Topic: RPM Range for Best MPG

[Popwood]

I'm just curious about an ideal RPM range to optimize fuel economy. Assuming the bike is properly tuned, where's the best mileage found; low, mid or high rpms?

If it matters, I'm riding two CBs, both '75s, a 550K and 750K.

And for that matter, what RPM range were these bikes built to run at in "normal" street and highway speeds.

[pjlogue]

As a rule of thumb, low RPM, high manifold pressure gives the best fuel economy.  The high manifold pressure means the throttle is open more. This is generally true as long as the engine isn't lugging.  This was demonstrated by Charles Lindbergh.   

-P.

[754]

I think Charles had different carbs than us..
 I used to find 6300 to 6800 was pretty good .. I don't get anywhere near the Mike age I got with that, I have a re F motor and it revs really  high I am turning  around 5500 to 6000 at same speeds as before and crappy mileage  lucky to get 45 US now.

[Jarrett_Honda]

You can bump up your front sprocket by one tooth to drop the rpms and to get a smoother ride. Your top end speed will got up, buy acceleration will go down.

You could drop the rear sprocket by one tooth. However changing the front has a larger impact on change.

The k5 cb750 has 18T and 48T sprockets.

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[scottly]

It's more a matter of speed and wind resistance than engine RPM. For instance, my 836 with Weber carbs, with 17/48 sprockets can get over 60 MPG when cruising at a steady 55-60 MPH on mostly flat ground and no headwind. Cruising at 70-75 MPH, I can get 50 MPG, but on the second Relay while battling a 40 MPH headwind across the Mojave Desert at 55 MPH, I only got about 40 MPG. The throttle had to be open more to make more power, so the manifold pressure was higher. 

[American Locomotive]

In general for a given load, lower RPM and more throttle will yield the best fuel economy. As you increase manifold pressure, pumping losses go down increasing engine efficiency. Additionally, as cylinder pressure increases, engine efficiency also goes up. So forcing the engine into a higher gear where you it needs more throttle to overcome the load will yield better fuel economy. As an example: 40 MPH @ 3000 RPM in 5th will give you better fuel economy than 40 MPH @ 6,500 RPM in 2nd.

Of course, as your speed increases the amount of power required to push you through the wind goes up too. When you double in speed (so from 20 > 40 MPH), it requires 8x the power (and fuel) to push the bike through the wind. So the amount of power required goes up dramatically as speed increases.  Shouldn't that mean the slower you go, the better your MPG? That is correct up to a point. Engines have really bad efficiency at low-loads. So most engine-vehicles tend to achieve peak fuel economy between 35-50 MPH. The engine is in its sweet spot in top gear, and wind resistance isn't too high.

If I really baby my 750 (shifting early, keeping it around 2500-3500 RPM), I can get about 40 MPG. If I just bomb around all over the place in low gear really revving it out (not going fast, just revving it out), I've seen as low as 25 MPG. My bike is also likely too rich on the carbs, so you should be comparing the differences rather than my absolute numbers.

[seanbarney41]

wow, American Locomotive, not sure we should be listening to your information about fuel mileage, as your numbers are really low.  I just got done putting nearly 700 miles on my bike in four days.  My mileage varied from 38-42mpg.  38 mpg ripping around Sleeping Bear Dunes area twisty's...lots of downshifting, full throttle, hill climbing.  42mpg averaging 65mph on the highway for a few hours.  And that's pretty low imo.  Other cb750's have been capable of near 50 mpg without excessive babying.  Mostly, I have seen 40-45mpg with the many cb750's I have owned.

Now 550's?...them things can really guzzle!

[scottly]

Anything below 4000 RPM is lugging an SOHC..
Sean, Mooshie has done in the 50 MPG range with Dash, IIRC? Of course, it should be mentioned that Dash is equipped with pods.  

[Jarrett_Honda]

Is there enough room for the front sprocket to go from 18t to 19t with damaging the case?

If so the rpms wouldn't drop below 4k would it? I hate riding at highway speed with the engine screaming above 5k. The vibrations get ready uncomfortable on long rides.

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[scottly]

5K RPM is not "screaming" any of the SOHC fours, it's just getting into the sweet spot.

[bryanj]

Too right Honda in owners manuals used to say it was permissible to go into the red zone, but not out the top

[ekpent]

 40's to 50 mpg on my basically stock set-up pre 77 Honda 750's is what I have seemed to notice in general all around riding.

[Bodi]

Mileage has more to do with load than RPM but gas engines are most efficient at the RPM of the torque peak.

[david 750f]

I agree with Ekpent. I beat the piss out of my 750F and usually get around 40..Higher if I’m cruising at 75mph (120kph)..

[Alaxy Galaxy]

Heh, on the 400/4 I pick a gear that lets me rev around 5500-5700 to stay at whatever speed I want to be at. It really isn't any fun below that.  I never really checked my MPG, I gotta say.

