Highway gearing

IMHO, I think you are asking too much of the engine. These engines like, actually need, to rev to work well. It's just a quirk of a short stroke engine. Long stroke engines as in a Harley like lower rpms.
You say you have no problem riding around town in 5th gear. Unless you live somewhere with higher speed limits in town than the usual 30 to 35 mph, you are dropping the rpms down to somewhere between 2000 and 2500 rpm if not lower.
That's putting quite a bit of strain on the engine especially heat wise. I went up to a 17 front sprocket on my 78 650 and according to the speedo and tach, I get 10 mph per each 1000 rpm in 3rd gear which I usually run in town which gives me 3000 rpm and plenty of power for most hills.
I am putting a 38 tooth rear sprocket on if the damned weather ever warms up enough to make it comfortable enough to work out in the garage so I'm not freezing my arse off. I'm doing that because I do most of my riding out of town where the rpm's will be higher. I'm hoping to ride the bike down to Pa. from here in Maine this summer and it will save some gas money. But, at a cruising speed of 65 to 70 on I95 I'll still be in the 5000 rmp range which in my opinion is optimal for these engines.
Like I said these are just my opinions.
Ride safe.

Last year I changed my ratio from a 16-41 setup to a 16-40. Not much but I reduced my RPM from ~5500 to about ~5300 at 70 MPH.
I estimated that I increased my fuel economy by about 1MPG. Again, not much but it gives me another ~4 miles per tankful, which has come in handy when we're burning fumes "up north" looking for a gas station. Last year on one trip we frantically searched for gas for fear of running out. When we finally found gas near Mackinaw City I put nearly 3.8 gallons in my tank, which was rated at 3.7 gallons! That extra few miles of range really saved out butts!
Another benefit of lower RPM's is the reduction of 'buzz' in my arms (back, shoulders and arse) that can take a toll over a long day of higher-speed riding.
It all depends on what you want, your riding style and your weight. Since my bike is used primarily for touring around with my wife, fuel economy and wear on our bodies take precedence over speed and torque. Also, my wife and I aren't big people (together we only weigh about 260# soaking wet) so we really don't need too much lower end torque.
There is a cost associated with whatever you do...another MPG or two is great but may takes years to offset the cost of the new sprocket, depending on how you ride. For me, it is the reduction of RPM buzz that is of primary importance, but the extra MPG helps too. Next time I change sprockets I may increase my front sprocket a tooth to 17...but only if it is a cost effective change and it can benefit my old arse!
That's my 1%

Of the 32 miles I ride back-and-forth to work each day, 4 are on 35mph city streets, and 28 are highway.

My chain broke, and the sprockets were worn, so they had to be replaced; so doing so was routine maintenance. My only real cost here that needs to be recouped is the cost of the second, 17t front sprocket. 20 bucks.

These bikes were geared to deliver a balance of performance and economy; they were the sport bikes of their day. I'm not interested in performance nearly so much as I am in economy on my daily commute. I bought the bike to save money and decrease my carbon footprint.

Lowering the gear ratio from 2.63:1 to 2.35:1 is not, in my opinion, too extreme. It's like adding another gear to the transmission (which it should have, anyway. Every 600 made today, which all have similar bore/stroke ratios, have 6 speed transmissions). It's like the second overdrive every manual Corvette, Camaro and Firebird has had since 1995. Both 5th and 6th are overdrive.

Regardless of where the engine operates most efficiently, since I began short-shifting the bike and loafing at lower rpms in-town on flat streets, I have seen an increase in gas mileage, and the oil shows no signs of heat wear when I change it.

I've been running an 18 tooth front sprocket for months now and I didn't need to remove the u shaped guard either. still plenty of torque and power even on hills in top gear.

Kingcobra wrote:

I've been running an 18 tooth front sprocket for months now and I didn't need to remove the u shaped guard either. still plenty of torque and power even on hills in top gear.

That's interesting. My experience was just the opposite. I had to slip the clutch like crazy to get it to launch, and it had no torque in top gear. The slightest hill required a downshift.

Maybe it's my size; I'm 6'5" and 290.

Wait, you didn't say anything about the rear sprocket. I was running 18/40 for a gear ratio of 2.22:1. If you're still running the stock 42 tooth rear sprocket, that's 2.33:1. My 17/40 comes out to 2.35:1, so maybe that's it.

yea... stock 42 rear... and now since I've switched to the harley wheel I'm runnin a 43 (thats the smallest rear sprocket that was ever available for a harley) but haven't noticed much difference. the bike has always been very powerful.. the reason I switched to the 18 was my 1st gear was useless. I had to shift into 2nd in like 20 feet... so I was just taking off in 2nd every time.

seanof30306 wrote:

Lowering the gear ratio from 2.63:1 to 2.35:1 is not, in my opinion, too extreme. It's like adding another gear to the transmission (which it should have, anyway. Every 600 made today, which all have similar bore/stroke ratios, have 6 speed transmissions).....

All the newer inline-4's that I'm aware of do have a 6-speed trans, but most of the V-twins (any given CC size) are still a 5-speed.
And IIRC, Kawi (and just about every other Asian import brand) didn't start adding a 6th gear to their 4 cylinder bikes until after they had eliminated the kick-starters in them.

Since no one's makeing a 650 I4 anymore (that I'm aware of), the comparison to 600 I4's was as close as I could get.

Regardless, the point was, going from 2.63:1 to 2.35:1 is not too extreme. All I can say is, I've done it, and so far, the results are very positive.

Suzuki recently upped the displacement on their GSX600 to 650 , probably has twice the HP of the old 650 at high revs . I'd bet that torque at highway cruising rpm is similar though .

OK, fine. I surrender. The analogy was flawed. I was wrong. I was in error. I capitualate. I stand here both bloody and bowed.

Let's try it again. Today, most inline four cylinder street motorcycles of between 499 and 651 cubic centimeters displacement have six-speed transmissions.

There. OK?

The point still is, however, that going from the stock ratio of 2.63:1 to 2.35:1 is not problematic.

On this I am not wrong.

I am not in error.

I am not mistaken.

I have done it to my motorcycle.

I ride my motorcycle.

I know.

In another thread, someone said it was appreciated here when people came back and posted results after posing questions in threads.

That is what I did (What a freaking mistake that was).

Here are the results:

I went from 2.63:1 to 2.35:1.

I have ridden the bike.

I like it.

Going from 2.63:1 to 2.35:1 is not too extreme.

How do I know this?

Because I did it.

Then I rode the bike.

I liked it.

Therefore, in my informed opinion, based upon the most direct experience possible, actually doing it, going from 2.63: to 2.35:1 is not too extreme.

Thank you.

If your gettting excited because you think that I was pointing out that you missed the Suzuki then you missed my point . It gives you a bike to make a gearing comparison to .

Anyway , what was your change in fuel mileage ? You stated in your orig post that economy was your primary goal .

Don't have exact numbers yet. The gauges are off being refurbished, so I can't track it precisely.

Before, it was getting 36 mpg. I was getting about 120-125 miles out of a tank before I'd have to switch over to reserve and start looking for a gas station.

Last week, I rode it back and forth to work 4 times (32 miles each day), and rode it at least 30 miles on my day off, too. I know I got 124 miles from the to-and-from work riding, and at least another 30 miles, which would translate to a 5mpg+ improvement. Won't know for sure until I get my gauges back.

Even if I am getting 40+mpg, that's still not where I think it should be. Lots of guys on here are reporting 50+mpg.

I've upgraded to electronic ignition, the carbs have been rebuilt to (near) perfection, and I hypermile it. If all I end up getting is 40-43mpg, I still have some work to do.