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· Registered
168 Posts
Discussion Starter · #1 ·

I have a 1981 CX500 after reading a post about how fast other people can get the CXs going I have a few questions. My CX has a Ural sidecar attached, at 4,000 rpm I am going 45 mph, at 5,000 rpm going 55 mph. All the time getting about 35 mpg. Does this seem about right to you? I do not like to rev. the bike much past 5,000 rpm and would like to cruise between 4,000 and 4,500.

Would the 18 inch rear tire help? What could I expect with that? Will this bike really rev 9,000 and if so who would want to?

Thanks Ken

· Super Moderator
10,530 Posts
The rev line has been brought up many times, and although most of us won't run there all day, the engine will if you want it to......assuming of course it is a properly maintained machine. If one had issues of unbalanced carbs, or any other lingering funks during normal running, I'd discourage going up to that range often.

Your mileage could very well be a factor of having the sidecar. The general range of MPG for the CX family is closer to the 40-50mpg range. Many factors can foul mileage.

Joel in the Couve

· Registered
3,847 Posts

Running them at 9 thou for anything other than a quick blip on the throttle

is probably asking for trouble.

I snapped a crank on my old bike by pushing it too hard for too long

They are all rather old nowadays and need a bit of consideration.

Having said that I'll ride mine to London and back at 80mph ish ( 7 thou) and it

feels quite happy doing that.

I do find that between 6 and 7 thou seems to be the sweet spot on mine though

Its a short stroke motor the pistons only go up and down 52mm/2 inches

so dont be afraid to rev it a bit.

I'd guess that with yours lugging a sidecar, it will be well within its comfort zone

at 4 to 5 thou.

As for wheels, the overall diameter is the telling point

I'm not sure of the difference in the overall diameter between an 18 rim and skinny tyre and smaller rim with a fatter tyre.

An 18" rim with a 120 tyre may give you more speed at lower revs

If it does, and you ride at the same speed as usual then you may find you go

further on each gallon of fuel.

That also depends on how efficiently the motors working of course.

With it properly serviced and setup and run at its sweet spot with no labouring or thrashing you may get better MPG too

· Registered
258 Posts
I ran my GL500 for days on end at 8k rpm doing IBA rides and other trips. did just fine.

I run my GL650 at 7+k rpm All the time.

With the added weight of the side car, you might find you are running the motor to far out of it's power band to get good mpg.

by letting the bike pull easier, you can get better mpg, when running more weight.

You may find better performance and mpg running your sidecar rig if you co with a Smaller rear tire and/or keeping the rpm in the 5500-6500 range more.

By smaller tire, I mean say a 120/90-16 or a 130/80-16. maybe even a 120/80-16.

As an example, I used to work for Ford motor company as a quality assurance manager at a dealership in denton TX.

the ford ranger pickup was available in a 2.3l 4cyl, 3.0l v6 & 4.0l v6.

The automatic 4cyl was a dog and would get really bad mpg.

while the automatic W/ either v6 would do pretty good.

that is because everywhere someone would drive the 4cl W/auto, they would be full throttle until the next light then on the brakes. if you put your foot in it on the V6 W/auto like that, you spun the rear tires.

So those with the V6's were lighter on the throttle.

I was able to warranty/customer satisfaction a new rear diff for a customer with a 4cyl W/auto ranger.

He was getting poor mpg and compaining that was why he bought the 4cyl in the first place.

we dropped the rear end ratio one step higher. (I think it came with 3.45's and we went to 3.73's. can't remember for sure)

He lost 1 mpg on the hwy. (26mpg before, 25mpg after)

and picked up 5 mpg city. (18mpg before, 23mpg after)

He was using it to commute down the hwy to work all week and run errands at work most days.

he checked it by only commuting, no errands for 2 weeks, then leaving it at work and only running errands for two weeks. it was a case of making the vehicle pull the weight better and so staying out of the throttle more.

· Registered
154 Posts
The 18" rear should give you a marginal improvement, but your real enemies are weight and the size of the hole you are trying to punch through the air, which deteriorates the faster you go. Semi-Trucks for example that run curved cab top skirts gain about 18-20% on fuel economy due to the improvements in air flow. Try something similar
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