I previously posted a thread on hypermiling (i.e. obtaining the best possible fuel economy on your vehicle). Here's the thread link:
http://www.automotiveforums.com/vbul...d.php?t=760761
Without knowing specifics on your driving style and specifics on how your vehicle is maintained, it's hard for me to speculate on why you only get 25 mpg.
I have a 2000 Altima GLE and notice that my highway mileage is highly variable depending on a variety of factors.
(1) The ambient outside temperature plays a huge role in maintaining the proper operating temperature for your engine; true, colder air in your intake has a greater density of oxygen which mixes with your fuel for added compression; however, with colder temperatures your engine must compensate by working harder to maintain it's ideal operating temperature, especially at highway speeds as increased airflow acts to cool your engine.
(2) The speed you cruise at is typically the most influential. EPA tests have shown that fuel economy drops dramatically when speeds are in excess of 60 mph. If you can recall, the EPA rating for your Altima is 28 mpg for the highway, but what you may not know is that this rating only applies to a max speed of 60 mph on a flat and level surface.
(3) Tire pressure, as another member pointed out, is also a crucial element. Higher tire pressure translates to lower rolling resistance (LRR) which less work required by your engine to maintain a constant speed.
(4) Hilly terrain can work against your fuel economy unless you learn how to make it work in your favor. In other words, when driving up-hill, maintain constant throttle pressure (your speed will drop somewhat since you are fighting gravity) which will yield little change in fuel economy and slightly let off on the gas pedal as you coast back down the other side of the hill (you should notice a slight increase back to your desired cruising speed); once level, maintain constant throttle pressure until reaching your next hill and repeat the previous steps.
(5) A properly maintained vehicle is always important. Spark plugs in the Altimas are platinum, and shouldn't need to be replaced until at least 100k. I'm assuming the distributor cap and such are expected to last even longer (check your manual to be sure). Frequent oil changes with the proper oil and oil level is always a good idea. I use Mobil 1 10w30 full synthetic, which lasts about 7500 miles; Conventional oil should be replaced at the end of 3 months or 3750 miles, whichever is first. Air filters are typically 20k miles. Fuel filters are also at about 20k miles. O2 filters and PVC valves may go bad, but your service engine light should pop on if this occurs.
(6) Finally the obvious. Remove excess weight in your vehicle. Everything in excess of 50 pounds may significantly contribute to a decrease in fuel economy.
Hope this helps!
