Near the end of the movie after leaving lone pine mall (formerly twin pine mall) doc brown takes Marty back home and then goes to the future. The DeLorean has enough road to get to 88 miles per hour. But the next day when doc picks up Marty and Jennifer, Marty tells doc that they don't have enough road to get up to 88 when doc had enough the day before!

They now have three people instead of one. Increased weight will affect acceleration.