3.91 gears w/ 3.23 speedo

after my trip to a bodies only car b que the total mile on my new set up,(engine, trans, and rear) reached 1024 miles. i have 3.91 gears and my speedo is set up for 3.23 gears. does anyone know the conversion to find out my true mileage. also at 3k and 4k rpms what would my true mph be? also when can i start useing a synthetic oil, or oil additive like lucas in my motor? i plan on doing an oil change this comming weekend.