Mod to save weight? Plust braking safety
You are going to modify a Honda Insight in order to save WEIGHT? Are you KIDDING? This is a car that has a carbon fiber dip stick because it weighs less than a metal one. The engineers have been all over this car, looking for every possible way to save weight. Unless to rip out the passenger seat, I doubt you'll find many ways to make it weigh less without threatening the structural integrity of the vehicle.
It is already the lightest car commercially available in the US, with the lowest drag coeficient. The Subaru Justy used to weigh less, but that was inherantly unstable in hard breaking and only got about 33mpg, despite the 3 cylinder, 1200cc engine. It was a trashy little car. I know. I owned one until I realized that my neck problems were directly related to the fact that this car was made for people shorter than my 6'. My posture is still recovering from the years I drove that car.
The Insight is MUCH roomier, safer and more stable. When I buy a car, I eventually try emergency breaking on a backroad with no traffic just to know how the car is going to handle. The 1987 Justy was scary. veering way out of control as soon as the tires began to skid at all. It seemed to want to turn around backwards and required a LOT of active handling to keep it facing forward.
My 1992 Honda Civic, by contrast, remained stable and understeered (kept going straight) when the brakes locked up. I also learned how to pump the brakes to make the car hop for incredibly short stopping distances. Brake until the car squats (braking with its weight PLUS suspension momentum), then release the brakes when the tires start to skid, do corrective steering while the car hops upward, then slam on the brakes again when the car reaches peak height and make it squat again. Four pumps and you are stopped from 60mph.
The Honda Insight is something else. Because of the anti-skid brakes, I never actually skid. Instead, I stomp on the brakes, ignore the noisy vibration in the brake pedal and I continue to steer comfortably, except for the extreme G-force pushing me into my seat belt. I remember that the pressure of my hands on the steering wheel made me think of what it would be like to do push-ups with a large gorilla on my back.
If you want to lighten your Honda Insight, I suggest that you start by dieting. You are probably the only thing in the car that has not already been severely engineered for maximum strength with minimum weight.
Note: I had a 2 cyllinder, 4 valve Yamaha Verago, then a 3 cyllinder, 9 valve Subaru Justy, then a 4 cyllinder, 16 valve Honda Civic. I figured I couldn't buy another car until someone came up with something with 5 cyllinders and 25 valves, but I bought an Insight, instead. So much for mathematical progressions in my life.