The vehicle was heading south on the Harbour Bridge when the tyre came off.
No one was injured, but a lane was closed while a tow truck was called.
Huge turnout for this year's event
Thousands of runners were out pounding the streets for the marathon and its related events this morning.
Organisers said 17,000 people were taking part - the highest number in 10 years.
Of that, 3000 runners were taking part in the full 42-kilometre marathon, which kicked off at 6am.
That course ran through Devonport, over the Harbour Bridge to St Heliers Bay and back to Victoria Park in the central city.
Another 14,000 registered runners were signed up for other events, including a half-marathon, an 11km traverse, a 5km run and a kids' marathon.
Auckland Marathon said it was the largest turnout since 2014, with 62 percent of the participants running the event for the first time.
They hoped to raise $600,000.
