My 'o7 X500 is carbed. Purchased 2/08 and as of 12/21/08 it has 13,245 miles. On the freeway cruising at 70-75, occasionally passing trucks, and loaded for a wedding in New Orleans, I recently averaged 48mpg burning Premium. Most of the trip was interstate, and in addition to myself - 6'3" & 200lbs - she carried saddledbags, topcase, and barrel bag on passenger seat with a few cold weather items underseat; all in all about 70lbs.
I have never topped mine out, but have backed down at 85. That was plenty fast. consider though, that I get bored riding interstates,do not like trucks and other bored automobile drivers.
Most of the time, I take the longer back roads as the advantage of living in the central Texas hill country.
On the longeswt interstate trip I have taken, one day's total was 575 miles. even though I had to stop for gas around 140 to 150 miles, and plan my stops accordingly; taking a break from the saddle, and calculating stops helped relieve the boredom.
In Beaumont/Port Arthur, in November '08, there were still gas stations and restaurants closed due to Hurricane Ike clean up. Stopping for gas and finding them closed for everal miles was a moment of some apprehension that ended well with a great pancake and omelette brunch surrounded by local people who enjoyed sharing their stories with a man passing through.
the speedo is off about 10 percent, the same as several other bikes I have owned. In fact, the only accurate speedo on two wheels I have ever experienced was a calibrated magnetic job on a Cannondale racing bicycle.
My X500 fit me and my wallet so I am prejudiced. As a former motorcycle rider, what I appreciate most about maxi-scoots is the versatility. They are equally at home on city streets and fun on twisty bendy hilly rides in the back country. I admit my bias.