Help from other bike riders needed.

Either get a HRM or take MFP's calculation. MFP tends to estimate high for most people so I wouldn't add any more to it.

I ride a road bike with lots of gears and I pedal constantly. My HRM always shows less than what MFP estimates.

I've been riding my bicycle for exercise for a while now, but it is hard to accurately count my burn.
The trail I ride is about 8 miles and I ride it in 45 min or less, so I know that my speed is 10 mph.
I record this using MFPs 10-12 mph option.

My bike is a regular one speed (my peddling). My husband as a multi-speed bike and we both agree that I work a lot harder when we ride because I have to peddle constantly.

I assume that MFP biking calories burned option is for multi-speed bikes because that is what any sane person would be using to exercise.

How would I be able to tell how many calories I'm actually burning? Or should I just assume it is more than what MFP option says but still record it using that option?

(I really like to keep accurate record of what I'm burning.)

Cheap heart rate monitor. It'll be your best friend.

Buy an odometer for accurate results.

I used to work at a bike shop all through high school and college and I used to race cross country, dual slalom and for the last few years I have been riding road.

A common question about cycling is "how hard am I working?" While you say that you are pedaling constantly on your single speed, I am assuming that you are riding a beach cruiser basically from your description. Adult bike with one gear and when you pedal backwards it brakes, but you can coast along, as if you are going down a hill but not having to pedal. A fixed gear means that you can not coast... so the pedals are turning basically as fast as the rear wheel is. The gear is fixed to the rear wheel.

Where you really burn your calories is how fast you spin your pedals, rather than how fast you are riding, (one often leads to another but not always necessarily true). It is really all about getting your heart rate up and spinning your pedals...... which is why there are Spinning Classes with stationary fixed gear bikes that force you to keep your pedaling up.... that is the work out.

If person A and person B are riding beside each other on multi-speed bikes and both are going 15 mph the one who gets the better workout is the one whose pedals are spinning faster, like riding in an easier gear.

I hope that helps explain it a bit better. I am by no means an expert, but just someone who has been doing it for a long time, and this is what I have picked up along the way.

But like everyone here said.... its best to get a heart rate monitor!