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Discussion Starter #1
I ride about 180 miles a day, about 90% of which is on empty Interstate at about 85mph-90mph. I'm WOT the entire way. At this speed, I'm turning nearly 10,000rpm for about an hour. This is well past the power peak and probably not healthy for the motor in the long term, so I'm considering taller gearing.

This is a commuter bike, plain and simple. It sees no track time and I'm not the least bit interested in 0-60mph contests. I *am* interested in the bike being completely streetable and improving my mpg. Right now I'm getting about 50mpg, a good bit less than folks who keep their bike under 70mph.

Anybody got any experience or tips for such a thing? I'm also interested in any *reasonable* mod that would improve top speed without ruining the bottom-end torque. I don't want something that stalls at every light unless you rev it to the moon and slip the hell out of the clutch.

Thanks in advance for any suggestions!
 

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It will be slower, use more fuel and a pain to ride, cbr wont redline in top as it is,

Ive had mine to an indicated 166kph, still a needle thickness shy of redline ,down a 2km long hill with the wind behind me and I got a draught off a mate.. hell I was even Tucking but dont tell Sendler :)

Up the same hill the other way 150 is flatout.
 

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Discussion Starter #4
http://www.cbr250.net/forum/cbr250-performance/749-longer-gearing-available.html

I think a bike with a bigger engine would give you better mileage at those speeds, though
I was wondering about that. As much as I like the 250, I'm starting to regret not getting the 600. I got the 250 for the mileage and because it was dirt cheap, but if a 600 would get better mileage at a lower rpm, I'm missing out. I'd planned on keeping the 250 for about a year or so before getting a used 600 but I may not wait that long.
 

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Discussion Starter #5
It will be slower, use more fuel and a pain to ride, cbr wont redline in top as it is,

Ive had mine to an indicated 166kph, still a needle thickness shy of redline ,down a 2km long hill with the wind behind me and I got a draught off a mate.. hell I was even Tucking but dont tell Sendler :)

Up the same hill the other way 150 is flatout.
Redlining this motor is counterproductive as that's well past the power peak of 8500rpm.

Mine will do about 87mph on flat road with no wind. I've had it up to 94mph (indicated) with slight tailwind and/or slight downslope. Slight upslopes and the bike slowly drops to about 81mph. But all of this is past 8500rpm on the tach.

I'd be curious how the bike would behave running 8500rpm in 6th at about 90mph, assuming that's attainable.
 

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I ride about 180 miles a day, about 90% of which is on empty Interstate at about 85mph-90mph. I'm WOT the entire way. At this speed, I'm turning nearly 10,000rpm for about an hour. This is well past the power peak and probably not healthy for the motor in the long term, so I'm considering taller gearing.

This is a commuter bike, plain and simple. It sees no track time and I'm not the least bit interested in 0-60mph contests. I *am* interested in the bike being completely streetable and improving my mpg. Right now I'm getting about 50mpg, a good bit less than folks who keep their bike under 70mph.

Anybody got any experience or tips for such a thing? I'm also interested in any *reasonable* mod that would improve top speed without ruining the bottom-end torque. I don't want something that stalls at every light unless you rev it to the moon and slip the hell out of the clutch.

Thanks in advance for any suggestions!
If you are WOT and only getting 90MPH, getting TALLER gearing would make you go SLOWER at WOT.
Reason is simple; at any given RPM your groundspeed would be FASTER, right? But you will then get to maximum drag condition at a new, lower RPM, which is lower power. Where the drag and power curves intersect, you are at maximum possible speed.
If you told me you could easily get to 168KPH at redline, I would deduce that you have the excess power available to get taller gearing.
 

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It will be slower, use more fuel and a pain to ride, cbr wont redline in top as it is,

Ive had mine to an indicated 166kph, still a needle thickness shy of redline ,down a 2km long hill with the wind behind me and I got a draught off a mate.. hell I was even Tucking but dont tell Sendler :)

Up the same hill the other way 150 is flatout.

I easily redline in top gear in a full tuck. I'm 5'9, 175lbs. Riders who present more rolling(weight) or aerodynamic resistance(frontal area) might not however.

