Most fuel efficient RPM range?

I know our engines are not harmed by higher RPM's but what is the most fuel-efficient cruising range (RPM's) for a 750?

Is cruising at 4k lugging the engine? Should I be running in a lower gear at 5 or 6k?

 

Anecdotal evidence:

When riding at 65 all the time, mileage came in at low-mid 40s.

When some of the trips were ridden at 55 instead of 65, mileage spiked to near 50.

My personal opinion says that these engines were meant to cruise along at 4k in fifth gear.

 

Thanks dude. I'll give that a try. I've been commuting at 70+ and getting there in a hurry too.

 

I've found pretty much the same thing - 100 km/h at 5000 rpm, mileage is great, push it that little bit farther to 6000 and the mileage drops significantly.

I think as far a mileage goes somewhere between 4 and 5 grand is the key area. even though this will keep you slightly below highway speed

 

Go as fast as you can get away with! That way you're using gas for shorter periods of time!

mikeg

 

Its not RPMs that makes fuel efficiency its wind resistance.
The wind resistance force F = 1/2*m*v^2
Where m is the mass and v is the velocity (speed).
The force goes with the square of the speed.
Add 10% to your speed and you get 1.1 x 1.1 = 21% increase in wind resistance.

55mph is always going to be much more fuel efficient than 65 mph. Its just physics and math.

 

Now wait a minute, I was told there wasn't going to be any math...

 

HalfCentury is the only guy alive that actually kept some math from high school. lol

 

I would guess that half is an engineer of some sort likely mechanical or stuctural and remembers far more from college/ university than high school, but I could be way off it's been know to happen!.....half?

 

Electrical engineer. The math is from elementary physics.

Wiki article here

http://en.wikipedia.org/wiki/Drag_(force)#Types_of_drag

Consult any High School or first college text book about Newtonian mechanics.

Given a rider on a motorcycle, everything except velocity is a constant. The force is proportional to the square of velocity. That is the important point of the equation.

 

Half, do you have any Tylenol?
Mah head hurts! lol

 

Its all about aerodynamic drag. Master the concept and you too can be a drag queen.

 

#1) I get the concept, and the math. Just couldn't repeat the formula to you.
#2) Drag queen. Good stuff. lol

 

If your route is anything like mine the traffic flow doesn't give you much choice on speed... 75-85mph or you're gonna get run right over.

 

I'm gauging my MPG now that my fuel leaks are solved. I've been runnin around town in 3rd or 4th, (depending on how many cars I'm passing) at 45-50mph sittin at 4-5k RPM. Gone 50 miles and down to about half a tank i rekon. I'm shifting at 6k RPM. Maybe revvin her too high when I shift?
-SLKid

 

The harder you accelerate the more fuel you consume compared to accelerating at a cager pace.

Where I live its crowded enough to prohibit hard acceleration in traffic.

I usually do not rev much past 4k rpm from a dead stop up to 45 mph. I ride the bike much like I drive my car. I do not pretend I am drag racing. To each his own.

When I ride on the weekends I stay off the freeway and my speed is probably 50 mph average. I get 50-53 mpg driving like that.

 

If aerodynamics is what dictates fuel efficiency, then my girth is probably to blame. I really feel the wind push on me when I ride. I very much want to get that aftermarket cafe style fairing I posted under "For us Seca owners".

I get on US 50 from the New Carrollton Metro station. US 50 is a race track and I practically have to launch myself out of a cannon to get up to speed, and to the left so I can avoid the exit ramp for I-495. Not to mention, I like to get into the HOV lane to avoid all the traffic.

 

Actually that's the equation for the amount of kinetic energy for a given velocity. This is the energy that you need to convert to get to a certain velocity ideally (no friction/air resistance). This is also the energy that you need to get rid of when braking.

Concept is similar tho as far as being related the square of the velocity but is not related to mass. F_d is also proportional to density of the fluid and reference area (nice for wind tunnel tests) as well as C_d, which you can do something about (tho also non trivial, need wind tunnel or complicated software).
But it doesn't tell the whole story. It's true to say that going 55mph results in less aerodynamic drag than 65, but so does 45 compared to 55...all the way to 0. To maximize mpg, you need to minimize the frictional losses (of the drive train, engine, and rolling friction, which will increase with speed, but I don't know how to express this) and aerodynamic drag, which is a function of the square of air velocity(not air speed, its a vector), and air speed is usually different than ground speed, while taking maximizing the efficiency of the engine when producing the required force (torque). That efficiency is going to relate the the torque produced at a certain rpm, as rpm will be proportional to speed. It's almost not worth calculating because there are so many factors, head wind vs tail wind, uphill vs down hill, accelerating. Empirical testing is as good as anything.

