After years of buying used Miatas (and having other family members owning Miatas) I was sick of buying new batteries for them. It seemed like every time I turned around I was having a dead battery. The last few I purchased were expensive Optima yellow tops and I only found out after my first one died that they carried a 12 month warranty. I went through 3 or 4 Westcos as well. My wife's '91 smurf blue Miata now had a dead OEM Panasonic battery (out of a '99) and I took the opportunity to fit a cheap Wal-Mart battery in its place. I took some measurements and headed off to Wal-Mart to find something that was cheap, had a decent warranty, and actually fit in the car. What I found was an Everstart 58R-3. Everstart vs Westco:

Length - 9.25" vs 9.25"
Depth - 6.75" vs 5.00"
Heigth - 6.00" vs 6.25"
CCA - 690amps vs 475amps
Cost - $52.00 vs $110.00 (plus shipping)
Warranty - 72 months with a 2 Year Free Replacement vs ?????? (someone will have to let me know)

The BEFORE shot:

Here you can see the L-bracket, the battery tray, and the J-hook bracket:

A comparison of the length:

A comparison of the depth:

Since the new battery is 1.75" deeper, the tie down will be cut in half and some all thread will be added:

This is the gap I need to fill:

The all thread will be welded to the top of the regular bracket. If you can't weld you can probably get this done for $5-$10 at a muffler shop:

The L-bracket will no longer fit so I am cutting off a chunk of it:

After cutting off the chunk I am adding a sheet metal screw to keep it in a fixed position:

The battery clamp almost never fits on the batteries that I have owned. I use a small sliver of metal inside the clamp to take up the slack:

You can see where I added the sheet metal screw:

The J-Hook bracket has been turned around and mounted in the existing holes to allow for the deeper battery:

The AFTER shot:

The battery cover no longer fits like it did before. You can see where the male and female snaps no longer line up:

The carpet fitment really isn't a huge issue: