One way to know if your car is in need of servicing is to simply do a gas audit. Next time you fill up on gas, take a note of your mileage. The next time you fill up, note the new mileage. The third time you fill up, compare the mileage used from the first and second fill-ups. If there is a decent size disparity between the two, there is a chance your car needs servicing. You can also check your car's owner manual to see what it states should be your vehicle's average miles per gallon. Anything that is substantially off is an indicator that something needs to be tuned up.