 

Here's a real example of ultimate fuel efficiency in your XJ:

Load up your XJ in a truck, bring it to my house, and hand me the signed title.

Your bike didn't actually use any fuel during the delivery and...this is the bonus...you'll never have to buy fuel for it again.

Hyper-miling at its finest.

 

Deleting a few words in the above quote and you have a solution to your troubles.

All you need to know is:

Load up your XJ in a truck, bring it to my house, and hand me the signed title.

I'm such a good friend to take care of these issues for you folks.

You can thank me later.

 

Scratch that.

Wrong topic

 

All of the stuff you mention is for all intents and purposes a constant. The variable is your ground speed. There will be a "sweet spot" where you get maximum mpg. For most cars that sweet spot is 45-55 mph. I would bet real money that the sweet spot for an XJ is in high gear between 45 and 55 mph.

 

I agree there is a sweet spot for certain conditions...but but conditions can vary, 45-55 is probably right.
You can change some constants, like C_d. I would assume a fairing could help, but aerodynamics are tricky.
RPM does play a factor as it relates directly to your speed and affects engine efficiency. This is why idling in top gear won't be the most efficient speed.

 

The lowest rpms at any speed in the highest gear will help MPG.

E=MC­­²

 

Flash, as the Guinness people say, "Brilliant!" Helped me get a whole lot more out of my Maxim

 

I learned this weekend that I defintely ride my bike hard. Even with the ups and downs of the mountains in northern MD, I was sipping fuel as we tooled around at 35-55 mph. A constant highway speed of 65 mph was pretty efficient too. I made 41.11 mpg on that run.

I seem to lose my efficiency when I launch myself for my workday commute, and the fact that I really blast down US 50.

 

Lol, thats just cause its FUN! I hit these little back side roads on my way to work every morning tucked into the country. 2-3 mile stretches with no cars, no medians, not even road lines. I usually cruise about 65 down those roads. Sometimes higher if I'm feelin Frisky! lol. But I know its eatin my gas so I usually keep my driving modest.
Penny Pinchin man
-StreetLEGALKid
Ha ha

 

Maximum fuel efficiency is achieved at the engine's torque peak rpm.

P.S> the hotter the fuel mixture is (without causing detonation), the better the fuel efficiency, too.

 

These bikes were set up to get the best MPG at 55 back in the day. They will certainly go faster but I feel the gearing is for the 55 MPH national speed limit we were on at the time. So I agree the MPG is from 45 to 55

 

I think what chacal is referring to is max engine efficiency, which increases until you hit peak torque rpm. But aerodynamic drag is also increasing with air speed (i.e. ground speed with zero wind), so there is a point where they balance and you get the max mpg.

Does anyone know if the efficiency changes if to torque required to remain at a constant speed is less than the max torque at the rpm of the engine at that speed?

 

I think the problem in this thread that it mixes two things which are barely related. The original question mixes up RPM range which is an engine variable and MPG which is an overall variable. They don't really connected to each other because we got the transmission

First, fuel efficiency of the engine. As Chacal wrote, the engine is most fuel efficient at peak torque RPM. Important note: it doesn't mean low fuel consumption. It means that the engine uses the wast amount of fuel in the most efficient way.

Second, MPG. How many miles can you do for a gallon? The most important factor is the wind resistance. http://en.wikipedia.org/wiki/Wind_resistance
If you look at the section "Power" in the article there is an equation where you will see how much power you need for a given speed to stand the resistance of the wind. Note a thing. The 'v - speed' is at third power. Guess what ...

What does this mean? Faster you go, more fuel you're using.

For example: luckily, to drive to my workplace and back can be a relaxed drive. A good 20 miles with a few crossings and a few stops but I barely go over 40 mph and 4000 RPM. I rarely hit the brakes, just using the engine to slow down. I'm driving with the flow and the mileage can be up to 65 MPG. In the town.

On the downside, when I'm in rush on the open roads doing 80 - 100 MPH or above my mileage drops to 30 MPG.

So, after all science doesn't helps at all if you are over 70 you're mileage starts to drop dramatically

 

That's how it is on the big roads here in the garden state. 295 and the turnpike both have average speeds of around 80 mph.

