It doesn't override existing grids, because it is not possible to programmatically tell whether a vehicle mod already has size settings for its grid, and I would not want to override those settings.
Making a blacklist of mods that shouldn't be overriden would be simple enough, but vehicle mods can add grid settings at any time (which has happened before, with the Aircraft mod, I think it was).
Making a whitelist of mods instead, that should always be overriden would be significantly more time-consuming, since most vehicle mods don't add grids to begin with.
Regardless of feasibility, I personally feel that if a vehicle mod already has a specified grid size in place, then it should not be up to me to override what that mod has set. This was more about standardizing the grid sizes for mods which didn't already have one (if people didn't like the attempt at balancing that was in the base vehicle grid mod)
This was always the one setting I never really wanted to touch from Optera's grid mod. Just consider it the de-facto replacement in every other way, except that one. At best, I could make another extension mod (so an extension to this extension) that specifically strips the vehicle grids from all detected vehicles, and then let this mod do its work automatically applying the default grids.
