2002 Ford Focus LX using a lot of gas?

I got the car a year ago. It has not much to it as I just needed something when I got it. It's proven to be really reliable and not once broken down as I've put on close to 10 thousand miles (for a total of 138000). I recently got a new job and have to travel about 40 miles round trip. The car has done fine on the trips but my average mpg has gone down drastically. I usually can go 2 weeks between fill ups. Now it's just 4 or 5, days until the car is really low on gas. I know it's old but I thought mpg was supposed to improve on high way driving. Do I need a tune up or something?

I think wheels tire to play a role in your mpg. Not much but anything can

DUH! Did it ever occur to you that the further you drive, the more gas you're going to use. OF COURSE you need more gasoline to do a 40 mile round trip than you needed to do a 10-mile round trip.

What kind of fuel mileage are you getting and what methodology did you use arrive at that figure?

Fuel economy does increase on the highway. But obviously if your commute double or tripled in length. You're going to be using more fuel because of the dramatically increased distances you're driving.

5x40 is 200 miles. How many gallons are you needing to fill it after 5 days? You can't judge gas mileage by the number of days it takes you to use it up, more miles means more gas. I've filled the same car twice in one day many times.