If it is your very first time up Pinnacle I would recommend the ground up (hiking from Pinkham) option for a few reasons. If you are not already familiar with the climb then leaving the car "at the top" presents issues should you have to bail due to weather, getting off-route, taking too long, etc. If you climb "ground up" you can retreat and hike back down... vrs. pushing on in deteriorating weather... Just saying, it is Mt. Washington, and it is always easier to go down then up. Also, it is more classic to climb a great route like that by the normal approach/descent.
That being said, I utilize the auto-road for most ascents these days, however;
1) The road doesn't open until at least 8am. You could hike from Pinkham at 6am and save $30 or so and get to the base at the same time.
2) I only use the road during very good weather conditions
A fast party, from the Cow Pasture parking, can hike down the Huntington Ravine trail, rope up, and climb Pinnacle, and return to the cow pasture, in 2 hours. That's fast, and knowing the route. Onsight, first time, probably take 4-6 hours... maybe more adding in a trip to the summit...
Why add the summit if using the road? That seems.. odd considering you've driven almost 4,000 feet up the mountain already... but meh, to each there own... hope my advice helps a little...
