I know the general rule of thumb is if you go to a 17 tooth in front, it can give a bit better gas mileage, and going to a 15 tooth give a bit less.  However, what if 99% of my driving is in stop and go city traffic for only a short distance of less than 5 miles, and I don't ever get higher than 50 mph.  Would having the 15 tooth help get it up to speed easier and I'll then just cruise in 5th or 6th instead of 4th or 5th when I'm going 50mph.

I don't know what the theory behind this would be, but it seems that it might be better having the smaller sprocket in city traffic for gas mileage.  If no one knows, I already ordered the sprocket, so I'll report back with what the difference is, if any.

Also, I'm trying to find a what can help with mileage, because I've been getting an average of 30mpg (us gallons) (and yes i'm calculating it correctly).  For this last tank, I've changed my driving habits, adjusted the valves, fine tuned the jetting, aired up the tires, not engine braking, fixed an air leak on the left carb, and probably a couple other things I'm forgetting, so hopefully this all helps.

A 15t sprocket will make it easier to pull off and accelerate to speed.If you are having some fuel mileage issues, check the gas pump for a sticker that says some thing like, fuel is blended/mixed with x% of ethenol.Avoid these if you can.I had issues with blended fuels and engine performance, or lack of.I doubt the sprocket change will effect low speed fuel mileage.That usually depends on throttle control,and rpm shifting points.

So at least the 15t won't make it worse.       I'm pretty sure that the stations around me don't have ethanol mixes, but I'll double check next time I fill her up.  I think a lot of it was my driving style, and the engine braking.  Since I've stopped those things, riding is a lot more enjoyable; especially on my wrists.  And it makes it more fun when I do rev her high, instead of revving high constantly.  But even with those things, you wouldn't expect the mileage to drop down to 30mpg though, right?

Anyone with 15t experience want to join in on how it affects city gas mileage?  Anyone have other ideas on why gas mileage is so bad?

Around town it shouldn't make much of a difference.On the highway you will see a 3 - 10 mpg drop.mainly depends on how hard you ride. 

I get about 60 mpg overall all summer long on my stock 02 GS500, no carb, exhaust, intake, or sprocket mods.  Strictly stock with about 59k miles on it now.  It will drop as low as 50-55 mpg overall in the winter or when pushing it extra hard in the mountains running with the big boys.  I never felt the need for a sprocket change on the 02 GS with it's 3 circuit carbs.

Previously, I got a little better gas milage on my stock 97 GS500E which I put 80k miles on, about 60-65 mpg in the summer.  I put a 15T front sprocket on it at about 37k miles and replaced with the same up to 80k miles.  It was very helpful in getting the bike moving without revving as high and slipping the clutch as much and was particularly helpful in getting moving on uphill grades with my 240# butt aboard.  Otherwise there was little change in performance or gas milage with the 15T, a helpful and harmless change on pre 01 GS's in my opinion.

Peak HP is reached at about 8500 rpm and max torque at about 7000-7200 rpm on stock GS500 engines.  I can run my fastest pace keeping it between 7k and 9k rpm even though it's redlined at 11k rpm.  Rev a little past the HP peak and shift dropping to the torque peak.  I haven't redlined a GS500 more than a few times in over 130k miles of spirited riding.  A good job of carb, intake, and exhaust modification will extend this usable power range to 10-11k rpm and add a few HP and reduce gas milage and increase oil consumption.  Oil consumption increases considerably past 40-50k miles and can be too excessive after that if continuously pushed beyond 9k rpm.

I don't very often do a lot of city riding, day in and out it's a short suburban distance to the country roads.  I take the twistiest and hilliest roads available here in NE Ohio, not great but better than Florida.   

Okay, great!  So the 15t won't hurt at least.  I wondering if I have a fuel leak.  That seems like the only explanation for such bad mileage.  I have to switch to reserve at about 75 to 80 miles (96 model so smaller tank).

Where are all the possibilities for a fuel leak?
• tank breather tube
• tank petcock
• fuel lines
• frame petcock
• fuel line going into carbs

That's it right?  Am I missing any places that I need to check?  And can fuel seep out of old fuel lines?  Maybe I need to replace mine.  I have 43k on the bike.  (99% by previous owners)

Hey Taco, I'm running low mpg on short commutes too. I think that the main thing is that they are *short* commutes, which means the bike has to warm up for a couple o' mins, and then you ride for a handful of minutes. Assume an average speed of 30mph while you're riding. When you're at a stop light or stop sign, you're not putting miles onto the odometer. This means That you can chalk this up to the not-moving-but-burning-gas time. Assume, then, that the actual ride (5miles at 30mph) is 10 minutes. With 2-3 minutes of warming up at 3k/4k+ rpm and then a handful of minutes idling at stop lights (.8-1.2k rpm unless you like revving your engine), it's not hard to see how commuting short distances cuts into mileage. 

Smart Alec answer: The 15T sprocket will cut into your mileage because you'll be accelerating quicker off the line and shifting more aggressively. I dunno if this is the case for you, but I know it would be tempting for me if I had a 15T sprocket.  Ben2go's right about the highway of course, since then you essentially loose a bit of top gear.

I know I'll take advantage of the 15t's gearing, but during my work commutes, I'm generally riding conservatively.  I know that the short commutes is killing the mileage more than anything, but I didn't think it would be this drastic.  I think all the stuff I've done over the past 2 weeks and the way I've been driving will help.  If I can get it to about 40-45 for commuting, I'll be happier.  The actual mileage for my commute is 3.2 miles.  Almost short enough to ride a bicycle.  I guess I should be happy with whatever mileage I get since in my Tacoma I only get 11 mpg commuting.

