Apart from monitoring my fuel use from tank to tank, the only real testing I've done was to note how long it takes for my cars to slow in neutral from 50mph to 40mph on a nearby road.
Although both my cars are similar in shape, the VW took 34 seconds to slow to 40. The smaller car did it in only 14 seconds...
The VW is much heavier and it has bigger wheels (less rolling resistance?) but I was quite shocked at the difference.
I did wonder if the little cars brakes were binding. In fact they weren't, but I found the amount of load/drag when trying to turn a front wheel by hand was quite significant. I immediately changed the oil in my wife's manual transmission to fully synthetic to see if that will make a difference. But I can't say I noticed anything afterwards...
She does a fairly long commute every day so, if she can stand the 'weirdness factor', I'd like to try skirts and a Kammback on her car to see if it will help.
She'll be driving with the Ultragauge in her car (out of sight!) for the next week to monitor her real-world consumption. Then we can do comparisons with the mods installed.
Like always, it will be adjustments to the nut behind the wheel that will make the biggest difference - but motivating her to change her kamikazi driving style may by a bigger hurdle than modifying the car!
My little automatic car will be getting a new set of tyres this week - Michelin Energy E3B Green X 165/65 R13 T (77)
Not cheap - but they are one size up which I hope will help with the gearing, and have low rolling resistance which may also help the MPG....