Fuel economy down after 18/38 sprocket swap

[TheRadBaron]

Over the winter I swapped the stock gearing of my '84 FJ over to the 18/38 combo in search of some more MPG.  However, at the same time I did a number of other mods that could have an effect on mileage as well.  I put on a 17" later FJ front wheel and the 17" GSXR rear wheel.  I also switched to blue spot calipers.  Oddly enough, my mileage went from about 38mpg before the mods to around 33mpg afterwards.  I have a adjustable bicycle speedometer that I calibrate using the mileage markers on the interstate.  It was accurate before the mods, and I recalibrated it afterwards.  I understand that there are a huge amount of variables here, but has anyone heard of mileage going down after making the gearing taller on our bikes?  I ride it in the same manner as I did before I did the mods.  That is, medium-aggression, mostly 75mph, highway riding.
I thought that I'd pick up a few MPG, not drop 5.  Any ideas?  One thing that occurred to me is that maybe the lower RPM has put the engine into the less efficient midrange with my 4-1 exhaust.


[edit] I corrected the title of the post from 18/83 to 18/38. Hope you don't mind Sgt.

[rexsk8er]

you should measure overall diameter of the new tire and old tire.


chances are your new overall tire diameter is smaller now with a lower profile tire, even though its a larger rim.


after the sproket and chain your tire diameter will influence working rpm vs mph

[yamaha fj rider]

Changing the gearing like that shouldn't lower the MPG. Must be some thing else. I like the wheel circumference question. How is the jetting?  Hope this helps?

Kurt

[Pat Conlon]

I agree with Kurt, something else is going on.... Have the carbs been sync'ed lately?  Fresh plugs? If not, then perhaps it's time.
I would run several tanks of gas thru to get a good base mileage data.

[ian.z9]

I've been running 18/38 for a wee while now.FJ1100 with 1200 motor and 17"wheels,standard pipes and airbox. Normal work commute in town and 80mph on the M-way gives me 55mpg (uk gallons).Same route on my fj1200 engined z900 with filters and 4into1 gives me 43mpg.It was 38mpg before I played with the needles.18"wheels on the zed and a 33 rear sprocket/17front.70mph at 4000rpm ish on both.

[ELIMINATOR]

My chain and sprockets are nearly due for replacement. Am I correct in thinking that the standard gearing for a 1990 3cv is 17/40?

Well that's what it shows on here. http://www.gearingcommander.com/

[andyb]

Eliminator:  Yes, 17/40 is stock for the 89-90 US models.

A drop in mileage right at the same time you changed brakes?  I know where I'd be looking first, a dragging brake can easily trash your mileage as well as your brakes.  That said, the idea that you're running less rpm and the engine isn't as efficent down low is also possible.  Also, to get the same amount of oomph will require a lot heavier use of the throttle, which won't help either.

[Pat Conlon]

oooooh, very good Andy....a dragging brake pad.....good catch amigo!   

[TheRadBaron]

I had thought of the dragging brake thing, and there might be something there.  I can hear the pads slightly dragging when I roll the bike around the garage, and it seems to be just a bit harder to push than it was with the old brakes.
However, I can't notice any dragging while the bike is coasting, and there's no undue heat from the brakes or unusual wear.  Is it normal to be able to hear the pads dragging (while rolling the bike with the engine off) with the blue spots?

[Pat Conlon]

yep, with the HH pads I run there is a slight dragging noise when I push my bikes around....that's no biggie.
Coast to a stop and feel the rotors...if they are hot, then a pad is dragging, a piston is not retracting.

[andyb]

Safer, and easier:

Come to a stop using the rear brake, check the feel of the fronts.  Check rear brake after using fronts to stop.

A little bit of contact is normal, as they don't have little retractor springs to pull the pads back, though some (car?) calipers have had that feature I understand. 


I dunno though.  My daily ride gets about 45mpg in normal riding, and 51mpg if I ride like my hair's on fire.  Needle settings are actually slightly rich (intentionally) on it, too.  I can only think  that the motor is running up in a more efficent area when I'm out hooning about like that, so it's getting better mpg. 

[rexsk8er]

So,

A slight drag on the breaks is typical and actually good, they wear more evenly if they make proper contact and a slight drag means that. a simple test is just to mash the break hard and if its difficult to roll at first that's a problem.

Back to my first post

If all being equal the stock vs stock tire size for fj and a 94 gsxr 600 are 150/80r16 and 185/55r17  with od's of 79.95 and 78.58, respectively. So, unless your tire sizes were much different I feel like your cruising at a higher rpm than before per given MPH even with the new sprockets because there pretty close to stock, equivalent to a tooth smaller rear with stockers. 1 tooth wont make up for  1 and 1/4 inch tire difference.

This is all subjective to tire size however, of new and old tires. check it out!!!!

http://www.tacomaworld.com/forum/tirecalc.php?tires=150-80r16-185-55r17




looks like OD is about 1.7% difference you be the judge 

[Mark Olson]

did you switch to radial tires?
what pressure are you running?

[oldktmdude]

So,

A slight drag on the breaks is typical and actually good, they wear more evenly if they make proper contact and a slight drag means that. a simple test is just to mash the break hard and if its difficult to roll at first that's a problem.

Back to my first post

If all being equal the stock vs stock tire size for fj and a 94 gsxr 600 are 150/80r16 and 185/55r17  with od's of 79.95 and 78.58, respectively. So, unless your tire sizes were much different I feel like your cruising at a higher rpm than before per given MPH even with the new sprockets because there pretty close to stock, equivalent to a tooth smaller rear with stockers. 1 tooth wont make up for  1 and 1/4 inch tire difference.

This is all subjective to tire size however, of new and old tires. check it out!!!!

http://www.tacomaworld.com/forum/tirecalc.php?tires=150-80r16-185-55r17




looks like OD is about 1.7% difference you be the judge 

I think you mean circumference not O.D.    1.25"difference in circumference is nothing to worry about.

[Klavdy]

I reckon it's because since you hotted the bike up, you're getting up it more.
You are more confident and riding it harder.