FYI; total drag is proportional to the frontal area, rolling friction, the coefficient of drag of the bike/rider combination, and the SQUARE of the speed (for example at 160KPH you have FOUR times the aero drag as you do at 80KPH)
 

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Top speed

Top speed will occur at the power peak or just after with the right gearing and has nothing to do with redline. The power curve of the CBR250R is fading rapidly after 9500 rpm. This bike would give much better fuel economy on the highway with longer gearing and probably won't lose any top speed. If you can tip toe through the beginning and end of your commute with a loud muffler, you can gain 9% horsepower all along the curve from 3000 to 8700 rpm with a slip on. Gearing longer by 10-15% to take advantage of this would improve fuel economy and might even give an increase of top speed as to the square root of the power increase. Less fuel, wear on the engine, and a 3mph increase in whatever your top speed is now at a lower rpm. If you really want to fine tune the engine after that you can then add a fuel controller and auto logger to dial in the best compromise of fuel usage and power. 15T and 16T front sprockets are coming eventually but no one has tried them yet. My sprocket builder has the design done but said the first batch he cut got ruined by the shop doing the hardening. Then promised to put this project at the top of the pile but that was three weeks ago and I haven't heard back yet. I have also seen a 15T/ 36T kit from Thailand but can't find a way to buy them.
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There is no 600 that will beat the fuel economy of the 250 at any speed although they will be able to top out much higher if you really have room to run over 100mph with out getting arrested. An SV650 might come close at 90 and have more left over 100 but they don't even sell those anymore. Check out fuelly to see what riders are getting.
 

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I'm with sendler, I think taller gearing will be usable, mostly because you are currently running significantly above the power peak. An addition tooth on the countershaft should drop your revs about 500, but you may want more.

I have a naked '06 SV650 (2nd gen, F.I.) that's geared-up 1T on the countershaft. It gets 55 MPG on 87 Ethanol-free gas cruising at an actual speed between 70 and 75 (closer to 75) reving about 5500. The SV has enough torque that some have even gone up 2T from stock without a problem.

I don't think there are many I-4 600s that can beat the SV's mileage, so it may be a decent way to go if you are looking for something larger. The naked models were discontinued a couple years ago, but the "S" or "SF" (full fairing) models were still available. There is some talk that they may not be on the roster for 2012 though. '07 and later SV650s got dual-plug heads, and may give even better MPGs.
 

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Discussion Starter #10
If you are WOT and only getting 90MPH, getting TALLER gearing would make you go SLOWER at WOT.
Reason is simple; at any given RPM your groundspeed would be FASTER, right? But you will then get to maximum drag condition at a new, lower RPM, which is lower power. Where the drag and power curves intersect, you are at maximum possible speed.
If you told me you could easily get to 168KPH at redline, I would deduce that you have the excess power available to get taller gearing.
Not necessarily. The 250 motor has peak power around 8500rpm. I'm turning 9500rpm at 90mph, a part of the powerband that has *less* power than lower down. Optimal gearing is where drag-limited top speed coincides with the powerband peak, and that's not the case with the 250 and stock gearing. The gearing is shorter than optimal for high-speed highway cruising, a consequence of gearing it for around-town riding.
 

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Discussion Starter #11
Top speed will occur at the power peak or just after with the right gearing and has nothing to do with redline. The power curve of the CBR250R is fading rapidly after 9500 rpm. This bike would give much better fuel economy on the highway with longer gearing and probably won't lose any top speed. If you can tip toe through the beginning and end of your commute with a loud muffler, you can gain 9% horsepower all along the curve from 3000 to 8700 rpm with a slip on. Gearing longer by 10-15% to take advantage of this would improve fuel economy and might even give an increase of top speed as to the square root of the power increase. Less fuel, wear on the engine, and a 3mph increase in whatever your top speed is now at a lower rpm. If you really want to fine tune the engine after that you can then add a fuel controller and auto logger to dial in the best compromise of fuel usage and power. 15T and 16T front sprockets are coming eventually but no one has tried them yet. My sprocket builder has the design done but said the first batch he cut got ruined by the shop doing the hardening. Then promised to put this project at the top of the pile but that was three weeks ago and I haven't heard back yet. I have also seen a 15T/ 36T kit from Thailand but can't find a way to buy them.
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There is no 600 that will beat the fuel economy of the 250 at any speed although they will be able to top out much higher if you really have room to run over 100mph with out getting arrested. An SV650 might come close at 90 and have more left over 100 but they don't even sell those anymore. Check out fuelly to see what riders are getting.
I've followed your threads elsewhere in the forum about changing sprockets. I'm eager to hear how that turns out. It's a pity we can't easily change the rear sprocket to get what we want, and I'm concerned about clearance changing the front one. I hear there's not a lot of room.

