus, This Is How Much Temperature Impacts EV Range - YAA

 

In other words, you need to move to Orange County, CA. ;-)

 

As opposed to what, driving on the local roads or highways?

 

it certainly depends on highway speed, and the type of local driving. it can go either way

 

Part of that goes back to a misconception that a bunch of auto writers had back in the earliest days of hybrids. They were thinking "hmm, gasoline engine like other cars, what's different is this weird electric stuff, regeneration and so on, so we think the MPG benefits will all come from stop and go driving when it's using the weird stuff."

What they missed is that a straight hybrid (not a Plug-In or a Prime) is a completely gasoline-powered car. Any energy that gets recaptured and reused in the weird electric parts had to come from gasoline to begin with, and with battery losses on both charge and discharge, the car is more efficient when it can just use that energy than when it has to shuffle it in and out of the battery. Yes, shuffling it in and out of the battery is better than losing it forever like a non-hybrid, but it isn't the principal win that those early writers thought.

The principal wins in the design are to be able to right-size the engine for the car's average power needs, instead of having to oversize it for peak acceleration demands—the weird stuff can chip in at those times—and to be able to make it an Atkinson-cycle engine, which is not like in other cars. It is more efficient. It lacks the peak power the other cars have, but doesn't need it because the weird stuff can chip in when peaks are needed.

So the car is darned efficient when it is using its Atkinson-cycle engine in steady cruising where it doesn't have to incur losses shuffling energy in and out of the battery—in other words, on the highway. That's what always surprises people who expect the only benefit to be in stop-and-go. Yes, it does also have a benefit in stop-and-go, where it can at least reuse some of the energy other cars would forfeit. But that's the benefit people expect, so it doesn't surprise them.

 

Two (of many) factors that can wreck fuel economy with any gasoline powered vehicle are high speeds and unnecessary braking (including Prius, as ChapmanF explained). In the EPA City test, accelerations and braking events are moderate, compared to the way many people drive, That lets the hybrid system recover significant energy with its regenerative braking, which is part of the reason it does so much better than a non-hybrid gasoline car in that test. (Reducing engine-idling time is another reason.) It also scores better in City than it does in Highway because lower speed means less aerodynamic loss. If you're braking a lot in your non-freeway trips, that eats up fuel, even when some of the kinetic energy is recovered. .Another factor may be trip length, if your non-freeway trips are short; the warm-up routine consumes fuel.

 

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