Well ,I think I've made a text book error in finding out why my average speeds have plummeted of late. The winter bike I use is a fairly chunky Boardman mountain bike fully kitted out with mud guards/disc brakes etc. I do though have some 1.50 slick tyres fitted ,and in all fairness they've coped pretty well down the filthy lanes I cycle down (roll on summer for the road bike again).
I haven't been checking the tyre pressures of late with my gauge. I've only been pressing the tyres with my thumb as a quick test and seem to be fairly good.
With the pressure gauge today I check them and was very surprised to how little pressure they had in them even though they felt good with my so called thumb test.
I usually inflate them to around 80 psi but they where down to around 45!!!.
After pumping them up again and a 15 mile ride ,my average speed was back up to around what I would call normal again
Still got home knackered though lol :-) :-)