Regarding a slip-on, I've listened to a few of them. They're just too loud and the pitch is too annoying for me to consider them for an hour ride each way. I saw a video of a Remus exhaust on a 250 -- it was for another bike but modified to fit the 250 -- and the sound was delicious, but it's obviously not for sale (yet). They're also expensive.

Last, logic would seem to say a 600 can't get better mileage than a 250, but I wonder about that. I've heard the 250 motor makes 27hp, which works out to 108hp/liter. The 600 motor makes 102hp, which is 170hp/liter. The latter is obviously more efficient than the former by a whopping 57%. The 1000RR makes 178hp/liter, making it even more efficient. Some of that's got to be in the higher compression (greater thermal efficiency) and probably better breathing (greater volumetric efficiency). Frictional losses of an I-4 should be higher than a single, but that all depends on manufacturing materials, tolerances, and other stuff I can't get data on. Long story short, though, I could *potentially* get better mileage out of the 600 than the 250 if for no other reason than the 600 is a more efficient motor.
 

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Discussion Starter #12
Long story short, though, I could *potentially* get better mileage out of the 600 than the 250 if for no other reason than the 600 is a more efficient motor.
It's worth noting that I should really be talking about BSFC, not hp/L, but I don't have BSFC figures for any of these motors. I'm just going with what I've got. The 600/1000 motors could be running much richer to get their power improvements over the 250, thus negating the better mileage argument. I doubt they are, but I can't prove it.
 

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I'm concerned about clearance changing the front one. I hear there's not a lot of room.



Last, logic would seem to say a 600 can't get better mileage than a 250, but I wonder about that.

Adding one tooth to the front sprocket should still give the chain about 5mm of clearance with the casing, which is probably enough.

Most engines get very thirsty at WOT compared to say even 85% throttle, as the ECU sees to it that it runs richer than it should, using parameters other than the narrowband O2 sensor. The difference can be pretty big. My first bike (a CRF150 derived engine) would do about 100kph on a good day and the mileage was significantly worse than than what the CBR250 turns out at that speed, using half it's horses. I'd agree that the engines are 3 generations apart, though. Similarly, a 600 using just a quarter of it's horses at 95 mph should not be too far off the 250 at that speed
 

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revs

It's not efficiency that brings more power per liter it's tuning. The CBR250R makes it's first torque bump at 6000 rpm and is tuned for fuel economy. Something like an R6 makes it's torque much higher on it's way to a 16,000 redline due to more extreme valve overlap ect. At the expense of fuel economy. Check out fuely. There are no 600's breaking 50 mpgUS. And very few 650's. Most CBR250R's do at least 70. I get 87 in good weather and the best I can do on the Ninja250 is 66. There is no other bike capable of running 80mph that will be even close to the Honda.
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You need longer gearing for sure. You would reduce fuel consumption at 80 and may actually gain 1 mph. And I wonder how much you would really hear a slip on adding to the road and wind noise at 85. It may actually just fit in to ALLOW you to even hear what the engine is doing at that speed. The real problem with the added noise would be as you start and end your trip through town.
I've followed your threads elsewhere in the forum about changing sprockets. I'm eager to hear how that turns out. It's a pity we can't easily change the rear sprocket to get what we want, and I'm concerned about clearance changing the front one. I hear there's not a lot of room.

Regarding a slip-on, I've listened to a few of them. They're just too loud and the pitch is too annoying for me to consider them for an hour ride each way. I saw a video of a Remus exhaust on a 250 -- it was for another bike but modified to fit the 250 -- and the sound was delicious, but it's obviously not for sale (yet). They're also expensive.

