___CAIís add HP but in the case of the Insight, efficiency falls fast as you go much below 60 degrees F. AITís of ~ 70 - 80 degrees F is the ~ equivalent. Below 40 degrees, the lean-burn capability of the 5-speed and HCH is all but gone. Unless that is you have a WAI and/or radiator block installed. You can receive Lean-Burn all the way down to almost 0 degrees although it is very faint and short lived in those temperatures.
___The second item is the coolant, block, and interior temperatures themselves. You will have problems with this in the winter as well. Hypermilers almost always run with the least amount of fuel burned in any number of circumstances. In the case of the 5-speed Insight while driving for maximum mileage, this is less then 1 oz. of gasoline per minute. Now consider this. The Insight ICE might be somewhere along the lines of 33 - 37% thermally efficient and you have less then a thimble full of gasoline to burn every minute to warm the coolant, warm the interior, warm the oil in the crankcase, drive the pistonís, spin the cranks, spin the selected gear and final drive, spin the axle, spin the MGSet if the pack needs a small charge, and drive the wheels to propel 2,100 #ís of automobile and driver down the road at 50 mph. Remember, this is less then a thimbleful! There isnít much fuel being burned so heat retention becomes a serious problem in the winter. Your diesel is basically running lean burn all the time and with a thermal efficiency approaching 40%, there is only 60% of that small amount of fuel making heat to keep your block, coolant, and interior temperatures warm. Again, imagine a thimbleful worth of fuel in an Insight. Light it off with a bic lighter. Will it keep the interior of the insight warm in let us say 10 degree F temps? I did not even mention warming up the coolant and crank case oil and spinning the cranks, transmission, MGSets, and wheels to drive you down the road. Heat retention is the number 1 priority in the winter and for us Hybrid drivers, Autostop should be avoided when approaching these sub-freezing type temperatures at all costs. Even using interior heat is a luxury an Insighter had better reconsider if he is considering Hypermiling year round.
___Under stealth, you are not emitting any CO2 at all but your short trip mpg average will still come into play when the ICE charges the pack and provides propulsion later on. 39 mpg for a short 3.5 mile trip in a Prius II sounds about right overall so that is what you should expect. Donít let a small detail like CO2 output cloud over what a Hybrid or other automobile can do in terms of reducing GHG and SMOG emissions overall. Just drive whatever you are behind the windscreen of to the best of your abilities. Doing your best is the most anyone could possibly ask of you given the driving techniques and setup you have learned here.