The entire Si drivetrain as in engine and transmission? If that's the case I think it would be pretty fair to just use the Si numbers. I doubt the VX body would improve your FE very much.
Their are some benefits to the VX (no sound deading/aluminum/some aero mods/no sunroof/no power anything). So basically, this is what you'd get if you put an Si on a diet.
I may use the Si numbers, but I'll note it somewhere so people can make their own conclusions.
I added the combined MPG for the 92 CX. Just got another 70+MPG tank, despite a rainy, 63 deg day thanks to Alberto.
I was comparing the dimensions of the Insight tire (165/65/14), vs. my current tire (175/70/13), vs. my OEM tire (165/70/13) on the Discount Tire site and the Insight tire falls right in between the widths of my current tire and my OEM tire. The Insight wheel is 1 inch wider than the OEM wheels I am currently running on my car. I'll consider getting a set of Insight wheels and tires for my car whenever my current tires wear, out but since the Insight tires aren't any skinnier than what I am driving now, the main advantage of going to the Insight wheels and rubber would be the LRR tires and a greater availability of tires in that size.