Last, logic would seem to say a 600 can't get better mileage than a 250, but I wonder about that. I've heard the 250 motor makes 27hp, which works out to 108hp/liter. The 600 motor makes 102hp, which is 170hp/liter. The latter is obviously more efficient than the former by a whopping 57%. The 1000RR makes 178hp/liter, making it even more efficient. Some of that's got to be in the higher compression (greater thermal efficiency) and probably better breathing (greater volumetric efficiency). Frictional losses of an I-4 should be higher than a single, but that all depends on manufacturing materials, tolerances, and other stuff I can't get data on. Long story short, though, I could *potentially* get better mileage out of the 600 than the 250 if for no other reason than the 600 is a more efficient motor.
 

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I doubt a CBR600RR can get you better mpg. I think 50mpg cruising on the freeway might be the best you can get. A bigger engine will have an easier time at that speed but the bigger engine also consumes more fuel. In the end, the amount of fuel consumed by any engine is probably directly correlated to the wind resistance/drag the bike/rider created. More wind resistance = more fuel/energy needed. Unless you can find a bike with 1) better aero dynamics or 2) better engine efficiency.

On Fuelly.com, you can search a variety of motorcycles and its reported real-life MPG. CBR600RR gets 45 (only 1 bike). A Suzuki GS500F would get you over 50 mpg but cruising at 80 might net you less mpg.
 

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Discussion Starter #16
There are no 600's breaking 50 mpgUS. And very few 650's. Most CBR250R's do at least 70. I get 87 in good weather and the best I can do on the Ninja250 is 66. There is no other bike capable of running 80mph that will be even close to the Honda.
My last tank got me 54mpg after running WOT for the whole trip. I'd love to get in the 70's but don't think it's possible with the engine running flat out the whole time. I could, of course, slow down, but...I don't want to :)

As soon as a good source of reliable sprockets is available, I think I'll try one. So long as I keep it near the power peak, I'm betting I won't lose much -- if any -- top speed. Might even improve it a bit.
 

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Discussion Starter #17
In the end, the amount of fuel consumed by any engine is probably directly correlated to the wind resistance/drag the bike/rider created. More wind resistance = more fuel/energy needed. Unless you can find a bike with 1) better aero dynamics or 2) better engine efficiency.
Drag increases at the square of speed, so going faster is always going to hurt fuel economy in a bad, bad way. That said, which motor will use more fuel over a given time, a 600cc motor turning 4,000rpm or a 250cc motor turning 10,000rpm? If all else was equal, they'd likely turn in very similar mpg numbers. However, the CBR600RR *is* a more efficient motor if for no other reason than its increased compression ratio. It will extract more energy per unit fuel than a lower compression motor.

In the end, the measurement that truly matters is Brake Specific Fuel Consumption (BSFC). That is the *real* measurement of engine efficiency vis-a-vis fuel consumption. The rest is Cd (coefficient of drag) and frontal area, which I'm betting is very similar between the 250 and the 600. I can't find BSFC, Cd, or frontal area data for either bike.
 

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Discussion Starter #18
While we're at it, WTF is up with Honda refusing to publish HP numbers for the 250? I can find hard data on the 600 and 1000 motors, but Honda's page doesn't list HP for the 250.
 

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Forget the 600 for fuel economy. None will ever match the single for fuel economy at any equal speed. The single has so much less wasted heat energy in it's radiator and exhaust gases. At 54mpgUS for 85mph you are already beating the best mileage figures that 600 riders are posting for normal speeds of 75. If you could borrow or rent a 600 for 1 trip at 85mph I'm sure you would find that your number is in the low 40's. An SV650SF could hit 50. If you write to owners on fuelly they will write back. BMW's get outstanding economy at 65mph but they are expensive and big so wouldn't hold much of their numbers above 85 either. My first gen Honda Insight could do 85mph all day long and bring back 50 mpg. I did one tank on the CBR at 75-80 and still brought back 68mpg. If you change gearing and just slow down to 80, I think you could get back up to the mid 60's. If you have to go 90 you may as well buy an Insight.
 

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What always surprises me most when I look at fuelly is the number of Interceptors. That must be like every one of them that were ever sold. And they must all spank their bikes to get them under 40. That is a V-Tec engine. The CBR250R is a quantum leap over every other highway capable bike ever made when it comes to fuel economy. This is the first bike that can beat my car. Including the Ninja. That's why I finally started riding again.
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