The problem with your question is that trainers vary a lot in their resistance, and many trainers have variable resistance. So the speed on the trainer is a non-applicable quantity. You want to know wattage, but of course you don't have a $2000 power meter. The answer is that if you rode the same length of time at the same average wattage as on the road, you'd burn the same or similar amount of calories. The problem, you see, is determining the average wattage. Lacking any measure of that, you'll have to fall back on your Rating of Perceived Exertion (RPE). If your RPE is about the same, your burn should be about the same per unit time. Many people think that time-wise, you only have to do 75% on the trainer of what you'd do on the road. But I don't think that works out for calories burned.