Declining MPG on my commute

[kumatae]

I’ve put little over 12,000 miles on my commute and those miles are very straight forward, 30 miles of freeway to work at 70mph and 30 miles back at about 50mph average. I pump my gas at the same Costco gas station. Have Madstad windshield with a top case. I track all my fill ups on fuelly. I always shift before hitting 4000 rpm and is very easy on the throttle. Initially I averaged in the 60s, never got in the 70s like some have here but I was still happy. I’m also on 17t front sprocket. I would say I’ve seen a slow decline and now I get about 55 mpg. Bike is 2014 and now have 16k miles. Haven’t changed the air filter. Tires are shinko 705s and changed the chain at about 10k miles. Other than that I just ride and love the reliability of the bike. Anything that I’m missing on the cause of my decline? Still happy with the efficiency but just wondering if there is something I’m missing here. Thanks!!


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[SilverRocket]

Is the current tire size the same as your original tire size?
Higher profile tires, with lots of rubber, would give better mpgs.
Adding a high profile windsceen and top case could reduce a few more mpgs. Have those been the same the whole time?
Any additional weight (rider or accessory)?

The thing that I notice that really effects my mileage is going fast. Over 80 and the nice average I got on the first half of a tank immediately starts going down.

 

[Deleted member 1183]

There are only two possible causes I can think of. One is that the tyre pressure might be too low and the other is the obvious one, the air filter.

 

[GgarryP]

Just a thought. Did you get a good quality chain and keep it lubricated? Are the sprockets getting worn? Chain and sprockets can eat a lot of power.

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[670cc]

Since the Shinko 705 rear tire does not come in the standard OEM size, either of the two close sizes will affect the gas mileage calculation. The 150/70 size has a larger circumference than stock. In addition, your 17t sprocket further messes with your numbers. The effect is not from the bike necessarily being more or less efficient, but rather from odometer error in your mpg calculations. Your bike is traveling further per countershaft revolution than what the odometer is measuring/assuming, hence your gas mileage is actually better than what you’re calculating.

I’ll expect many people to jump on the air filter as the culprit, but I’d doubt it’s the cause. The filter is very large. The primary air flow restriction is the throttle plate, which is a small opening most of the time since you rarely run at full throttle. Unless you ride in a dusty environment, or a rodent dragged stuff into your air box (well worth checking) that filter should easily be good for 24,000 miles.

I recently experienced a 10% drop in fuel economy on my GL1800. After a couple tanks of low mileage, it eventually turned on the MIL. The root cause was a bad oxygen sensor in the exhaust system. The engine was running richer in open loop mode. I have never heard of an oxygen sensor failure yet on an NC, but I mention it only as a very, very remote possibility.

 

[kumatae]

Thanks for the replies! I feel really dumb as the culprit was an obvious one. Silver Rocket and 670cc got it right. It’s the tires and the sprocket. Changed sprocket May of 2017 and both tires got replaced Aug 2017. That coincides with my declining MPGs. The rear shinko is a 150/70 and not sure if the front matters but it’s a 130/80. I’ll have to do the exact math when I get to work but I’m worrying about nothing. Thanks for all of your inputs, really appreciate it!!



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[mreric]

16k is also due for a valve adj.