I just installed an 18 tooth front sprocket on my '99. It has a stock 43 tooth on the rear. My goal was to get my speedo to read true and gain some MPG at highway speeds.
The speedo is now dead-on when comparing to an on-bike GPS. Previously, I was off about 6%. As a Mech. Engineer (by degree), that bothered me. I don't ride very aggressively, so a slight loss of acceleration is not even noticeable to me. I will gain some some MPG, but difficult to measure because the ODO was previously off as well.
If you aren't doing track days or need to hang with the fast guys, I definitely recommend this mod.
When I needed a chain and sprockets, I was going to go 18/46 for longer driveline life, but I could not find anyone making a STEEL 46 tooth rear sprocket except chain gang in Australia, and it was very expensive, plus shipping, etc. so 17/43 it was.
I tried a 17 tooth sprocket but it made pulling away from a stop uphill, fully loaded, a bit hard on the clutch. In fact I'm trying to find a 45 tooth rear sprocket to make loading the bike in the toyhauler easier. So far no luck.
Whoops, I mean an 18 tooth rear sprocket. Duh. Also saw no difference in mpg.
As was pointed out to me in a different thread, (thank you VFRCapn!) while the speedo read a bit high stock, the odo was pretty much dead on. If your speedo is now dead nuts accurate, your odo is going to be off by the same amount. I went the opposite direction, going to a 16t sprocket. I added a speedohealer, and right now it's calibrated pretty much perfectly for speed. My odo is now off by a coupla miles per 50, so I'm going to adjust it so that the error is smack between the two. BTW, I don't necessarily know that you'll gain mpg by lowering the rpms. If you move the engine below it's designed efficiency RPM, you'll have to give it more throttle to make up for the lack. Then again, maybe you will. I find that mine gets awfully happy right between 5000 and 6500, and trying to lug it below that at freeway speeds is counterproductive.
You bring up a good point regarding the odometer. If it was accurate prior, it will record miles approx. 6% low now with the larger countershaft sprocket. I will have to do a measured course run to verify the odometer situation.
My MPG will really only improve at steady top gear interstate speeds (65 - 85 MPH), as I'll turn lesser RPM's. At a true 70 MPH, the 5th Gen with 18T front sprocket, will turn about 5K RPM. The MPG gain will be slight. Also, I won't be able to accurately measure MPG since the ODO is now off.
G.
[image: View Post]whodat90, on 27 May 2012 - 09:58 PM, said:
As was pointed out to me in a different thread, (thank you VFRCapn!) while the speedo read a bit high stock, the odo was pretty much dead on. If your speedo is now dead nuts accurate, your odo is going to be off by the same amount. I went the opposite direction, going to a 16t sprocket. I added a speedohealer, and right now it's calibrated pretty much perfectly for speed. My odo is now off by a coupla miles per 50, so I'm going to adjust it so that the error is smack between the two. BTW, I don't necessarily know that you'll gain mpg by lowering the rpms. If you move the engine below it's designed efficiency RPM, you'll have to give it more throttle to make up for the lack. Then again, maybe you will. I find that mine gets awfully happy right between 5000 and 6500, and trying to lug it below that at freeway speeds is counterproductive.
Just for reference the gen 5 at 5000 rpm in 6th gear, with a 24.9" rear tire diameter (typical) and the std 17/43 combination has a calculated speed of 72.9mph. Going to an 18 T front sprocket would bump that speed at 5000 rpm to 77.2 mph.
This applies to gen 5 only since gen 6 has a 16/43 original tooth combination and has different trans ratios to compensate. For gen 6, the 6th gear is an overdrive.
FYI...
I have an .xls spread sheet that calculates speed in all gears if anyone wants a copy. There is a separate sheet #2 for gen 6. It's pretty elementary. I have tried to put this on the forum site but an .xls file is not allowed. If an administrator wants a copy to put up on the site just let me know. I don't know of an easy way to share the file with a link but if anyone else does i'll be glad to send it to them.
As a data point, my bike with a 16t front sprocket turns about 5300-5400rpm at a GPS verified 70mph in 6th, brand new tires @40psi. I still find myself downshifting to 5th to get the revs up on a regular basis. However, I ride pretty much bolt upright fully in the wind with either side cases or a trunk, so I probably have more drag than the average skinny joe.
[image: View Post]whodat90, on 31 May 2012 - 08:24 PM, said:
As a data point, my bike with a 16t front sprocket turns about 5300-5400rpm at a GPS verified 70mph in 6th, brand new tires @40psi. I still find myself downshifting to 5th to get the revs up on a regular basis. However, I ride pretty much bolt upright fully in the wind with either side cases or a trunk, so I probably have more drag than the average skinny joe.
Good info! That says the effective tire diameter is closer to 24" than 24.9".
If fuel economy is your game then you might consider a Vacuum gauge. So that when you're cuising the more vacuum you're pulling the better fuel economy you'll get. Sure you may have lowered your RPMs but how much throttle opening is used to maintain that speed.
I went a tooth larger on my 4th gen counter sprocket about 40,000 miles ago and think it is a fantastic mod. While I did not see any significant change in mpg it did make my speedometer spot on as per my GPS. I did note that the odometer reads 3.6% low now. As far as engine speeds and rideability I would recommend this change to anyone who does lots of miles.
End of thread · 11 posts recoveredWayback Machine snapshot 5 Jul 2012