Quote from: quiktaco on May 08, 2008, 02:02:50 PM
Okay, great!  So the 15t won't hurt at least.  I wondering if I have a fuel leak.  That seems like the only explanation for such bad mileage.  I have to switch to reserve at about 75 to 80 miles (96 model so smaller tank).

Where are all the possibilities for a fuel leak?
• tank breather tube
• tank petcock
• fuel lines
• frame petcock
• fuel line going into carbs

That's it right?  Am I missing any places that I need to check?  And can fuel seep out of old fuel lines?  Maybe I need to replace mine.  I have 43k on the bike.  (99% by previous owners)

Tank vents thru the cap.Here's my write up on how to clean and grease it. http://gstwins.com/gsboard/index.php?topic=41061.0
Sticky carb floats will cause fuel to leak in to the cylinders and air box when it's sitting.You would have hard starting with this, a strong smell of gas in the oil,and gas draining out of the air box drain.Other than that maybe a bad tank seam or pin hole in the tank.

I've never had a problem with the vents on the gas tank that I know of, but since I have 43k on the clock, it's probably worth checking out.  I'll also take a look around for a small pin hole somewhere, or a cracked seem.  Thanks for the ideas

Hey Kids,
I have about the same riding conditions as the OP, I also want to do a 15t or 14t, however I spent 2 hours searching for a sprockets with no luck.. I have 2007 GS500F will sprockets from previous years fit, if so what years and does anyone have some links to online stores??


Thanks

Quote from: ruger on May 11, 2008, 11:53:45 AM
Hey Kids,
I have about the same riding conditions as the OP, I also want to do a 15t or 14t, however I spent 2 hours searching for a sprockets with no luck.. I have 2007 GS500F will sprockets from previous years fit, if so what years and does anyone have some links to online stores??


Thanks

96 and up will work.Google sun sprockets.

I get front sprocket and DID chain from Chaparral, they carry JT sprockets now.

http://www.jtsprockets.com/52.0.html?&L=0&sel_uid=5026&p=

Jt shows same sprockets for 07 model as for 04-06 GS-F so all fronts from 94 on are JT516 and all rears from 89 on are JT823.

http://www.chaparral-racing.com/Chaparral/product_family.asp?family_id=16780&mscssid=D47D97C9B1C7478488B341185A8030E1&Gift=False&GiftID=

Hey guys.  I got the 15 tooth installed this weekend, and took it on a test drive to work this morning.  I have to say, all the hype is a little excessive.  I guess the powerband might be shifted down a little, but is almost unnoticeable.  The great thing that I found from it, however, I don't have to slip the clutch to get started off the line anymore.  I used to have to slip it out till about 10 mph, now with the same throttling, I can have it all the way out by around 4-5mph.

I have to get a new chain soon anyways, so I'm thinking of going up a couple teeth on the rear sprocket sometime in the future.

I'll post gas mileage with the 15 tooth when I fill up again.

I just thought of something, what affect does a different hight of tire have on the gearing.  I don't remember what the numbers are, but my rear tire is a little taller than stock.  Would that essentially make it like having a bigger front sprocket?

If so, then putting on the 15 tooth is just like going back to stock gearing.  I might be thinking of this backward, can someone confirm which way it is?

I have new stock size tires to put on at my place, just haven't gotten to it yet.

All of california uses at least 5.8% ethanol, this is the oxygenate of choice since mtbe is banned for contaminating groundwater. I've spoken with a bp rep before that says it's been upped to 7.5% but face it ALL gas in California is ethanol blended. What year gs500 are you using? If you have a newer one with 3 circuit carbs you can definitely be in 6th gear by 50 with stock sprockets I was usually in 6th by 40 mph revving about 3000 rpm.

I checked the station, but I didn't see anything that says it was using ethanol.  Doesn't mean they aren't though.  I have a 96.  I usually drop it to 6th around 40-50 mph.  Seems to run fine at the low rpms for cruising.  Only problem is, I'm at those speeds for only 30 seconds or so, then I have to make another turn, or get stopped by a red light.

Quote from: quiktaco on May 12, 2008, 09:13:24 AM
I just thought of something, what affect does a different hight of tire have on the gearing.  I don't remember what the numbers are, but my rear tire is a little taller than stock.  Would that essentially make it like having a bigger front sprocket?

If so, then putting on the 15 tooth is just like going back to stock gearing.  I might be thinking of this backward, can someone confirm which way it is?

I have new stock size tires to put on at my place, just haven't gotten to it yet.

If you have a 140/80-17 (i.e. slightly larger than stock) rear tire with a 15 tooth front sprocket, the gearing is almost identical to stock.

I put a gearing calculator on the wiki. You can download it and change the inputs to play around with the ratios. The spreadsheet tells you top speed for each gear, and gives plots of road speed [mph] vs. engine speed [rpm]
http://cgi.stanford.edu/~sanjayd/gs500/Main/GearingCalculator

Hope it helps!

HOLY CRAP I have a tall tire.  I just went out to look and I have a 130/90-17.  Stock is 130/70-17. And you said 140/80-17 with the 15 tooth is like stock gearing.  The tire I have is over 2" taller than stock!

130/70-17  --  24.17" tall    /    75.92" circumference

130/90-17  --  26.21" tall    /    82.35" circumference

140/80-17  --  25.82" tall    /    81.11" circumference

So with this tire and the 16 tooth sprocket, I've probably been geared about the same as a stock tire size with a 17-18 tooth sprocket.  When I get these new tires on, there's probably going to be a major change in drivability!  I wonder why the previous owner got such tall tires.

Those taller tires are cheaper is why you've got them. I'm probably gonna get a metzeler marathon when my cheapo kings wear out, I'm currently running 140/70-17 kings and the metz's are 140/80-17.