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Discussion Starter · #1 ·
Hi guys.
I changed my rear sprocket size from 42t to 39t cause I do lots of highway riding only 30%off road. Cruising at highway at 130km/h is so much better after the change as the rpm is lower. Somehow I notice my fuel consumption become more den our stock 42t. Anyone out there with the same setup as me. Would like to hear from you guys out there
 

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If you have not installed something to correct your speedometer/odometer (which I suspect are read off of the front sprocket/main shaft, which is likely to be inaccurate now due to the gearing change) that is likely why you are noticing lower mileage than you are used to seeing.

On a side note: how many rpms did you drop at 130kph in 6th gear? I have been considering swapping in a larger front sprocket to drop the rpms on the highway. Going up one tooth on my last bike got me about 1000 rpms in top gear, and that would be pretty nice on this machine too, I think.
 

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Discussion Starter · #3 ·
At 130kph is about 4200rpm. Smoother ride and engine seems less stress on highway especially long distance ride compared to stock. Slight lost in pickup but still feel OK to me.
Before I change the sprocket, my speedometer on our bike is faster den my GPS speed reading. Example if GPS reading is 90km/h den on bike it reads about 100km/h.
After changing the 39T sprocket, speed reading seems to be the same as GPS.
One things I notice which seems to puzzle me shouldn't we see the improvement in fuel consumption?

STOCK i get about 20km/L
New setup about 18km/L
Same riding style as stock.
 

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Hi Andrian,

keep in mind that while the Speedometer is inaccurate on all vehicles by some consicderable percentage (7% approximately for the Hondas) for a good reason, the odometer is absolutely accurate!

So, now that you have changed sprocket, you do have probably correct speedo readings but way off odometer readings. Your odometer is registering LESS km than the ones you do.

You need to correct it via a device like the Healtech Speedohealer, or calculate it at least.

Edit:

BtW, you shall not nesseserily see an improvement in consumption. The bikes may rev lower for a given speed, but the engine may stress more to reach and keep that speed thus using more fuel.
 

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Making an already tall-ish final transmission ratio even taller may well increase the fuel consumption as you will need more often to use 5th gear and also you will have to open up the throttle more in 6th to get to a higher speed. I have installed a 43 tooth rear sprocket on mine and I find that the consumption is effectively the same and the 6th gear more usable/responsive.


For information, I also own a VFR VTEC and I have installed a 44 instead of the standard 43 tooth rear sprocket and this resulted in a decrease of fuel consumption of approx. 0.5lt/100km because the 6th gear has became much more usable.
 

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Making an already tall-ish final transmission ratio even taller may well increase the fuel consumption as you will need more often to use 5th gear and also you will have to open up the throttle more in 6th to get to a higher speed. I have installed a 43 tooth rear sprocket on mine and I find that the consumption is effectively the same and the 6th gear more usable/responsive.


For information, I also own a VFR VTEC and I have installed a 44 instead of the standard 43 tooth rear sprocket and this resulted in a decrease of fuel consumption of approx. 0.5lt/100km because the 6th gear has became much more usable.
I guess there are a number of variables that eventually defines the impact on Fuel Consumption, and definitely the bike itself is the major one as well as the actuall usage.

Generaly, I find the final ratio on the ATAS rather close to beeing short, not tall. This is obviously because I drive 100% on asphalt. I would prefer an overdrive 6th gear.

But, changing the sprocket makes all gears taller, thus I would expect that overly the consumption would suffer a bit.

On the other hand, on 2 out of my previously owned Varaderos, I had shorten the drive (15 vs. OEM 16 front sprocket = 3 teeth at the back sprocket). I did this because the Varadero needed the extra pull the gear change provided. However, while traveling on mountain twist road the consumption was lower than stock, on long highway stretches there was a substancial negative impact.

Given the fact that the noise was also increased and the riding comfort was somehow reduced (for the character of the bike), I returned to the stock gearing.
 

