Clydesdales/Athenas (200+ lb / 91+ kg) - Cadence calobration?

So I picked up my neww Caad 95 and took a couple of spins (the weather still stinks in the NE) and I found that my cadence was sticking around 70-75 on flats at a comfortable pace doing 17-20 mph.
I thought I was out of shape for biking so I hit the gym and got on the Lifecycle and spun at 90-100 rpms at level 14 and was pretty comfortable.
I then went to spin class again and once again found my comfortable cadence in the low 90 range.
Since I don't feel like I'm biking at 20rpms slower on the road, why would my cadence on my bike computer vary so much from the gym equipment? I tried a 70-75 rpm cadence on a spin bike and it was sssllllloooowwww, so I know that can't really be what I'm doing on the road bike
Any advice on how to calibrate my bike computer? Is it possible that it isn't accurate?

Check the display against the gearing using this: http://www.sheldonbrown.com/gears/

It could be that if your magnet and pickup aren't properly positioned, you're missing revolutions. Just a thought. Use the gearing chart above to get an idea if this theory holds any water.

Set your cycle computer so it displays the time with seconds. Count pedal revolutions for six seconds and multiply by ten. This number will be within 10 rpm of your actual.

So I picked up my neww Caad 95 and took a couple of spins (the weather still stinks in the NE) and I found that my cadence was sticking around 70-75 on flats at a comfortable pace doing 17-20 mph.
I thought I was out of shape for biking so I hit the gym and got on the Lifecycle and spun at 90-100 rpms at level 14 and was pretty comfortable.
I then went to spin class again and once again found my comfortable cadence in the low 90 range.
Since I don't feel like I'm biking at 20rpms slower on the road, why would my cadence on my bike computer vary so much from the gym equipment? I tried a 70-75 rpm cadence on a spin bike and it was sssllllloooowwww, so I know that can't really be what I'm doing on the road bike
Any advice on how to calibrate my bike computer? Is it possible that it isn't accurate?

Most sensors have a mark on them, the magnet should be centred on the mark, if it isn't then your sensor could be hit and miss, resulting in a lower RPM. Unlike wheel revolutions there isn't any real math to cadence, it basically counts the number of times the magnet passes the sensor in a given amount of time, if the time period is say 5 seconds, and it gets 8 revolutions, then 8 x 12 = 96 so it sets the display to 96, and repeats the process. You can tell the test period by coasting and seeing how long it takes to get a 0 on the display. Different computers use different periods, my Filzer takes about 15 seconds, and then it doesn't zero it shows average cadence.

You do need to check the sensors once in a while, they sometimes get vibrated out of alignment and stop reading properly. This is common with sensors that are zip tied in place, one help is to wrap a small piece of an old inner tube around the bar before installing the sensor, to give it a little better grip.

Most sensors have a mark on them, the magnet should be centred on the mark, if it isn't then your sensor could be hit and miss, resulting in a lower RPM. Unlike wheel revolutions there isn't any real math to cadence, it basically counts the number of times the magnet passes the sensor in a given amount of time, if the time period is say 5 seconds, and it gets 8 revolutions, then 8 x 12 = 96 so it sets the display to 96, and repeats the process. You can tell the test period by coasting and seeing how long it takes to get a 0 on the display. Different computers use different periods, my Filzer takes about 15 seconds, and then it doesn't zero it shows average cadence.

You do need to check the sensors once in a while, they sometimes get vibrated out of alignment and stop reading properly. This is common with sensors that are zip tied in place, one help is to wrap a small piece of an old inner tube around the bar before installing the sensor, to give it a little better grip.

Never thought of that...thanks for another tip, Wog. I have had some trouble with the sensor for a Garmin 705 moving around. Because it picks up one magnet from the crank for cadence and another from the back wheel for speed, the position is pretty important.

So I picked up my neww Caad 95 and took a couple of spins (the weather still stinks in the NE) and I found that my cadence was sticking around 70-75 on flats at a comfortable pace doing 17-20 mph.
I thought I was out of shape for biking so I hit the gym and got on the Lifecycle and spun at 90-100 rpms at level 14 and was pretty comfortable.
I then went to spin class again and once again found my comfortable cadence in the low 90 range.
Since I don't feel like I'm biking at 20rpms slower on the road, why would my cadence on my bike computer vary so much from the gym equipment? I tried a 70-75 rpm cadence on a spin bike and it was sssllllloooowwww, so I know that can't really be what I'm doing on the road bike
Any advice on how to calibrate my bike computer? Is it possible that it isn't accurate?

The cadence readings are probably 100% accurate in both cases. You are just working at a lot lower resistance on the trainer compared to the REAL bicycle. It's just like a treadmill, sure you can run at 8-9 mph on a treadmill for an hour. Just try it on the REAL road and you will see what I mean.

Your speedometer can be mis-calibrated due to having wrong roll-out distance. This wouldn't apply to a cadence measurement, though.

I doubt you're missing revolutions. If it works like a speedometer, you're either picking up everything or nothing.

Your speedometer can be mis-calibrated due to having wrong roll-out distance. This wouldn't apply to a cadence measurement, though.

I doubt you're missing revolutions. If it works like a speedometer, you're either picking up everything or nothing.

If the switch is just at the edge of the magnetic field, and there is a little flex in the crank arm or wear in the BB, then it is possible to miss some revolutions, but not all. Of course the thing to check is that the centre of the switch and the centre of the magnet match up.

The cadence readings are probably 100% accurate in both cases. You are just working at a lot lower resistance on the trainer compared to the REAL bicycle. It's just like a treadmill, sure you can run at 8-9 mph on a treadmill for an hour. Just try it on the REAL road and you will see what I mean.

+1 on this. I can spin 95-100 on a stationary like nothing, but spinning my fat butt on the road, I typically average 82-87