1968 Mustang Battery Size
One of the nice things about older cars, is that they've been around so long, it's given the automotive aftermarket plenty of time to make lots of parts for them, especially popular cars, like the 1968 Ford Mustang. In the case of that particular Mustang, OPTIMA offers several different battery upgrade options that we consider to be direct-fit replacements, meaning they should be able to get dropped right in where the original battery was installed at the factory. Those options include our Group 34 and Group 35 batteries in both REDTOP and YELLOWTOP configurations.
The Group 34R REDTOP differs from the others in that the positive & negative post terminals are reversed when compared to the other Group 34 options, while they are of the same orientation as our Group 35 batteries. In terms of REDTOP versus YELLOWTOP, most older cars, like the '68 Mustang are relatively-simple from an electrical perspective and only need a battery to start the engine. In those applications a REDTOP will work just fine. However, if a vehicle has significant key-off electrical demands, like an aftermarket alarm system or large stereo, a YELLOWTOP might be a better option for it's ability to be deeply discharged multiple times.
Another situation where a YELLOWTOP might be a better choice over a REDTOP would be in a race car, like Doug Obermeyer's 1968 Ford Mustang that we spotted at a vintage racing event at Road America. If a race car doesn't have an alternator to keep the battery properly-maintained or there's a chance it could fail and the battery would be called upon to deliver electrical current to critical accessories (like an electric fuel pump), then the OPTIMA YELLOWTOP is the correct choice in either a D34/78 or D35 group size. You can take a look at all five of those Mustang battery size options here.