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Hi guys.
I changed my rear sprocket size from 42t to 39t cause I do lots of highway riding only 30%off road. Cruising at highway at 130km/h is so much better after the change as the rpm is lower. Somehow I notice my fuel consumption become more den our stock 42t. Anyone out there with the same setup as me. Would like to hear from you guys out there

You made a 7% change in your gearing. Since your odometer was correct before you made the change, you are now reading 7% low on your odometer. This means that your fuel consumption is now 7% better than indicated. For example, 50 mpg indicated is now actually 53.5 mpg. This means that for each 100 miles you put on your bike, you are actually only adding 93 miles to your odometer. This gives you 7% more warranty (based on mileage). This also means that you should do your maintenance services 7% sooner.
 

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Just bought a JTF1370 Rubber Cushioned Front Drive Motorcycle Sprocket 15 Teeth (JTF 1370.15 RB), these sprockets have the rubber chain cushion like the original Honda part but at less than half the price delivered from the other side of the world, I got it from Chain & Sprockets in the UK, freight was slightly more than the cost of the sprocket
 

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Discussion Starter · #9 ·
Hi guys.
I changed my rear sprocket size from 42t to 39t cause I do lots of highway riding only 30%off road. Cruising at highway at 130km/h is so much better after the change as the rpm is lower. Somehow I notice my fuel consumption become more den our stock 42t. Anyone out there with the same setup as me. Would like to hear from you guys out there

You made a 7% change in your gearing. Since your odometer was correct before you made the change, you are now reading 7% low on your odometer. This means that your fuel consumption is now 7% better than indicated. For example, 50 mpg indicated is now actually 53.5 mpg. This means that for each 100 miles you put on your bike, you are actually only adding 93 miles to your odometer. This gives you 7% more warranty (based on mileage). This also means that you should do your maintenance services 7% sooner.
After two days of riding, instead of using my speedometer on bike to calculate distance travelled, I used my GPS instead to see the distance travelled and the difference is 7% difference spot on. That means if I travelled 214km on my bike it will read only 200km. ??????????
 

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At 130kph is about 4200rpm. Smoother ride and engine seems less stress on highway especially long distance ride compared to stock. Slight lost in pickup but still feel OK to me.
Before I change the sprocket, my speedometer on our bike is faster den my GPS speed reading. Example if GPS reading is 90km/h den on bike it reads about 100km/h.
After changing the 39T sprocket, speed reading seems to be the same as GPS.
Since those 130km/h are actually real now, that's the equivalent of reading 4200rpm while doing 140km/h on a bike with stock sprocket!!
Not bad!
 

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Hi guys.
I changed my rear sprocket size from 42t to 39t cause I do lots of highway riding only 30%off road. Cruising at highway at 130km/h is so much better after the change as the rpm is lower. Somehow I notice my fuel consumption become more den our stock 42t. Anyone out there with the same setup as me. Would like to hear from you guys out there
I am looking for a 39t sprocket where did you find one?
 

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At 130kph is about 4200rpm. Smoother ride and engine seems less stress on highway especially long distance ride compared to stock. Slight lost in pickup but still feel OK to me.
Before I change the sprocket, my speedometer on our bike is faster den my GPS speed reading. Example if GPS reading is 90km/h den on bike it reads about 100km/h.
After changing the 39T sprocket, speed reading seems to be the same as GPS.
One things I notice which seems to puzzle me shouldn't we see the improvement in fuel consumption?

STOCK i get about 20km/L
New setup about 18km/L
Same riding style as stock.
When you reduced the tooth count on the rear, you also reduced the rpm for a given speed which means your odometer is now reading fewer miles for the same rolling distance than before the change. If you use this odometer output to calculate your KPL you will get a smaller value. The difference will be 39/42 (0.929, or 93%) of original KPL. Your values are 18/20, or ~90%. Right where you would expect it to be.

If your speedo is now in line with GPS, then check your odo against GPS distance traveled, and try to do this in a straight line, to make sure they line up too. If they do, your new KPL numbers are the more correct of the two. If not, then note the correction factor for future use: it will be actual mileage/odo mileage...lets call it O-corr. Your actual KPL will be (odo distance * O-corr) / liters of fuel.

FYI, Ol' Red's MPG numbers are 41.60 MPG (17.69 KPL) and I'm running a 44 tooth rear sprocket. Your 18 KPL sounds closer to actual Adrian.
 
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