Iowa State's solar car slow but steady on day four of the North American Solar Challenge

AMES, Iowa -- It was slow but steady for Iowa State University's Sol Invictus on Wednesday, the fourth day of the North American Solar Challenge. Team PrISUm's solar-powered car traveled an average speed of about 35 mph to cover about 225 miles from Seneca, Kan., to Sioux City, Iowa.

The slower speed was due in part to clouds and sprinkles during the morning, prompting the team to conserve power and charge batteries as they drove, said team member Luke Martz.

Martz said the only challenge of the day involved the "chase van" support vehicle, which almost ran out of gas. Otherwise, Martz said they're "basically cooking now!"

The team is expected to make it to Sioux Falls, S.D. for Thursday's rest day around noon, although an approaching thunderstorm threatened a delay.

While in Sioux Falls, team members will replace the car's wheel bearings, seat tires and complete other regular maintenance.

The race between 13 teams ends on July 22 in Calgary, Alberta, Canada.