mpg dropping on 05 elantra gt
 

i usually get 16 mpg in the city. i just got an oil change, transmission fluid, and power steering fluid. i even just gave her a tune up. well today my mpg dropped to 10 mpg. i get gas at the bp station religiously. so my i want to know why the sudden drop? any suggestions would be highly appreciate it.

How many miles?
Did you change the timing belt?
How did you check the mpg?

41,600 miles. i just change some the belts. it just dropped from 16.9 to 10 in one day on the odometer

i don't know what kind of belts i changed but it was last year. where is the time belt anyway.

Timing belts are inside the timing belt cover. It gets changed every 60K, so you're alright.

Do a manual fill and calculate true mileage. Don't rely on the odometer thing.

where or how do i do a manual and calculate true mileage. this is my first car so i am new to this. thanks.
oh and one more thing, my sister said the car battery died earlier and she got a boost. could this be the reason and do i need a new battery?

Fill the tank, write down the mileage. Drive the car until gas is empty. Fill the tank, write down the amount of gas you put in and miles traveled. Divide mileage by amount of gas.
Thats how you calculate mileage.

No, battery going bad shouldn't effect the mileage. But the mileage calculator thing on the odometer is notoriously inaccurate. Do the manual calculation to decide if the car is not running right or not.

Replace the battery if it is the original one, it's time.