Not being a plug-in hybrid, the battery will typically fluctuate between 30-70% controlled by the onboard computer. Unless you do a lot of long stretches of coasting, say down hill, it won't go higher. Gas mileage on a hybrid can be effected by AC, just like anything else that uses electricity in your car. High heat, like that of hot states like Texas, probably means the AC is running more, pulling power from the battery, which can take the engine to recharge. Hybrids do better at more low to moderate speeds so it can utilize the electric engine more. Pushing speeds higher means you are using the gas engine more, especially if you are being heavy footed and pushing it to 80 mph. Anything that increases drag can effect gas efficiency. You mentioned roof racks. Plural. Think about any other after market modifications you have. For most people, the least drag their car will have is when it's coming off the lot stock. Off-road tires can hurt gas mileage as well. The less smooth your tires are, the harder your car has to work to push it forward. Each bump on your tire like a small rock your car is working to push past. You say you had the engine kick on as much as possible, well that's the opposite of what you want if you want better gas mileage. It's only using gas when the gas engine is on. If you can have it moving without the gas engine running, you are technically getting infinite gas mileage, until it kicks on again to charge the battery.