The Optima Red Top is the pwn. I've got it in all of my cars(except the X5, it's too new). It's small, dense, heavy, cranks like a champ, and lasts forever. I had one that I bought used last for 5 years.

Don't let the 800 CCA fool you. They guys at Interstate Battery hooked it up to show me they are actually over 1000 CCA.

YMMV of course.

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'65 Mustang
'94 RX-7
'02 M3
'05 X5