To the original poster I would rest the ecu and clean the maf. While your at it I would pull off the elbow from the airbox that goes into the fender it is very restrictive and you will get a bit better throttle response and power.

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I put 275/70 R16s on steel wheels on my truck from stock 265/70 R16s on Alu wheels and I went from 20-21 to 18 and thats not much of an increase so yes going from a 265/70 R16(31" tire) to a 285/75 R16 (33") tire will really hurt your mileage because your gear ratios are way off. You really need to regear if you want to run those tires. A more moderate step is just going to a 265/75 R16 for about 32" which isn't as drastic a step. Also make sure you do a mileage correction for the tire size differential.