Deka has a large line of batteries, so I'n not sure exactly which one you are referring to:
http://www.dekabatteries.com/assets/base/0083.pdfA miata battery is minimal at 320cca and 25 lbs.
A replacement battery for a Volvo 240 is 600cca extra 7 lbs.
Deka suggests that using a battery with less than the minimum cca will shorten the life of the battery.
More cca is always better as long as the cables are adequately sized.
The charging system keeps your accessories going.
The battery is for starting and to keep you moving when the charging system fails (reserve capacity).
Fewer cca also means less rc so while a 240 may only need 450cca, the amount of accessories in a volvo push the rc requirement up and the cca with it. A 5.0L mustang needs only 580cca.
The Odyssey 680 is around 220 cca apples to apples (i.e. 30 seconds at zero F):
http://www.odysseybatteries.com/odyssey101.htmA PC925 is appropriate for a miata 1.8l.
A PC1200 is appropriate for a stripped down volvo 240 2.3l.
Check out the specs at the bottom of the page:
http://www.odysseybatteries.com/batteries.htm
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Miata UBJ: ES-2074R('70s maz pickup)
Ford IFS
viewtopic.php?f=5&t=13225&p=134742Simple Spring select
viewtopic.php?f=5&t=11815LxWxHt
360LA 442E: 134.5x46x15
Lotus7:115x39x7.25
Tiger Avon:114x40x13.3-12.6
Champion/Book:114x42x11
Gibbs/Haynes:122x42x14
VoDou:113x44x14
McSorley 442:122x46x14
Collins 241:127x46x12