treadmill gradients

Treadmill, I could not manage without.  I am 66y old and suffer with quite a lot of arthritis and sore joints, I have an artificial hip which replaced the arthritic one ten years ago.  Running on the road a lot really causes quite a lot of pain, any 'efforts' on the road cause a lot of pain and or injury.  I find the best for me is to mix the treadmill work with the road or off-road. I do my long run on the road each week.  I will jog/walk with my dog for about 3m once or twice a week or when the evenings are light I will run with my club.  The remainder of my running is on the gym treadmill.  I use a minimum of 2% gradient and do a mix of intervals, tempo runs and 'hills' some of which are in the 'fell runner' style.  I do a Pilates session once or twice a week for the stretching and to help with mobility, and some core and (a little) upper body work.  I walk the dog each day and we do agility (that's the dog not me).  I find this keeps me fit enough to do a half marathon (sub 2h) or a 10K (sub 55m) each month so although very slow I get what I want from my sport and am able to enjoy running with other club members.  I am also tired most of the time except when I rest for a few days before a race when I feel great.  Older runners with similar problems might find that less road and more of the treadmill could keep them going for a few more years rather than give up.

Cam someone explain to me why my times are quicker on the road than the treadmill.. For example I do 40 minutes on the Treadmill I do 5 miles approx, but for example if I ran a 10km race I would complete that around 40 minutes... It seems the treadmill is slower than my road speed.

 is this correct ?

am I going mad ?

No idea?

Is the treadmill calibrated correctly?

 Are you slower on any treadmill or one in particular?

I've only ever resorted to treadmill work when it was so snowy outside that I physically could not run on the roads.

I used to put it on the programme and at maximum, I'd run on a 4 - 5% gradient. I usually did a negative split and ended up running on a 1% gradient at speed. It's hilarious to watch all the other treadmill-goers go : 0 when you run really fast at the end like your life depends on it!

SO glad that I don't have to sweat it out in the gym anymore. Hooray for this lovely weather