Sudden Drop in Fuel Economy

[betoney]

 I am kind of at a loss on this issue and hoping can get some insight from you guys. I went on a 2 Day Rd. trip last Thursday and Friday and I noticed my current fuel reading was much lower than normal, highway riding at 70 to 75 mph it was reading 38-39 MPG. The fuel gauge dropped to half tank reading at about 90 miles and the F trip started flashing at around 155 miles, this was consistent  for the entire trip, approximately six tanks of fuel. 
 Four weeks ago I did a 3000 mile trip and the fuel economy was much higher, mid to high 40s and F trip coming on around 170 to 180 miles. 
 The bike seems to run fine, it’s not making any abnormal noises nor is it smoking or exhibiting any abnormal behavior, the only maintenance I have done since the 3000 mile trip was a clean air filter and an oil change. 
While on this last two day trip I only used premium fuel at a major name gas station like shell or Chevron.
 
Anybody have any ideas?

[nsmiller]

Check the obvious stuff first:
 
-Tire pressure good?
-Bike loaded normally?
-Abnormal head wind?
-Low temperatures?
-Chain properly lubed? Perhaps it is developing tight spots?
- Gas with ethanol vs non-ethanol?

[betoney]

Check the obvious stuff first: 
-Tire pressure good?
-Bike loaded normally?
-Abnormal head wind?
-Low temperatures?
-Chain properly lubed? Perhaps it is developing tight spots?
- Gas with ethanol vs non-ethanol?

 In the early mornings on both days the temperatures were quite low, on Thursday morning I left the house and the temperature was just at 40° and Thursday morning when I left the hotel in Oregon it was 33°, however by late morning temperatures had risen back to normal, mid to high 50s to low 60s. 
 
Tire red pressure is the same as I always set it, 35F/ 36R, normal E10 premium fuel, bike loaded normal for an overnight trip. 
 
@piotrek. The battery has not been disconnected for any reason, prior to leaving on Thursday morning, I removed it from the Battery Tender that I had placed it on 2 days earlier. 

[jonno12345]

Have you actually checked your MPG by the actual mileage and gallons of fuel used? My on board fuel calc is always a good few MPG out (although typically low..), just so you can tell if you really have a problem!

[roadrash83]

It could also be the change to winter blend gas. I don't know what part of the country you are in, but it does cause a MPG drop.

[estell]

I have noticed a drop in fuel mileage every autumn when the morning temperatures drop. Then the MPG rebounds each spring when the morning temps rise up again. Winter gas could have something to do with it. I have suspected that it has more to do with the rich fuel mixture that the ECU/EMM commands when the coolant temperature is low.
 
I used to do my own tuning of a turbocharged car that I owned. I know that the fuel mixture had to be richer when the coolant temps were below 160 degrees F while it was warming up. That caused higher fuel consumption.
 
I noticed this morning during my commute that the FJ-09 coolant temperature was less than 150F. If the FJ-09 has a thermostat then it doesn't seem to work very well to let the coolant warm up. I suspect that it was running a rich fuel mixture and therefore consuming more fuel. I have intended to test that theory by blocking off a portion of the radiator surface in order to raise the coolant temperature and then compare the MPG calculation. I'll do it someday. I am looking for a black plastic semi-rigid sheet that I can cut to half the size of the radiator and tie-wrap to the radiator shroud.
 

[betoney]

I have noticed a drop in fuel mileage every autumn when the morning temperatures drop. Then the MPG rebounds each spring when the morning temps rise up again. Winter gas could have something to do with it. I have suspected that it has more to do with the rich fuel mixture that the ECU/EMM commands when the coolant temperature is low. 
I used to do my own tuning of a turbocharged car that I owned. I know that the fuel mixture had to be richer when the coolant temps were below 160 degrees F while it was warming up. That caused higher fuel consumption.
 
I noticed this morning during my commute that the FJ-09 coolant temperature was less than 150F. If the FJ-09 has a thermostat then it doesn't seem to work very well to let the coolant warm up. I suspect that it was running a rich fuel mixture and therefore consuming more fuel. I have intended to test that theory by blocking off a portion of the radiator surface in order to raise the coolant temperature and then compare the MPG calculation. I'll do it someday. I am looking for a black plastic semi-rigid sheet that I can cut to half the size of the radiator and tie-wrap to the radiator shroud.

You may be on to something there, I was monitoring the displayed coolant temperature and it hit 160 only a handful of times over the 2 days, most of the time it was at 149-159. 
 
I was going to mention the exact same thing about blocking the radiator. 
Wasnt it common back back in the day to cover half of the radiator to kind of trick the thermostat?
I seem to remember guys taping cardboard over part of the radiator on cold days to impede the air flow.
I may have to try the same thing myself.  

[duhs10]

My '15 runs about 100°F above ambient temperature pretty much all the time while moving... our engines are pretty compact and sit pretty much in the wind so the ratio of surface area to mass is fairly high (I'm making an assumption), therefore there is a lot a surface area to dissipate heat.
 
Even if the thermostat remains closed in cooler temps, I'm willing to bet that the cool air rushing around the engine will be enough to keep it cool while moving down the road at any decent rate of speed.
 
While stopped at traffic lights the temp on my FJ will run all the way up to the point the fan kicks on. Mine stays between 47-49 mpg year round. My throttle usage determines the mpgs for the most part.
 
Maybe someone with better science will comment and tell me I'm full of it.

[2and3cylinders]

Is you ECU flashed so the O2 sensor is dead and it runs an open loop only, T-stat set to 205, speed limiter removed?
 
Black corrugated plastic zip tied only to the lower half of a radiator guard screen will raise temps; yellow will remind you on...
 
I have a heavy right wrist and am used to more powerful bikes but even when I try to restrain my enthusiasm I still don't ever see much more than 40 mpg on 91~93 E10 when the display mpg is showing 43+; and recently I was able to run 2 tankfuls of 91+ E0 and the mileage did not improve.
 
Check your valve lash and plugs.
 
Mileage will drop after 20,000 miles...

[micah2074]

I'm pushing 50K miles and my mileage is pretty much the same as when i bought the bike new in 2016. Like others my engine temp is always around 100F above the outside temp when moving. Mileage does drop slightly when they switch to winter blends. Maybe that's what you're experiencing.

[bugie]

I'm pushing 50K miles and my mileage is pretty much the same as when i bought the bike new in 2016. Like others my engine temp is always around 100F above the outside temp when moving. Mileage does drop slightly when they switch to winter blends. Maybe that's what you're experiencing.

Same here. I live in a sunny country where ambient temp is 30 C (86 F) and the bike runs at 89 C (192 F) when moving and goes up to 105 C (221 F) in heavy traffic. We don't have highways here since the island is less than 50k long so am constantly working through the gears.