This is kinda odd...last year when l bought my used scion xb - l was averaging 32 mpg.......Now - l average 24mpg....I am easy on the pedal, tires are all inflated properly, oil changes regularly.Here is the odd part....when l tow my little trailer w/ our luggage for going up north - I am averaging at least 28mpg.....any ideas why?And....what else can l do to improve my gas milage??
oh - and I've changed the air filter - and spark plugs...but l did not change the spark plug wires...should l do that too??Should l change out the fuel filter???

Probably because when you are going up north you are spending more time on the interstate which means less energy wasted braking. Generally cars get about 4mpg on highway EPA ratings than city though for some cars (hybrids especially) it can be less and for some cars it can be more.

On the decline in fuel economy in general you've got spark plugs, an air filter and plenty of other things that could wear over the course of 4 years.

Sorry but a box on wheels can only get a certain amount of fuel mileage.

take it to the mechanic & have them check it out. could be something wrong with the ecu which causes the car to use up more gas since the a/f ratio might be off. it could also be the type of gas ur using/oil u use, but it should not affect ur mpg this much.

some things u can do r coast to a stop & then lightly apply the brakes. saves brakes n if u have a manual transmission it saves the transmission as well.

honestly, the car is a scion so most of the parts should last u a long time. u have a really odd problem, u should go & also check out some scion xb forums. they have a lot of knowledgeable members on their that could definitely help u out with ur question/problem.

Sounds like a fuel issue, possibly a fuel filter. What is the service interval for this on an '04 xB? Might also be gummed up fuel injector and/or a dirty throttle body that are bad enough to affect mileage, but not bad enough to trigger your check engine light. It could be as simple as adding some fuel injector cleaner to the gas, or have a dealer/mechanic give it a fuel injector cleaning.

http://..com/71129123613AA1o620

Change fuel filter + clean injectors

depends on driving environment as well.uphill most of the time? stop and go traffic? manual or automatic?
i have a 2006 xB and l get about 34 mpg, but it is mostly interstate driving.when l drive uphill, l go down to 30mpg.
i had added a aem air intake and borla exhaust (not the loud noisy ones)

this seemed to help my mileage go up from about 30mpg on highway to my 34.*shrugs*