2003 Mazda6 S 3.0L 6Cyl. Car shakes between 20-30mph, 3rd gear. Car has 94,000 miles on it. I'm also getting very bad gas mileage, I have an 18 gallon tank and I'm getting 240-260 mph highway and city, very light driving. Help would be appreciated, thanks.

Bad bearings in gear shaft. Requires transmission rebuild or replacement. If only 15,000 miles, it should be under warranty.

One tank of gas is not enough to compare mileage. Or are you saying the computer's readout of mpg estimate is showing less? Try a tank or two of premium and see if that helps. Keep track of gas mileage. Driving techniques have a lot to do with gas mileage, as does long trips vs. short trips, highway cruising vs. city driving. For best mileage, do regular maintenance and keep the car in top running condition-good spark plugs and clean filters and clean oil ! Attend to minor problems before they escalate.
If you want to check actual miles per gallon, start with a full tank. Write the odometer reading down, and go. Everytime you put gas in, write down the amount. When you are close to 1,000 miles from the time you started, go fill the tank and include that amount of gas to your previous list. Now add up all the gas used and divide it into the miles to get miles per gallon. (976 miles/31.8 gallons = 30.69 mpg ) . Only one tank will skew your numbers, might be off cause the tank was filled more to the top than the previous fill-up. About a thousand miles of driving will give you a very accurate figure of miles per gallon for your car.

air filter? checking the octane rating of your fuel? tire pressures? theres so many things that alter mileage. is ur mechanic jeep certified?

Well first, is the overdrive on or are you in the overdrive gear, also sorry to tell you but these v-8 jeeps get real bad gas mileage, i think highway mileage is 18 and city 14,if you were getting better mileage and the trany is actting up then that will cause the lower mileage, good day.

THAT MODEL CAR YOU COULD GET 18 MILES TO A GALLON OF GAS.BUT IF ENGINE WORN HAVE POOR COMPRESSION. YOU WILL GET POOR GAS MILEAGE.

If you do not know what the codes are, it is O2 sensor. This means the computer is not reading the right information from the faulty sensor. (there is a set range the sensor is meant to show, if one goes over or under the computer shows check engine). The diagnostic is not too bad, give the car a tune up, plugs, wires etc. and replace the O2 sensor. This normally clears up any problems. Let me know if you need further clarification or help.

Try having your fuel injector cleaned, you can start with running fuel injector cleaner though your gas tank and if that does not help then you have to get it cleaned out professionally or even worst have the injector's replaced if they are bad.
The other thing that will give it bad fuel would be a bad air intake temperature sensor go bad, as for the ruff idle that is common to a dirty or stuck EGR valve.
Good luck and keep me posted, by the way, what kind of Garage did you take it too? And are there and DTC codes from the ECM?

I have the same issue of low gas mileage. My 2008 Kia Rondo with V6 (2.7Lt) gets 20-21 mpg in average. Even for highway mileage, it is around 21-22mpg. Very poor gas mileage. Disappointed. I hope it would improve in the future.

Sometimes a bad engine temp sensor (not the coolant gauge sender) will cause the ECU to stay in 'choke' mode.

do you notice the gears not switching? Does the car start off in 3rd no matter what gear you tell it? It may also be old gas... :)