Ill tell you this, a 85 vw golf diesel with a group 34 battery can go 70 miles highway, but be warn the brakes suck power for the lamps. Killed it nearly twice, but it was a stick so bump started.
Whats the reserve capacity printed on top of battery? its listed in minutes and it is a good estimate assuming you limit your accessories and its in good shape. A bad battery can last minutes. If its a bad alt, the battery maybe at a reduced capacity already.
