sudden big drop in MPG

[trc.rhubarb]

Bike has ~52.5k miles on it and runs great. It's a bit noisy and has been since I picked it up a year ago. One day I'll get around to ordering new cam chain tensioners.

Anyhow, I've generally gotten in the high 30's and sometimes even mid 40's for mileage but the last 3 tanks of gas (over the last 5 days) have been in the low low 30's. Even the bike knows something is up and shows 31.x mpg on the screen after a reset of the average.

It runs well and accelerates fine, no carbon build up on the bags or muffler.

air filter changed, plugs checked, valves & tb were done and checked good at 48k. FD fluid was changed then and there is no freeplay. No noises in the driveline when rolling in neutral. Checked for drag on front and rear wheels. Brakes aren't hot after riding. New rear tire was put on just before the issues started but the tire looks good and is still at 40psi; front good too. The only other change is that it's been raining.

What's next to check?