If it's only eight miles or so, then your best bet would be to get some slicks (around 1.5s) for your mountain bike. This still leaves you the option of going off-roading by simply swapping the dirt tires back on.

If you're under-geared you can replace the chainrings. I've got a Shimano Deore chainset with 48/36/26 for about fifty bucks which works great on my grocery-getter.