Tracking calories from cycling/bike riding

Most accurately is with a power meter. It isn't even close. A power meter will tell you how many calories you put into the bike to within +/- 2.5% of god's honest truth.

The good news is that for weight loss you don't need that kind of precision. If your riding is fairly consistent you can ballpark it and then adjust based on what the numbers predict your weight should do vs what the scale says it's doing. 🙂

In the example you gave, I would be inclined to trust the Fitbit's numbers more but it depends a surprising amount on the bike, and on how fast you went. Also, some fitness things tell you how many calories they think you spent on exercise, and others tell you how many they think you burned during that exercise - the difference being your BMR. It probably took 3+ hours to ride 60 km (?), so your BMR would be a big contributor. I don't know if they're telling you different things, it's a possibility though.

I'm thinking that must have burned a lot more calories than it would have on a road bike! Awesome effort! 🙂 👍

My calorie estimates have evolved over the last eight years or so. Looking back over that time.....

Spinning bike in a gym (no power meter) was pretty reasonable but a gross calorie estimate, correcting that myself to net estimates I would say for me it was pretty good.
Ramping up the mileage mostly on road hybrids I used HRMs and Strava app - perfectly reasonable for weight loss/maintenance but probably a bit on the high side.
MapMy suite of apps I found very high for everything.
Garmin bike computer with HRM input for a few years - again pretty reasonable. (Maybe the Strava and Garmin reasonableness was due to me having pretty dreadful aerodynamics and a low HR?)
Power meter equipped indoor trainer - very accurate if you did the maths manually or connected a bike computer but told big lies if you trusted their algorithm.
Comparison between HRM and accurate power meter indoor training showed steady state estimates by HRM entirely reasonable but a significant over-estimate for intense intervals.
Power meter equipped road bikes - very accurate most of the time. Very likely to be significantly under-estimates for winter riding when it's a struggle to stay warm.

For dieting or maintenance none of those estimates have the numerical power to disrupt my weight control, people get a bit too obsessed over accuracy. Someone would need a very large volume and a very large error delta over an extended period of time to be a big issue. Reasonable really is good enough for purpose.

---

I don't use a Fitbit but is that 1500 just for the exercise alone or is that a reconciliation you see between Fitbit and MyFitnessPal? (The reconciliation isn't just exercise.)
If you are eating to your daily variable Fitbit goal I'd suggest simply carrying on and apply an adjustment if your weight trend indicates you need to.
Did you have the HRM going during the gym workouts? HR is a dreadful metric for weights workouts as it's neither cardio or steady state so a long way from where HRMs can be reasonable. Some wearables have a mode for weight training which should switch off using HR for calorie estimates.

I get results from using "Ride With GPS," my little stem-mounted cycle computer and MFP. All these numbers differ. I tend to believe RWG more than the others. As the pirates said in "Pirates of the Caribbean," " ... these is more like, um, guidelines ..." and that's how I treat them. I measure actual progress by the bathroom scale and live as best I can by the other guidelines.