If you are running 4.62 gears... consider jumping up to a set of 4.88 gears. It isn't an exact science, given the fixed availability of gears to us, versus tire size availability, but you can achieve a great improvement.
When I ran 235 series tires on my Tracker (with factory 4.62 gears), my GPS indicated I was moving 2 MPH faster than my speedo indicated. I swapped my factory 4.62 gears for a set of 4.88 gears and went to 30" size tires and now my GPS indicates I am moving 2MPH slower than my actual speedo indicates. That's a difference of 4MPH while running the larger tires, just changing to the 4.88 gears in my differentials.
In my case, I prefer to be slightly below my indicated speedo (for heavy foot reasons) PLUS the 4.88s restored more of the original mechanical advantage (and then some) and my Tracker now has a very usable 5th gear. I actually prefer the "power" these 4.88 axle gears provide me over the stock tires and gears the Tracker came with.
As far as MPG... I typically get around 24MPG out on the road now versus the 25-26MPG I was getting in the stock configuration. Plus, the deeper gear set also improved my lo-range gearing as well and the deeper gears are easier on my entire drive train because the drive train pressures are better distributed.
You should see an even better improvement on your X90 stepping up your axle gears because of your smaller initial tire size, not to mention the additional mechanical advantage "power" you will regain from your 1.6 liter.
