If anything I would put a heavy duty higher cold cranking amp battery in your Honda. And also do some extra grounding.
One thing I disliked about my 06 Civic EX was its charging system sucked. The battery was way to small in terms of size, cranking amps and over all capacity. Honda already use's a small battery. Any smaller and the car would have been a worthless yard orniment.
09 HCHII, w/Navi
07 Mazda3 S Touring, 5MT
Mild Hypermiler or Mad Man?
Nothing quite like a car that won't start on a cold winter day, due to a battery that's not up to the job. If that happens with a motorcyle battery in the engine compartment, whoever gives you a jump start will probaby laugh.
Currently getting +/- 50 mpg in fall weather. EPA is 31/39 so not too shabby. WAI, fuel cutoff switch, full belly pan, smooth wheel covers.
prolly not much, maybe like 2-4 lbs? try this: find your cars battery on a rack at walmart (or one the same general size) and then compare its weight to the small lawn and garden battery. thier both heavy (not to mention you would have to cut off your stock cable clamps and crimp/solder on new ones to fit the mower battery) but most mower batteries dont put out enough amps.(the cars ive delt with need 4-600 CCA while most mower batteries are 2-300 CCA) it wouldnt start the car or if it did the battery would die quickly from being totally drained each time its started.
if your car starts easly even in the winter, then you might be able to get by with a smaller battery, but insted of going with a motorcycle, or lawn mower battery I would think going with a racing battery might be better, as they are designed to be able to run a starter, and are small and light weight to boot, of course making your electrical system more efficent is alwas a good idea, get rid of energy wasters where possible, make sure that your connections are all in good shape with a few extra ground straps and silver soldered lugs.