So I'm at the point in Final Testimate where the players are on their way to capture and use the ordinatus. Now they have been tried and convicted by commissar Saleri and sentenced to execution before the attack. Now they have stolen a chimera and headed off to the ordinatus during the confusion of the attack.

At some point I would like to have the players run out of fuel and need to find alternative transportation (such as Talleran Mukaali). My question is how long would the fuel in a chimera last and is this a good idea?

You could just play it that Chimera had only minimal fuel left to get them to certain point. After that it's players ingenuity that will keep them going towards their target.

By quick checking, modern tracked IFVs seem to have operational range of 300-660km. Wheeled IFVs can go up to 900km. But I think this is when they drive on the road or similar flat surface.

Most ground vehicles today seem to be designed to run from 4-6 hrs endurance at cruising speeds. I would use this as a guideline or just decide when the chimera runs out of "Gas"

Thanks for the replies, turns out it was not needed for my game as the vehicle basically broke down at the point I wanted them to run out of Gas so now they are continuing the rest of the way on Mukaali. Still, a good reference for the future.

I recommend that you don't spring something like this on your players. Either warn them that you're actually going to take fuel consumption into account, or have your scenario start with "you've run out of gas in place X" as part of the boxed text. In my experience, just pulling the rug out from under your players leads to much irritation.

Well They had fair warning that the vehicle has a certain fuel range, I make them take it into account at the beginning of each extended mission so they can plan ahead on what they need to requisition. I just never really had them have a problem with running out of fuel until recently. Turns out their tank was too damaged to go any further anyway so it didn't end up being a problem.