MPG 16-17, with 93 octane in my Hyundai XG 350 L-WHy?

The owners manual probably says no. Most say 89 octane is the lowest. Look in manual if you have one or they are typically on-line. With that said, test your vehicle with that fuel. Next time it is very close to empty put 2-3 gallons in the tank and drive. If the octane is too low the engine will "spark knock" or "diesel" (a rattling sound from the engine upon acceleration caused by fuel pre-ignition/detonation). If that happens put an equal amount of premium fuel in the tank and the spark knock will go away. Short-term a spark knock won't damage an engine, but longer term can by causing over heating, vibrations, abnormal stresses ... hesitation on acceleration - poor mileage and poor engine performance. If you are able to use 88 octane-15%, without getting a spark knock, make sure you check your mileage and see if, or how much, it drops. Lower octane and higher ethanol both reduce mileage. What I always told my gas customers was to check your mileage with different octanes and see what octane gives the highest mileage (and then do the math and see if its cost effective). Say you use regular normally and you get 30 MPG. Then you try the mid-grade and get 33 MPG, so you try premium fuel but you still get 33 MPG. In that case the mid-grade is your best option, assuming that your 10% increase in mileage cost you less than a 10% increase in cost! But you do always stay away from fuel that causes a spark knock.
The rule of thumb is: The higher the octane the cooler the combustion, therefore the lower the octane the hotter the combustion. Higher ethanol means that the engine is running a little leaner and a little hotter.

The Fuel Economy of the 2004 Hyundai Santa Fe is listed as being 16 mpg in the city and 22 mpg on the highway.----------------------------------------------------------------------------------
Digiprog3 Odometer Correction Tool

I'm not sure what the problem is specifically, but I can assure you this--you're basically throwing money way (and possibly exacerbating the issue) by using 93 octane fuel. Your car is timed and tuned to run most economically and with best performance with 87 octane fuel. The higher the octane, the slower the fuel burns. Longer burn time can equate to more power, but ONLY if your engine is designed and tuned for this application.
Couple of suggestions..checkto make sure your exhaust is "free". A plugged catalytic converter can really rob mileage. Tire pressure and air filter are worth mentioning as well.

Cause you're driving a bus. lol. Mileage will vary depending on what fuel you use, how you drive, and where you're located. typically what i've seen on trucks like yours, i'm asuming it's a 6.0 liter, is anywhere from 12-19 mpg. you should see around 15mpg for the most part. if you do a lot of in town driving, expect a lot less. @9-12. if it's less than that, i'd start lokking in to it. try running a K&N air filter and use synthetic engine oil. I've seen noticeable improvements with power and mpg's with that. Also, ensure that your tires are properly inflated as that can make a difference in mileage. use the recomended fuel octane and change fuel filters.

what are you getting? The only thing you can do is drive easier, coast, dont accel hard ect. If you wanted good mileage, you bought the wrong car.

when the weather gets cold fuel mileage and power suffers due to idle time to warm the vehicle and also the fuel is mixed(blended) for the winter months one way to offset this is to use a block heater when parked for an extended amount of time,also use a higher octane gas,or even try a fuel/octane booster with injector cleaner and line antifreeze.hope this shed a bit of light on your problems good luck.

you need to change the spark plugs and wires to properly burn the gas fully then you can just get agood oil change and you would get better gas mileage

I don't know from where you bought the car but do you feel certain that typical causes such as fouled plugs or clogged air filters are not at fault?
Most dealers, even the shlocks don't normally skimp on the simple stuff but who knows about a private source?
That does seem low, especially the city numbers. I don't think Hyundai is any leader in gas mileage but they are better than what you are getting.  

You can use 87 octane, but first let me give you some food for thought, when you use lower octane than recommended the knock or ping sensor will ****** the timing to accommodate the lower octane fuel and prevent piston damage, this causes a loss of fuel economy and power, so the real saving is very small if at all, but yes your car will run fine in all but very hot climates, like the southwestern states. I hqave a Corvette, it calls for 91 Octane, I have run 87 in it to save money, but the fuel mileage drops 3 MPG, so I gave that up.

I have the same problem as talked about above. 2002 Hyundai accent gsi. HORRIBLE gas mileage. I only get about 320 km per full tank. My driving habits are good. i took it to the garage and they put it on the computer and no error codes showed up. Question: Can a MAP sensor be faulty but NOT show up on the computer diagnostic? I had my O2 sensor checked out and my exhaust, and changed the flex pipe. Car runs pretty good just horrible gas mileage and kinda crappy power. Changed fuel and air filter. Cleaned the fuel system entirely ($80 at garage not $4 bottle).

Anyone ever use a resistor in the MAP sensor?