loss of efficiency 2015, what to look for?

[galen]

my last 3 tanks have ranged with 92o
486 km
381 km
182 km latest, rear tire was low pressure

what should I be looking for to get the efficiency back up?

PS also the last two tanks I've been noticing a high pitch whine

[An Orange Crush]

With that low of efficiency and a whining sound I`d replace the gear oil in the final drive, check the belt, and see that none of the brakes are sticking.

[Ruffus]

@galen,  high pichted noise means almost always a rotten bearing to a technican.
So, where do we have some.
As OrangeCrusher stressed out already,
-gear oil low or not there at all
-clutch bearing howling
-wheel axle bearing dry and whining
All of this has to be checked right now to avoid further demage.
Broken gear bearings and ignored means most of the time MOTOR change, because bearing seats are wearing out.
The same for axle bearing (new support)