Which event was it, and what time did you do? I've found the AI mostly too slow for that to work, but maybe it can work on the odd race if it's a simple oval and you don't have to stay behind them for very long to save enough fuel.
Edit: Okay, I have tried it for some events, it can be a little bit faster, the simpler the track the better as the AI are so bad. But it's extremely RNG because you're so dependent on the pace the AI sustain for long enough to save enough fuel, and they so often take each other out, or pit crazily early. I must have tried about 10 times on one event without it ever working again after it saved me about 1.5 seconds one time. So for an easy life, just drive away from the AI and pit, but if you really want to save every last second, it can be beneficial to save fuel behind the difficulty 5 AI, and particularly so if it's a completely basic oval.