adobian

I was wondering what can be done to improve the gas mileage MPG, and thought that maybe bigger wheels could help some.

What are your favorite set of wheels ?

I have the stock double rim and so far I like them. But it seems my old Audi A8 with 18 inch chrome wheels ride better, and strangely, the Sienna van feels better than the AllRoad (maybe it's taller, that's why).

awdjunkies

I'd hate to say it but putting bigger wheels on an allroad has the opposite effect on MPGs. Even if it's 1-5 lbs heavier, the rolling inertia of the wheel can have a big impact on having to lug that wheel for a period of time, especially in stop and go traffic.
Your sienna is a totally different vehicle in that for starters it's not as heavy, also (Depending on which model) it is lighter due to it being a FWD vehicle not all wheel drive/quattro. Suspension components are different as well thus giving you the feeling of having different driving characteristics.

all(over)road

You can increase the tire pressures to 36-38 lbs and get a slight increase in MPG.

A good engine clean, seafoam etc, along with a throttle body clean and a gas/injector cleaner treatment might get you a few MPGs too. BTDT on my high mileage allroad.

As said, bigger rims will do nothing for MPGs.

Teutonix

If you do a fair amount of highway driving, going to narrower, smaller diameter rims with taller tires would probably new you some MPG. You could use 16" rims with something like a 205/65/16 or 195/70/16 tire and probably gain a few MPG on the highway.

adobian

Let's say that I keep the stock 17 inch rims, what is the tallest tire I could use ?