Your MPG @ 100mpg is 100mpg.
The engine turning at 2200rpm/100mph in a reasonably aero car is why you get o.k. mileage at those speeds, and I think the reason your car has gearing that tall, and that 1st to fourth shift solenoid iirc, is to avoid the guzzler tax. If you regear while keeping the speed down to ~65-75mph you should be capable of ~40-50mpg highway.