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[Eagle of Fire]

I just tryied this myself and vehicules can run about forever without replacement. The trick is that it's reliability will plunge toward bottom as it age and in itself should be the reason why you would buy a new vehicle to replace it. I actually kept road vehicles running 10 years past due in a recent game so I presume you could possibly run them forever if you wanted to.

If you think you are about to get a new version of the vehicle considering to keep it running a little longer than the life time date might be a good choice even if the real saving would be in time and not in money...