I have a 2003 Toyota Highlander, 4 cylinder, 2 wheel drive. I don't drive crazy, I typically accelerate a little fast on occasion, but I never go too far over the speed limit and drive pretty normal for the most part. My average MPG for city is 18. If I mix in highway driving, it might get 19. If I do mostly highway I might get 20, if I am lucky. 20 is the most I have EVER gotten, so the majority of the time its 18-19, although it has been down to 17 at times. I am just wondering, is this the average MPG for a 4 cylinder, two wheel drive, Highlander? Or should I perhaps replace something to try and get better mileage? Since I purchased it last year, I have replaced the spark plugs and air filter in hopes to get a little bit better mileage, but it didn't do much. Any information regarding ideas I can try to improve gas mileage or if this is indeed the correct mileage would be greatly appreciated! :)

Thanks so much!