tyees2002

iTrader: (1)

Those tires are only a half inch or so bigger than stock so i dont think thats your big issue. Weight, nearly 30# heavier than stock, wider footprint and more rolling resistance are more likely the causes. That combined with your level.
I do think that kinda mpg is a little much though
Like you said though, until you return it to stock, you know what answer you're gonna get from the dealer.

fieldoc

You have to look at rev's on each tire diameter so you know the percentage diff between the two and then do math and add that to your miles to be accurate.

zx12-iowa

Level will hurt mpg too.

ian515

Definitely, I am just sure all of that combined wouldn't knock me down 10mpg. but if the speedo hasn't been calibrated, than that my fix some of my problems!

sullyman

Calibrating not only will fix the speedometer, but more importantly, it will adjust your shift points, etc. You should see some improvement.

The shudder and poor mileage also had me thinking you might need to recap your spark plugs. Seems it is fairly common for them to be way over gapped, causing this exact issue and poor mileage.

jwanck11

The gap issue is not only that they are over-gapped, but that the plugs are gapped significantly differently. Additionally, the plugs are platinum tipped which are not the best for boost applications. They can be prone to "blow-out" at high boost. Consider switching to iridium plugs (FoMoCo upgrades).