With any vehicle that has a tall profile and the aerodynamics of a brick, you can expect your mileage to be affected by the wind you encounter while driving.

Tire size will factor on your mileage as well.

I was getting 16 mpg city/highway with my I-5 over the course of 90k miles.

Now with my '09 Alpha, getting an average of 15 mpg.

 

I'm a bit of a half tank freak.


198.6 miles // 16.8 mpg
242.2 miles // 17.7 mpg
204.9 miles // 18.7 mpg
218.1 miles // 17.3 mpg
155 miles // 17 mpg
This is east to Ohio, forgot to write down where I filled up.
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This is pretty much all 1-80w

166.7 miles // 14.6 mpg (OH) TRAFFIC, ARRGGGG
177.1 miles // 16.8 mpg (IL)
188.9 miles // 16.2 mpg (IA) With city driving
151.7 miles // 15.9 mpg (IA) With city driving
138.8 miles // 16.1 mpg (NE) Cruise control - 60mph
149.9 miles // 15.7 mpg (NE) Cruise control - 65 mph
143.6 miles // 15.6 mpg (NE) Hammering it doing 80-85mph
(**** you nebraska!)
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109.1 miles // 15.3 mpg (CO) I-75w (Still had nebraska in the tank)
170.7 miles // 16.7 mpg (CO) I-75w with city driving
207.5 miles // 17 mpg (WY) I-80w
228.5 miles // 17.3 mpg (WY) I-80w
191.3 miles // 20.3 mpg (AZ) Highway driving
177.7 miles // 17.9 mpg (NV) More highway driving

I think that averages to 16.8 mpg of pretty much all highway. Just a base model with it's standard tires and I thought I was pretty light on the pedal, also about 300lbs of stuff in the back.

Highest I've gotten ever was 26 mpg, still don't know how the hell it happened, and I triple checked and it wasn't an error on my part, tho I was doing about 45mph on the highway for about two hours because of construction and traffic.

 

13.5MPG city and 17.5MPG Highway