I'm considering getting a Mazda 3 GT this year. I want to do some upgrades to the car myself (simple stuff). First off- does the new 2014 still use the 35 series battery as the older models do? I'd like to get a AGM type battery which lasts longer and better supports a real stereo in the car. Secondly- assuming the GT is using a Bose system (I know its 9 speakers), I'd like to replace the speakers with all Alpine variants. I already have a deck, sub, and amps waiting to go in. I don't usually like stock stereos. Does anyone forsee any problems removing the stock head stereo unit in favor of an aftermarket unit?

Thirdly- the stock GT runs 18 inch tires. Will this car support 17 inch wheels and tires for winter use or do I now have to run a minimum 18 inch wheel? I'm not sure what the brake caliper requirements are on the new car. I'd like to get some true winter tires and find 18" a bit big for winter tire use. Can I run 17"s?

Hopefully these mods won't ruin the warranty on the car, but I'd imagine they might complain. Any word on that?

 

With a car that new, you'll have to ask dealer about the 17" rim clearance.

Decks normally don't just fit in, you may need a new back plate, "metra" kit.
see: Metra Online | Welcome to Metra Auto Parts Online Warehouse might be too early as again, new car, is it available.

AGM battery good choice, make sure you wrap it in insulation. Heat kills batteries but you find out when it's cold. The insulation will keep hot air off of it and cold air off of it!

With an amplifier make sure you run an extra ground wire from the battery to car ground and to engine ground (2 extra grounds on top of grounding your amp).

 

I don't have access to Metra kits, but we can get Scosche dash kits here. I'm assuming the GT has the 9 speaker Bose system. Mix of 6x8s and possibly 3 1/2" speakers. Have to check that still.

The smallest rim size the dealers have been putting on the new 2014 GT is 16". So the correct tire size should be 205/60/16. This is based from a 215/45/18 tire size stock.

Another interesting tidbit is the battery. If its just a regular GT with no tech option package, it still uses the 35 series battery. If it has the tech option package (with i-loop), it has a completely different battery that has internal capacitors inside. No aftermarket option is available for that as far as I know.

 

If decide on the Mazda 3 for my next car (sometime in 2015) I would probably get the i Touring without the tech package, then replace the stereo with something better than the Bose Mazda offers.

Edmunds has some advice for how to shop for after-market car stereo, linked below:
How To Shop for an Aftermarket Car Audio System

 

Two additional grounds on top of the amp ground? WTF is the point of that?

 

The stock ground on most vehicles is not meant to handle hi-amperage amps. I knew of a person that didn't do this and fried his stock ground wire (wasn't big enough), and he wondered why his bass was sounding very poor.
Grounding an amp to the body at mid car, or rear of car, is half the circuit, you need to add another ground wire from body to negative terminal to complete the circuit. You may get away without extra ground wire on a small amplifier, but relying on stock ground wires is foolish.

The engine block ground is just insurance as well. Grounding never hurts your car, but only can help your car. People even ground their throttle body directly to negative terminal for supposedly quicker response.

Chart is a guide line to wire size with total amp draw. As amps go up so does thickness of wire (lower the number, thicker the wire).
So if your amplifier draws 130 amps and your ground wire is 11 feet directly to negative terminal of batterry you need 2 gauge thick wire. Thicker the wire, the better the bass. I found this out by experience as my wire wasn't thick enough either.

 

Cool.