[PeWe]

You hear when fuel comsumption is low. Sssssss or rrrrrrrrrrr.
When drinking... Baaaaaaa

I can cruise fine in lower speeds below 4000 rpm (100kmh/60mph)
It even charge OK at +3000 rpm. Have voltmeter connected sitting between gauges.

70-80kmh is nice on 5th gear 18:48 gearing (CB750 K6). If engine should have been stock  17:48.

This need a correctly adjusted engine, carbs jetting/sync and ignition to fully enjoy.

This is what I have done a lot lately when test riding after carb jet updates which now are OK

Planned dyno run to see how the A/F chart will look for good jetted carbs delivering good power when twisting and even low lift behaviour.

[Deltarider]

Mileage has more to do with load than RPM but gas engines are most efficient at the RPM of the torque peak.

^This. On German Autobahns I used to maintain 7000 rpm (140km/h or 90M) for hours*. No vibrations anywhere, mirrors crystal clear and motor was happy. 7000 max torque for a CB500
* Having said that, I needed to refuel every 1¹/₂ hour.

[754]

Heh, on the 400/4 I pick a gear that lets me rev around 5500-5700 to stay at whatever speed I want to be at. It really isn't any fun below that.  I never really checked my MPG, I gotta say.

Really ? Do you ever travel? If you are on backroads without being sure were next gas is can get you in trouble. I had to quit running a 400 4 tank on my 750 because of that..or start packing fuel.
 I would rather not stop every 110 miles if I don't have to 140 is more like it.. like 2 hrs riding or more. In Utah 110 miles is 1.5 hrs or less..

[754]

 I was riding with a girl on a CB 500 and suddenly she pulled over, whipped out a fuel jug and put it in her tank.  Small reserve I said ? She said no.. it just went on reserve.. I said how much you got on reserve?, knowing the next gas was 7 or 8 miles ahead. . Shecsaid I dont know , i just pull over and put this gas in when it goes on reservevand look for gas.. .ha.. wtf..

[ekpent]

 Does reserve pull gas down off the bottom of the tank ? Might be a good idea to keep a dirty or rusty tank from going into reserve if that is so though a clean tank is a happy tank !

[Mooshie]

Anything below 4000 RPM is lugging an SOHC..
Sean, Mooshie has done in the 50 MPG range with Dash, IIRC? Of course, it should be mentioned that Dash is equipped with pods.

Don't start a pod thread now Scott!  People have a lot to say on that subject!   Yup, dash gets about 50mpg regularly.  Head winds do mess it up though.  He likes to RPM at about 6000-6500

[Jarrett_Honda]

Maybe I need to check my steering bearings or wheel bearings. I can't see anything in my mirrors at 5k rpm.

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[PeWe]

Autobahn had really good signs with info about next gas station  and if there is a restaurant or only cafe beside. Plus info about next station after next.

Easy decision if reserve has not yet been switched on.
My bike could take closer to a liter/10 km when twisting
I think around 180kmh caused that.
Choice between very slow traffic in max 90kmh behind stinking trucks or next lane where min speed was 170-180kmh to avoid headlight flash in the mirrors. That is forbidden driving style nowadays.
Autobahn Hamburg- Munich- Austria to head further to Zell am See.
More uphills from Kassel area where trucks were really slow, had to  be sure to ride in right lane in time to avoid crashing into the slow traffic in maybe 50-60kmh, sometimes even slower.

[HondaMan]

In the 750's "round top" carbs, the best MPG happens when the slides are open to just below the wasp-waist in the throat (about 48% throttle). My K2, when heavily loaded with camping gear and a wife, gets better MPG in 4th gear at 55-60 (48 MPG) than in 5th at that speed (40-ish), and it drops slightly above 75 MPH either loaded or not in top gear. When riding solo, I have used a 19T front sprocket (judicious trimming needed here and there on the sprocket and engine's sprocket cover) with the same 48T rear for 80+ touring and found it did best at 75 MPH, but still about 40 MPG. Most of these 750s will do better MPG with the original 16/45 gearing (but hard on chains) or the 17/48 gearing. The 18/48 gearing is very tall for the torque curve!

[Popwood]

Good information here so thanks for all the info. Looks like I've been riding like a pokey until recently when I've tended to stay in 4,000 to 5,000 range most of the time. What prompted me to ask this question is this: my trip meter was sitting at 100 and my experience has been the reserve tank switch would be needed with 15 more miles or so. So I topped off the tank and holy crap-- it only took 2.4 gallons (550K1), so about 42mpg. In past I'd be in the low to mid-30 mpg. And I think I need to stay out of high gear unless pushing on the interstate. And just FYI, no changes to set up or even a tune-up so it has to be riding in higher rpm range.