I keep seeing discussions on this topic referencing stock group size as a criteria (e.g., Josh's comment above)and I keep wondering why. As I recall, the rule used to require stock group size, but now all it says is "similar amp-hour capacity and weight." When they made that change to eliminate the "group" language, my interpretation is that it now means: automotive batteries with amp-hour ratings to similar to stock. Based on the discussion above, I don't think anyone could define what "similar weight" really means (since for starters, I 'm not sure anyone could tell definitively what "stock weight" is.) Or put another way, the rule is now (roughly) "no lawn tractor batteries".

Does anyone still think that a non-standard group size battery would be deemed illegal (assuming it has similar-to-stock amp-hour rating?)