Fuel economy down after 18/38 sprocket swap

[yamaha fj rider]

This maybe unlikely but could your pilot jets be lean witch causes you to open the throttle a little more hence forth using more fuel. Hope this helps?

Kurt   

[Mike Ramos]

Gentlemen,
Right on the money Kurt.
Many miles ago I also geared as high as possible.
As my original carbs wore out, the mileage began to drop off until at normal highway speeds (mid-70's) mileage was in the high 30's.
When I replaced the carbs (mainly because of the costs of new diaphragms) the gentleman from RPM rejeted to larger than stock.
Since then, the milage at normal highway speeds is at least in the 42 mpg range. The lowest I ever get is 38 mpg, no matter how I ride (including road conditions, time of day or temperature...?!).
Cruising Highway 50 on my way to Gunnision last week, at night it was quite cool, probably in the 40's, and at cruise speeds in the 90 mph range, I averaged 38 mpg. On my return during the day and warm temperatures, the same mpg.
Also check the float levels as well, the wrong setting will use more fuel, even to the point of your engine running on 3 cylinders (regardless of the temperature, road conditions or time of day). randy at RPM has tips on the proper method(S) to use. And believe it or not, the correct float level will keep you from getting kicked out of hotels...! Apparently not a problem at the KOA.
And as an aside, because of an upgraded oil cooler, not a big difference in oil temperature either, day or night.
Stay in good humor & Ride carefully,
Mike Ramos.

[Pat Conlon]

Right on Mike! You a funny, funny man!

[Mike Ramos]

Hi Pat,
Funny? Not true! But I am Goofy (and have the ears to prove it).
As the record shows, there is an ex-wife who unequivocally sates that I am " [expletive deleted] Goofy!"
Ho hum...
Ride safe,
Mike Ramos.

[markmartin]

I just checked gas milage again on the last two fill ups.  I'm getting 46 - 48 MPG. 18 tooth front, 38 tooth rear. GSXR rear wheel, new carbs, recent valve adjustment, carbs synced, new wheel bearings, new blue spots.  I very rarely go much over 5000 rpm. 91 octane if it matters.  60,000 mi on the odometer.

Back when I first got the bike and was getting on it at every opportunity, I was getting in the 36 to 38 MPG.

YMMV.

[ELIMINATOR]

My brain is frazzled trying to work that out.

     1Imperial gallon is 1.2 US gallons. Your rear tyre is a 17", mine is a 16"................ melt down!

[markmartin]

I was getting the similar milage - mid + 40's -- with a 16" rim.  The tire circumferences between the 16" and 17" tires aren't much different.  The 16" has a higher sidewall.  I am enjoying the lower rpms with the 38 tooth rear sprocket especially when cruising, so I'd advocate for a 38 tooth rear.
I guess my point would be that my milage varied more by the way I drove(accelerated etc) than by changing the ratio of the sprockets.  Again, YMMV.

[Arnie]

A stock rear wheel/tire combo is about 25.5 inches high.
a 180/55x17 wheel/tire combo is about 24.8 inches high.
So, the circumference of each is (pi * D) is 80.1" OEM and 77.9" with the 17" wheel.
This gives you a difference of ~ 3% pretty close to the 4-5% change of a single tooth on the rear sprocket.

Hope this helps your argument :-)

Arnie

[yamaha fj rider]

 Yes the big front sprocket and small rear are great for cruising, but the small front and large rear make the front end real light and save on front tire wear this can be hard on fork seals. Too much fun  !!!!

Kurt

[markmartin]

I've noticed my MPG is the same or slightly lower since I've switched to 18/38 from 17/38.  I know it doesn't make sense and this is only over 5 or 6 tanks of gas.  For what it is worth, I've been commuting and cruising at 3000-4000 RPM with the 18/38.  If I recall from last summer, some of my best mileage was on a trip that I was cruising faster and with the 17/38 gearing, the RPM's were more in the 4000- 5000 + range.  

I still enjoy the lower RPM's.

[movenon]

I've noticed my MPG is the same or slightly lower since I've switched to 18/38 from 17/38.  I know it doesn't make sense and this is only over 5 or 6 tanks of gas.  For what it is worth, I've been commuting and cruising at 3000-4000 RPM with the 18/38.  If I recall from last summer, some of my best mileage was on a trip that I was cruising faster and with the 17/38 gearing, the RPM's were more in the 4000- 5000 + range.  

I still enjoy the lower RPM's.

I to enjoy the comfort level of lower RPM's. But the engine efficiency is probably better closer to the peak TQ RPM. I think she likes to spin....
George

[Thmsdoyle]

I have the same mods as move on and markmartin, my mileage has dropped from the 40 to 42 range to 38. However; I'm riding a lot more aggressively! So Im sure that is a lot my lower mileage. May take her out for 400 mile rode trip this w/e, so I'll see what she does on the open rode. I can see the lower rpms making the engine work harder do to lower torque. However at higher highway speeds (75 to 80) it may be back into power band of the engine??? Makes sense but I'm just guessing, haven't actually calculated anything out.

[Mark Olson]

volumetric efficiency is the key .

[aviationfred]

I went on a 350 mile ride over the weekend. I have the 18/38 sprockets with a FZR750R front wheel w/standard size tire. A GSXR rear wheel out back with a 180/55/17". My mileage used to be between 40 to 42 mpg. Saturdays ride gave me 44.8 mpg which would give me 260 miles on 5.8 us gallons.

Fred