bike runs at 5000 rpm on highway

[blazercrx]

I have a cb900 custom with a dual subtrans. The bike runs great but once I run through the gears and get to fifth it seems like the bike is reving to high, and I don't have the greatest mpg. at 55 I'm at 4500rpm, and at 70 I'm just over 5000rpm's! Is this normal for this type of bike? Is there a different gear that I can replace in the subtranny? or is the problem in the tranny? Any info is greatly appreciated.

[dieziege]

What's a dual subtrans?

AFAIK the CB900 is chain drive... you could perhaps swap sprockets.... but it is also an inline 4, right? 5,000RPM isn't out of line. My 1100 (inline 4) runs about 4400RPM at 70MPH.

[TechTMW]

Are you sure you're not in Low range??

[flynrider]

If you're used to something that revs lower at highway speed, then 5,000 rpm might seem high. That I-4 should be quite comfortable at that rpm going down the highway. My 750s I-4 runs 5K to 5.5K going down the freeway and I like it just fine.

The CB900 custom was a shaft drive bike, so there are no sprockets to adjust the gearing. With the dual range final drive, you could change the final drive gearing to "low" for better acceleration in city driving and "high" for a more comfortable ride on the highway. As TechTMW noted, you might be still in the low setting. Experiment with the dual range gadget that's located near the left foot peg.

Revving lower isn't going to do much for your gas mileage. Your engine sucks gas according to how much power it has to put out to keep you going down the road at the speed you chose. If it requires x hp delivered at 5,000 rpm, it'll burn the same amount of gas if it delivers the same hp at 4,000 rpm.

[blazercrx]

I have switched back and forth from low to high and low is worse(of course) for highway driving. flynrider is right about the subtrans. w/the shaft drive which drew me to the bike in the first place, thinking that I could get great response in low and great milage in high gear.
If I run the bike in low gear(higher rpms) in order to achieve 70 mph the engine runs at 5500-6000 rpms, in high gear(lower rpms) at 70 mph the engine runs at 5000 rpms. If I change out the high gear with one that lowers the rpm even more would that not get me the results I'm looking for? the lower the rpm the better the gas milage, right?

[niterider]

My 750 v twin vulcan runs 5,000 rpm at 70 mph.

If you lower your rpm's you might lug your engine down forcing it to use more fuel to maintain the speed desired. Also you may end up having to down shift more often to go uphills and accelerate out of turns. Just my thoughts on the subject.

[JCS]

5500 rpms at 70 sounds like it is in the ballpark. The I-4 is built to run all day long at these speeds. Don't worry about it. A friend used to have the same bike as you. He put many troublefree miles on it. Enjoy.

[blazercrx]

If thats normal then I'm cool with it. I just wanted to make sure there wasn't something majorly wrong with my bike. thanks for the help.

[flynrider]

the lower the rpm the better the gas milage, right?

Nope. It's a common misconception. Go back and read the last part of my previous post. Gas mileage is determined primarily by the amount of power the engine is delivering.

Let's say, for example that a motorcycle going down the highway at 80mph and 5,000 rpm in top gear, gets 40 mpg. For arguments sake, we'll say it's producing 40 hp. That translates into a fuel consumption of 2 gallons per hour to deliver 40 horses.

Now, imagine yourself sitting in the driveway with the bike in neutral. If you rev the engine up to 5,000 rpm you're not going to burn anywhere near 2 gallons in an hour. Since you're not actually doing any work with the engine, real fuel consumption will be closer to .2 gallons per hour. BTW - this is hypothetical, since you'd probably melt the engine by running it while standing still for an hour.

When an engine delivers more power, it sucks more air which increases the amount of gasoline sucked into the engine. This is independent of the rpm.