What gas mileage do you get?

Nelson wrote:

well I just test rode an 81 KZ550... not!

It felt like a little underpowered toy that was going to bounce off the road at any moment. (no offense to 550 owners)

I owned one myself about 20 years ago, but I guess you can't go back. Looks like another 1000 or maybe 750 is in my future. Tall gearing and a tad lean jetting will save some bucks.

Try a 750 twin, it's a big bike with sufficient power (to get out of it's own way without winding the motor out) and excellent mileage.
I'm 6'3" 235-240 pounds and a 750 twin is comfortable for me.

Considering a higher gear ratio to increase gas mileage seems akin to selecting the easiest gear when pedalling a bicycle. The bicycle rider is the motor providing the power. Am thinking the most economical and efficient utilization of this human-power motor is whenever the least effort is being required to propel the bicycle under any given conditions.

Applying this logic to motorcycle gearing, the motor is imo operating most economically and efficiently whenever the least motor effort is being required to propel the motorcycle under any given conditions. And motor rpm is imo only one of several variables involved in attaining the most economical and efficient utilization of power available from the motor. State of tune is imo more critical toward maximizing gas mileage than arbitrarily reducing the factory designed rpm for any given road speed.

Ma Kaw designed these KZ motorcycle engines to spin happily at relatively high rpm's. Spinning happily along propelling the motorcycle with the least effort required is, I believe, the most economical utilization of the engine which will produce the highest gas mileage, regardless of whether a higher gear with a lower rpm may produce the same road speed from a harder working engine. Rpm is not itself the determining factor in how hard the engine is working. For example, a "lugging" engine is working way too hard -- much harder than a happily spinning engine at the same road speed. Using the bicycle analogy, the human motor is frequently using less energy when peddling a little faster but traveling at the same speed (peddling faster in low gear to go uphill uses less energy than peddling slower in a taller gear).

While a higher top gear may feel more relaxed at cruising speeds, I'm no stranger to hunting for it while on the road. And have experimented with a one-tooth larger countershaft sprocket to lower the engine rpm in any gear at any given road speed. The 5th gear cruising was indeed more relaxed, but did not improve cross-country gas mileage, and did noticably impair acceleration from stop or slow speed (started losing friendly drag contests otherwise won). And there was no increase in absolute top speed -- just slightly reduced rpm at the same attainable top speed.

Also experimented with a one-tooth smaller countershaft sprocket to increase the engine rpm in any gear at any given road speed. The 5th gear cruising was much more frenetic, and with about the same absolute top speed, (reminded of running in 4th gear with oem sprockets), and did slightly reduce cross-country gas mileage. But noticably improved acceleration from stop or slow speed (could win more of the friendly drag contests). Have long since returned to and kept oem gearing, which for me is best overall (as Ma Kaw intended.)

Rumor has it that top gear ratio is near perfect when a well-tuned bike will barely pull redline in top gear. Rumor also has it that absolute top road speed for most Z1's and large KZ's is about the same whether running in 5th gear or 4th gear. Guessing many of us have experimented by rowing between 4th and 5th gears while running at a pretty good clip.

Good Luck with whatever "best" gearing is decided! Have Fun! Be Happy! And Ride Safe!

36,000 miles is how far you would have to ride to save $1000(a reasonable amount to spend on an old 550) in gas if you rode a bike getting 50mpg instead of one that gets 35mpg .

50mpg@ $3.25/gal = 6.5 cent/mile , 35mpg@ $3.25/gal = 9.23 cents per mile . A savings of 2.73($.0273) cents/mile .


$1000/$.0273= 36630 miles

steell wrote:

Nelson wrote:

well I just test rode an 81 KZ550... not!

It felt like a little underpowered toy that was going to bounce off the road at any moment. (no offense to 550 owners)

I owned one myself about 20 years ago, but I guess you can't go back. Looks like another 1000 or maybe 750 is in my future. Tall gearing and a tad lean jetting will save some bucks.

Try a 750 twin, it's a big bike with sufficient power (to get out of it's own way without winding the motor out) and excellent mileage.
I'm 6'3" 235-240 pounds and a 750 twin is comfortable for me.

