I don't see how your tire size is relavent, except for that your engine is being worked a little harder. This may increase oil consumption slightly, but it certainly doesn't explain the type of loss you are experiencing.

Isuzu recommends 5W-30. While running AMSoil 0W-30, I was experiencing significant oil consumption, but switching to 5W-30 and fixing my intake manifold leak seems to have brought that consumption way down, from about a quart every 1000 miles to probably about a half pint (1/4 quart). It seems reasonable that in a warmer climate / season you might be able to run a bit heavier oil, but I wouldn't deviate too far from the Isuzu recommendation. I'm sure some others will chime in - and I trust this community's judgement over the best Jiffy Lube "mechanic's" any day.