Mr Turnbull said his relationship with the United States president is strong, and he discussed " a number of things, including some old mutual friends" during his time in Mr Trump's $1.5 million vehicle.
"President Trump and I get on very well," he said, "which is good, as is apparent."
The wives of Mr Trump and Mr Turnbull, Melania and Lucy, were also present during the ride on Friday night, after the first full day of the G20 meeting in Hamburg.
The eight tonne vehicle features five-inch thick bullet proof glass, kevlar tyres and is outfitted with grenades, pump action shotguns and tear gas cannons.
Mr Turnbull said he "felt very safe" during the ride.
"Well look it was very kind of him. We went from a reception to the concert theatre and he and Melania invited Lucy and I to ride with him in the car," he said.
"And it was the opportunity to have a good private discussion about a number of these issues."
The prime minister will head to Paris at the end of the Hamburg summit, which on Saturday is focusing on innovation, women, jobs, refugees and African development.
Malcolm Turnbull and other G20 leaders are set to unveil a final communique which acknowledges a split over climate policy and US President Donald Trump's "America first" stance on trade.
It is understood the summit's final statement, to be delivered by German Chancellor Angela Merkel on Saturday afternoon local time (11pm AEST), will reflect strong support for the Paris climate agreement and note Mr Trump's decision to pull out.
One paragraph, in a draft seen by Reuters, says the US will "work closely with other partners to help their access to and use of fossil fuels more cleanly and efficiently".
The statement would also be helpful for Mr Turnbull, who has pledged a technologically neutral approach to cutting emissions and taking pressure off the cost of electricity with possible investment in clean coal power in Australia.
The section on trade will, as G20 communiques traditionally have done, underline the importance of free trade but say members have the right to defend themselves against uncompetitive practices.
Chancellor Merkel has described the last-minute negotiations as "very difficult", with Mr Trump taking an "America first" position on trade and describing deals between countries as entrenching winners and losers.
A key meeting during the day will be between Mr Trump and Chinese President Xi Jinping, who will discuss North Korea, steel exports and America's sale of arms to Taiwan.
The leaders enjoyed a performance of Beethoven's ninth symphony at Hamburg's Elbphilharmonie concert hall on Friday night and a dinner at which Melania Trump sat next to Russian leader Vladimir Putin, who had a much-watched first meeting with Mr Trump on Friday.
Mr Turnbull will be hoping to hit some positive notes of his own in bilateral meetings with the leaders of the Netherlands and South Korea on the sidelines of the summit on Saturday.
The prime minister hailed as a success an agreement with Indonesian President Joko Widido to finalise an economic pact by the end of the year.
As well, the G20 members included in a statement on terrorism his call for the rule of law to be applied to tackle extremists using encrypted communications apps and social media.
The statement called for work with industry to provide "lawful and non-arbitrary access to available information where access is necessary for the protection of national security against terrorist threats".
Extra police are being brought in to handle what is expected to be protest rallies involving 100,000 activists, some of whom have been setting fire to cars and looting stores across Hamburg.