When I had my CB550, I was driving alot of suburban/urban Interstate commuting. It always seemed way underpowered on the merge/acceleration. I was still driving the same commute when I got my KZ750, I really noticed the difference, merging was no longer an issue. I don't do the commute anymore but she has plenty more to give, than I want to ask, on the hiway.

Thanks for the good information, Patton.

I initially played with the sprocket ratios to minimize the buzz at common highway speeds (65 MPH) around here. At 55 the SR was smooth as glass and at 65 the mirrors were useless. By going down three teeth on the rear allowed some use of the mirrors as the real buzz was moved to about 68 MPH.

Since I have a pair of '79 650s, I tend to think that they were geared for the infamous 55 MPH national speed limit (USA) in force at that time they were new/designed. Since they have been put back on the road the limits have been relaxed and adjusted upward. Still, my SR feels the best at 60 to 62 MPH and I'm reluctant to lower the final ratio further. I noted a slight drop-off of acceleration in 5th and 1st is a bit taller, but not harmfully so.

Sometime in the past I read that best fuel mileage is attained when the carburetors are wide open. I don't know if the author knew what he was writing about, however.

N0NB wrote:

You can go to a 17 tooth sprocket on the front of a 650 very easily. My SR has a 16/38 and the 650B has a 17/42 combo and I think the 650B ended up playing a bit better.

When I changed my sprockets last year I also wanted to replace my 16 tooth drive sprocket to a 17 tooth...but upon inspection of my front sprocket chamber I saw that the clearance was really tight. I didn't think a 17 tooth sprocket would fit without the chain rubbing against that thingy that surrounds the front sprocket chamber. So I decided to shelve the idea and instead focus on the rear sprocket only. I've seen others have installed a 17 tooth front sprocket but I don't see how that's possible on my engine. Does the KZ650B differ from the SR650 in this area of the engine?

17 will fit, it does get real close to the chain guard though

I think an 18 will fit if you remove the chain guard.

Steve is correct. The chain guard is the same on the SR and B engines as there is no difference between the engines used.

Anyone that has the 650 factory manual should pull it out and turn to the horsepower/torque/fuel consumption charts towards the front of the book. The lowest fuel consumption occurs where the torque curve is at its peak. Now you physics majors out there should be able to explain this for us physics-challenged guys, right ??
Regardless, the torque peaks are:
650B ~6800RPM where consumption is only ~215g/HP-hr
650D ~7000RPM where consumption is only ~210g/HP-hr

Now obviously no one wants to run at those engine speeds in 5th gear on the highway because of vibration, never mind the speeding tickets. However, reasonable consumption back DOWN the curve CAN be obtained at:

650B ~5800RPM where consumption is ~230g/HP-hr
650D ~6000RPM where consumption is ~235g/HP-hr

And don't anybody ask me to translate those consumption values into miles per gallon. I've looked through all my books and cannot find a conversion factor for such units.

The lowest fuel consumption occurs where the torque curve is at its peak. Now you physics majors out there should be able to explain this for us physics-challenged guys, right ??

No physics needed

The maximum torque is at the point where the engine is most efficient

Keep in mind that is at wide open throttle so it minimizes pumping losses.

IIRC, the torque peak is somewhere between 2800-3500 rpm on my 91 Ford F150 with a 5.8L, and it weighs 5000 lbs.
But I can get 20 mpg cruising at 1500 rpm at 65 mph. 3.08 rear gearing with 31" tall tires, just don't ask it to pull a trailer with any weight in anything greater than second gear.

There is something to be gained by gearing

So, for my 650D2 I can argue with the nice officer that I'm saving fuel by running at about 7400 RPM (somewhere around 80 MPH, by my gearing) and here's the chart to prove it?



I get what you guys are saying. I see that it also shows a dip at around 6500 RPM which translates to around 70 MPH for me. Examining those charts a bit more closely I can understand why my bikes seem to run very well between 65 and 85 MPH. It's where they run their most efficient.

Isn't the key thing here the hp-hr rating? You're running more efficiently for the HP produced, but you're sure as hell using more gas.