Thanx PK, you're right on target. The main problem is the tires-- you're running 255/55R18 (29" O.D.) which are smaller in diameter than the OEM tire of 245/60R18 (29.5" O.D.). Thus, your current smaller diameter tires are rotating 1.8% more than OEM causing your VX to consume more fuel (less MPG) for the same distance traveled.

Instead of changing the differential ratios, a better solution for more Highway MPG is to go to a tire that meets three(3) criteria:

1. Must be "tall" in O.D. between 32" -- 35" O.D. to act like a transmission Overdrive thereby reducing engine RPM's (saving fuel) for the same given forward speed of travel.
2. Must be "narrow" in width to create less drag/rolling resistance. The skinnier, the better.
3. Must be "lightweight" to reduce unsprung weight and use less fuel from standing start/launch.

Here are some optional tire sizes that meet the above criteria (tall, narrow, lightweight):

P255/70R18 (32.1" O.D./35 lbs) - bolt-on perfect with OEM rims.
P265/70R18 (32.6" O.D./44 lbs) - slight trimming required.
P275/70R18 (33.0" O.D./46 lbs) - more trimming required. (recommended tire: BFG - Rugged Trail)

To minimize fitment problems, utilize your stock OEM 18x7 rims for the above listed tires. Use only "P"-series tires (aka Passenger/P-Metric) which are lighter in overall weight, and stay away from heavy "LT"-series (Light-Truck) tires.

For the absolute extreme gas saving tire, you could use:
35/10.0-15 (35" O.D./48 lbs) - lifting required. (recommended tire: MickeyThompson - Baja Pro, which is a Sand-Rail tire and not DOT approved for the street. Use 15" super lightweight CenterLine brand wheels to keep weight to a minimum)

Focus on changing to a taller O.D. of your tires, not the gearing in your differentials. Then drive your VX like you are hauling very fragile eggs; be featherlight on the gas pedal, and let off the gas pedal early before coming to a stop. You'll be amazed at how much your MPG will improve.