Let this be an exercise in numbers and futility but if Ultron is at $874,989,000 and it hopes to pass Furious 7 which is at $1,466,420,000 (as of May 10th), Ultron has to do about $591 million to catch up - and Furious 7 sales need to stand still.
Furious 7 had the biggest box office opening and film in China @ $325-$326 million. Before that it was Transformers: Age of Extinction at $319-$320. I don't think it's going to be massive and completely crush the previous record by doing $400 million - but let's say it does - there's still $191 million left to make up. If Ultron did about $80 million this past weekend and take best case scenario of 50% drops each week @ $40, $20, and $10 million over the next 3 weeks there's still $121 million left.
Where does that money come from?
At the same time, the international haul for Furious 7 is $1,128,000,000. Ultron is @ $562,400,000. If it did $400 million there's still $166 million left on that front. Furious 7 will still have a larger international box office (by then all the large markets are done).
I'm not saying Ultron is doing bad numbers (because it's still $624-$625 million in profit) I'm just asking what those numbers could be if the movie appealed to a wider audience (because environments change) and who actually thinks it won't bow down to Furious 7's sales in the final tally?