We are thinking of getting the mont bell 800 #1 Womens for our winter bike tours. It is rated at -3 for comfort. Temperatures usually range between +5 and -12 though usually in the middle of that range.

Would the bag be too warm on those +5 nights? We already have an quilt for the coldest times.

Cheers

I doubt it would be too warm at +5 Celsius, but it could be a bit too cold at -12, since the "lower limit" rating is -9. A lot would depend on whether you tend to sleep warm or cold in general, and what clothes, if any, you wear inside the bag. For winter use, you'd likely wear a base layer of some kind in the bag, and could, of course, add more clothing if necessary.

Personally, I mistrust the ISO ratings. The best way to judge the warmth of a sleeping bag is by the amount of loft (thickness). As far as I know, Western Mountaineering is the only manufacturer who states the amount of loft for their bags. A visit to their web site, westernmountaineering.com, may help you compare. If you can see the Mont Bell bag in person before you buy it, you could check it.

I don't think it would be too warm at +5°C. It is not terribly difficult to cool off a sleeping that is rated to much lower temperatures: unzip, stick out an arm, stick out a leg, just cover your lower appendages. Your body is constantly circulating blood normalizing your overall temeperature. Your whole body doesn't need the exact same amount of insulation. you just need to get the overall insulation correct.