Wrong Battery?

1987 Ford F150, 302, 5.0, v8, automatic, 120k miles

I’ve been pricing batteries for my truck and the last owner had the wrong size. It is a Walmart brand Valuepower 24F. The correct size is supposed to be group size 65. I don’t know the difference between battery size numbers, as far as functionality, but is there a tremendous difference between the installed 24 and the correct 65?

There’s not a lot of difference between the 2, I would just keep using the 24F until it needs replaced,

Then get the 65 when it’s time for a new battery, The 65 has a few more CCA (cold cranking amps) and a little better reserve capacity.

The price for both sizes appears to be the same.

As far as I can determine group 65 is correct for your truck.
I like to proactively replace my batteries at 3-4 years, started doing this after a battery failed in the middle of the night! By replacing a battery before the old one fails you can shop by price, special sales. My car has a Duracell from Sam’s but my truck has a Motorcraft because of a rebate when on sale.
In addition to what It_s_Me said the group 65 will probably fit better in the battery box/holder. Generally within a group size there will be different CCA (see explanation below), higher CCA higher price, but over the life of the battery it’s negligible. The higher range batteries often have a slightly better warranty. As to brand, there are only a handful of manufacturers, that produce all the batteries available.
CCA:
CCA is a very important battery performance indicator. It’s based on how well a new battery will supply sufficient power for 30 seconds, at zero degrees Fahrenheit. The higher the number, the better.

Your new battery’s CCA number should match the CCA number in your owner’s manual, or the CCA of the vehicle’s original battery. If you live in a cold climate, a slightly higher CCA rating may be beneficial to your vehicle’s cold-starting performance.

Those Valuepower batteries From Walmart are only $49 and I assume are bought only by people who need to replace a battery to sell something, They only carry a 1 year warranty.

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My Ford truck uses a group 65 as I recall, which I bought at Costco, and that’s been working fine for 3-4 years now. My daily-driver Corolla’s battery is a relatively new, maybe 18 months, Walmart $49 special, and it’s been working fine too, no complaints at all, to add another data point to the mix.

store i dealt with said my car had a recommended cca and it listed a lower cca as acceptable. car had a 4cyl and v6 available but i recall store saying it might have had an impact on their recommended chart? i bet the price diff was very little. this was 20 yrs ago

I haven’t bought one yet, but it’s on the near future’s purchase list. It hasn’t been running well (stalling while driving) but I swapped some old parts, including the starter, and it’s been having trouble starting. The only thing I can think of is it being a battery issue, even though it holds a charge (checked with multimeter and charged with portable charger to 12.4 volts)

Battery should not cause a stalling problem unless it is almost totally dead.

“having trouble starting” could you elaborate? If it’s cranking fine, the battery is OK.

If it cranks normally your battery is probably adequate for the short term. On a 30 year old truck there are many possibilities for the problems listed.
Spark plugs, spark plug wires, fuel pump and/or filter, fuel system may be dirty (this is where our old buddy SeaFoam comes in), and more!
Good luck with your truck.

“Cranks ok but won’t start” is usually something besides the battery. It’s the “fails to crank” which is is often a battery problem. Here’s a test a diy’er can usually do to get a quick measure of the battery and alternator status. All that’s needed is an accurate volt meter.

Before first start of the day the battery + to - should measure approximately 12.6 volts. Immediately after starting the engine and idling, it should measure 13.5-15.5 volts.