 

i ride everywhere pretty much flat out, i havent done ther maths and i dont have a tacho but my wallet says i use too much fuel

 

plumber just change your license plate to W.O.T. I was going to do it but i often find my self short on time. Keep us posted on the number of traffic stops

 

Basically, you're not going to be able to do that much to your C_D unless you start putting big, fancy cowls onto your bike, so you pretty much have to take C_D as constant. That means that you can't really change the drag profile of your bike, and you can't change the fact that drag goes as dynamic pressure (1/2*density*velocity^2), so the only thing you really can change is where on the torque curve of your engine you ride.

So, obviously, this is a very complicated set of parameters, because at each speed, in order to keep from accelerating, your rear wheel torque must match your drag force. This means it's really pretty much impossible to calculate the most efficient speed/gear to drive at before hand; you'd have to measure it.

I'd say your best bet is to do what others have recommended and drive at the torque peak (my bets are with everyone who said 4500 rpm or so) in top gear.

 

i am working on a ram air design for the XJ if your interested more HP = better fuel milage

 

I've been messing arround with the idea of runnung two cone filters facing forward on top of the engine, running back into a 2 into 4 manifold on the carb, the faster you go the more air you will get.......

 

Not to be critical of ram air midnight, but the slight added air pressure will work against our carb design, in the same way that "pods" do, and I actually hooked parts of my Manometer, taped to my truck to measure the air pressure at 80 MPH, I get two inches of water, which is 2/27 of a PSI
or about .08 PSI of boost.

That won't impress the ladies.

 

I'm not sure what the issue with pods is, i'm running pods and an open pipe, i have never had an issue with this set up and i've used it on many bikes once you jet the carbs your all good, ram air would only require the same re jetting which is no big deal

 

.08 psi of pressure is great! i am curious what size tube were you using as the collector? what you may have forgot to do is hook up a vacuum gauge between your air filter and your carb to measure the pre-ram air design in hg you'll find that its negative pressure when off set at 5000 rpm in top gear increases fuel efficency... even 1 psi of vacuum can change the power band by up to 400 - 1000 rpm

it all has to do with how much air you can grab... re-jetting shouldnt be necescary ince it wount replace the factory design .... keep in mind this isnt going to by pass the air box, just add air pressure behind the filter so the engine is working as hard to pull air in the power band ..... if anything it will help the engine take a better metered ammount of air per cylindar.

if i were putting in say a turbo, or pods where i am changing the air flow design that may be true.

 

I did this a year ago because of a thread about ram air.
I didn't sample the air in the air cleaner, but that would be an interesting comparison. I made something like an airplane air speed tube taped to the antennae in clean air, away from the hood, then routed into the cracked passenger window, then into a "U" of clear tube taped to the dash.
All windows open slightly as the "ram" air is compared to the "average" air pressure inside the truck.

If the 650 Turbo at 15 pounds of boost makes 40% more power,( 30 HP) , then 1.5 pounds of boost would make 4% more power,(3 HP), and .15 would be .3 HP- - we're talking half that, so .15 HP gain at 80 MPH, roughly.

The air cleaner injests hot air from behind the motor. Ducting would give you cold air- - that more dense air would be a plus, so don't let me keep you from your project.

 

agreed it will be smaller difference ... obviously we arent making big numbers.... but it is theoretically possible not produce pressure but rather reduce filter vacuum by reducing air filter and air air cleaner vacuum.

 

And my guess for a 650 - 750 would be to lean out the pilots, way too lean, but still running, and drive at 3,500 RPM, or 40 MPH all tucked down like an idiot, and you should get 65 MPG, if you can stand it.

I got 49 MPG on my 900 running lean, but fast. Around 55 - 70.

 

But isnt Lean bad though?? Running your engine lean causes engine damage right?? Like my right side pops and I'm waiting for shims to finish fine tuning, and popping usually means lean. So regardless of what MPG your getting, running lean is supposed to be awful for yuor engine right???
(reminds me I'm going to post here tomorrow or so about setting idle mixture screws without a colortune)
-SLKid

 

My lean suggestion above would just be for a test ride to prove the max fuel economy, just barely twisting the throttle. Head down to reduce drag. Ride normally with the bike all leaned out and you WILL hole a piston or maybe burn a valve.

Isn't it amazing that using less fuel creates more heat?

Kid, once your bike is running, you read the plugs, and lean out the darker plugs in fine increments, turning the screwdriver in only a few degrees, then ride again. Put that silvery anti-sieze stuff on the threads and don't bother to really tighten them down until you're done.