New Battery Type (size) for the Fox Body


Founding Member
Nov 13, 1999
Palm Beach Gardens, FL
The Mustang's DieHard Silver just went bad yesterday. Voltage dropped to 1.0V on the cheap load tester.
It started right up a couple months ago & is always on a battery maintainer. It lasted 7 years, can't complain.

There are 3 US manufacturers of car batteries.
Costco, Diehard & all the others are just names they have these 3 place on their brand.
Johnson Controls

I just got a new one from AAA. They no longer spec the Group 58; it's now the Group 59.
Wasn't sure it would fit, since it's a little bit bigger. But it does fit & has a little more capacity for only a few dollars more.
I did have to change the location of the alternator fuse & the positive cable now goes across the top of the battery.
Might get a longer cable, but my current ones are marine grade.
All the online battery finders still say Group 58.

AAA has the best warranty of anyone: 3 years full + 3 years pro-rated.
The installer comes to where you are, checks out the old battery, removes it, installs the new, & checks the charging system, all for no extra fees.
I had already removed & checked min out.
My installer has been in the battery business for 25 years & he seemed to know his stuff.
He said Deka was the best & maybe the proof is the following:

It's turns out that AAA batteries are made by Deka.
Diehard Gold was Interstate: lasted 5 years
Diehard Silver was Deka: lasted 7 years
Both were used in the same vehicle under the same usage & conditions.

Deka info is here:
Searching for the Mustang still came up with the 658MF, so searched for the 659MF.
The 659MF is spec's by Deka for other years & engines of Mustangs.
I suppose they never went back 30 years to check.
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10 Year Member
Mar 2, 2003
Greenville, NC
I just put a group 65 AGM from Advance Auto in mine. Fits like a glove and has authority when I start the engine unlike the small group 58 it replaced.


Apr 2, 2015
Pearland TX
Optima Yellow top deep cycle battery. I think it is the 34/78 size. Worth the money, can be found cheaper on amazon than autoparts stores

John Dirks Jr

5 Year Member
Jun 28, 2013
Interstate group number is on the front, not the top.
That doesn't appear to be a 58 or 59.
59 specs:
CCA = 590
CA = 740
RC = 100

58 specs:
CCA = 540
CA = 675
RC = 80
I just checked the front of said battery and it says group 58. The same one I posted a pic of the top label showing 610 cca


My dad had a bra
SN Certified Technician
Sep 5, 2001
Seaside, CA
On my 7.3 Powerstroke, which has 2 batteries, I did the Walmart battery thing. Was looking at the Interstate replacements, but they were 3x the price of the walmart brand. I also found that the cheaper interstates out there have given up quality. With Walmart, warranty replacement is easy, and even if I had to pay full price again every 3 years, I have a hard time believing that the interstates would last the 9 years they'd have to to break even, and that's assuming I don't have a problem that drains and damages them in the mean time.

Now, on the mustangs, I've just been turned onto Northstar batteries. They're supposed to be 100% lead, and the word is that they have state of the art facilities and are the go-to brand for the the military. Their AGM group 65 battery, for example, is 1070 cold cranking amps. They're pricey, and they won't go into my truck, but they may be the best by reputation.

One anti-brand recommendation, stay away from the Optimas. I've just found that even when they're good, if they ever go too low, they're gone. I have had half a dozen of them, and I'll never buy another.

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