How to acheive lower RPM for highway riding

I have a stock 78' xs650E that runs around 4000 rpm in 5th gear to do 60 mph but the highways around where I live require me to do around 70-75 mph in order to stay safe.

Most of my daily riding is done around the city but on weekend road trips or the occasional commute to work the highway speeds are needed.

What rpm should I be trying to achieve for extended riding at these speeds to prolong engine life? Currently the bike is around 5000 rpm at those higher speeds.

What would be my best option to find a balance between a lower rpm on the highway but not affect my city riding?

I've heard of the overdrive 5th gear or changing sprocket sizes. Is there a disadvantage to doing either of these mods?

I'm new to the motorcycle world and the xs650 engine so any help is appreciated!

On a Standard with an 18" rear wheel, I like to drop the rear sprocket one tooth, from the 34T stock sprocket to a 33T. I know, a one tooth change doesn't sound like much, but it makes a real difference. It will do just what you want - better highway cruising without sacrificing the in-town low gear "umph".

Here is an Excel spreadsheet that will yield RPM vs MPH for all combinations of wheels, sprockets and gears. Use the yellow pull downs to select the desired numbers.

MPH vs RPM

Hi LPYYZ and welcome,
the XS650's "sweet spot" is claimed to be 4,400rpm but even 4,400 ain't all that smooth.
Balancewise the engine is an oversquare big single so the vibration is built in.
Vibration can be lessened with good carb balance and will drop by ~40% if you pour the big bucks into a rephase.
Like you say, there's two ways to lower the engine revs at highway speed (OK, there's three but the price of the Australian reduced ratio primary gearset puts it out of most folks comfort range)
The overdrive 5th gear lowers revs in top gear while leaving the others alone.
I reckon fitting 23 teeth in where 22 teeth are meant to go has to mean that the 23T gear has a sub-optimal tooth profile and won't stand up to prolonged use.
But that's my 40 years experience as a machine designer talking, just maybe the thing works OK despite the theory, eh?
The easiest way is to swap out the rear sprocket for a 32T or 33T.
Don't be tempted into an 18T front sprocket. It's too tight in there for an 18T to work nice.
And don't be tempted into swapping to a #520 chain & sprocket set either.
Costs 10% less, wears 10% quicker and unless you are fighting for ounces on a racebike the weight saving is neglegible.
Some have gone as low as a 17T/30T but you'll only be happy with that if you are real good at slipping the clutch and weigh 100lbs or less.

fredintoon said:

...the XS650's "sweet spot" is claimed to be 4,400rpm...

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Interesting that you would note that, Fred. Back in those early days, we were fortunate to have factory data on engine performance charts. This chart for the early '256' XS1-XS2 engines includes the BSFC (Brake Specific Fuel Consumption) curve for the engine, revealing its energy conversion efficiency.



The chart is in (Grams of fuel) per Horsepower per hour, and that 4400 rpm is near the middle of the 'sweet spot' for good fuel consumption, about 230 grams/Hp/Hr, or about 1/2 pound/Hp/Hr, which was the expected standard for 4-stroke engines. Since fuel weighs about 6lbs per gallon, that works out to producing 12Hp at 1 GPH. So, if you can get the bike to cruise at 70 mph on 12 Horsepower (1 GPH), then you'll get 70 MPG...

Thanks for all the great responses.

After consulting the chart either a 33T or 32T rear sprocket is just what I'm looking for. I'll be in the high 4k rpm range for most of my highway driving using a 32T. I'm assuming these engines were made to run all day at these higher rpms?

The bike seems to be burning a bit of oil so I might have to do a top overhaul this winter. No smoke on start or while running but the bike burnt roughly 1 quart of oil during a 600 km road trip. If that's what needs to be done I might do the rephrase while I have it apart.

Is there anyone in the greater Toronto area who has any experience rephasing?

If you re-phase you should put in the new 5th gear. It will lower your rpm's.

Took my 1973 for its biggest ride on the weekend, about 120 miles, with speeds up to 70 mph.

At 60 mph, i was on 4k revs, at 70 mph, about 5k revs. I could have pushed it further, but it was shaking quite a lot and I didn't really want to. What revs will these bikes comfortably sit on for say, 5 minutes? I was probably being too gentle.

18t front and 30t rear, 520 o-ring chain lasts forever. highway speeds are a breeze with many miles around the west and mexico. the trick for the 18t front is to trim part of the shift shaft guard arm so you can keep it in there, since a lot of people seem to think you have to remove it. I can run 80-85 all day on the interstates.

louis said:

Took my 1973 for its biggest ride on the weekend, about 120 miles, with speeds up to 70 mph.

At 60 mph, i was on 4k revs, at 70 mph, about 5k revs. I could have pushed it further, but it was shaking quite a lot and I didn't really want to. What revs will these bikes comfortably sit on for say, 5 minutes? I was probably being too gentle.

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you wont hurt your bike running 70 as long as you have oil in it, these arent tractor motors

Yeah, I didn't think so. Just gonna take some time to get used to that shaking! So it's okay to sit on 5k revs for extended periods?

I'm also running the 18/30 combo. It's been on for years without problems. Being the original owner of my 77, I've tried many things over the years to tame the vibes on the mighty XS,including the carb syncs, bar snakes etc. For me this gear combo is the only thing that really works. Do you lose some power, yes. Do you have to slip the clutch to start out, no. Can you ride for a distance without having to shake the numbness out of your hands, yes! It may not be for everyone but works great for me. The bike runs at 35-3600 rpms at 60 which is perfect for the two lane roads I ride. Also haven't weighed 100lbs since grade school 40+ years ago.