Why 3000 RPM at 75 MPH?

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#1

A fairly common highway cruising speed is 75 MPH. At this speed, in sixth gear, my MINI, and a lot of other cars, turns at 3000 RPM. My MINI’s non-turbo engine delivers good torque at 2000 - 2500 RPM. Why can’t the transmission be geared to run in top gear, 75 MPH, 2500 RPM?



Wouldn’t this result in reduced wear and increased fuel economy?


#2

Not necessarily. Usually the smaller the engine, the more RPM’s it has to turn to deliver acceptable power. Also many smaller engines tend to make more power at higher RPMs. Even if it had taller gearing, that doesn’t mean it’s going get better fuel mileage because it’s possible that with a taller 6th gear one would have to use more throttle to maintain speed on even modest inclines on the highway, which would lower the fuel economy. Trust me, the engineers thought this out.


#3

Using more throttle at low rpm actually increase efficiency slightly because the engine wouldn’t have to pull air through a closed throttle. Toyota engineers say that the Prius is designed to run at full throttle most of the time at low rpm. Then again, that car can change its ratio much faster than you in your 6 speed Clubman.

In a Mini Clubman, this is a moot point since it doesn’t use a throttle. It has the BMW’s nifty Valvetronic variable valve lift actuation to do the job of the throttle and that practically eliminates pumping loss.

Check this out to learn more about variable valve lift technology
http://www.autozine.org/technical_school/engine/vvt_5.html

Large throttle opening increase fuel efficiency.


#4

Good thoughts. With my 118 HP engine, I can see that I might have to downshift occasionally to maintain speed on inclines. There’s probably a “don’t frustrate or disappoint the driver” factor involved in the engineering decisions.

With the 173 HP turbo engine in my former MINI Cooper S, I think that would be less an issue. Mine was always ready to accelerate, even on the steepest mountain inclines.

I see that the gear ratio specs are different for the S and non-S versions…


#5

Your mini is running at 3000rpm at 75 due to the gearing that makes it enjoyable to drive the car around town. In short your statement about the mini producing good torque between 2000-2500 comes up short or it would not need the gearing it does to make it have decent acceleration. The car is underpowered.


#6

It has a throttle, a drive by wire throttle but a throttle nevertheless. If it has a throttle body then it has a throttle. Valvetronic is just variable valve timing for the intake and exhaust valves, many cars these days have this feature.

Generally speaking large throttle openings at lower RPMS will indeed increase fuel efficiency, mainly due to lower pumping losses, but there is a point where it becomes more beneficial to run the engine at a higher RPM with less throttle opening. For example you will get better mileage if you shift into different gears at moderate engine speeds while accelerating from a dead stop versus starting out in 1st gear, running it up to 20 MPH then shifting into your top gear and staying in 5th or 6th gear whilst flooring the accelerator until to get to 75 MPH.


#7

Why NOT 3,000 RPMs at 75 MPH? In 95% of the country, the speed limit is 70 MPH. Designing a car to run optimally at 75 MPH just doesn’t make any sense.

Also take into account that as you increase speed at that range, the force needed to counter air resistance increases. “…each 5 mph you drive over 60 mph is like paying an additional $0.24 per gallon for gas.” (http://fueleconomy.gov)


#8

Okay, how about 70? When I’m in South Dakota next time I will hold back traffic.

At 55 mph I get 43 mpg. At 65 I get 40. At 75 I get 38. A 5 mpg loss for 20 miles farther every hour. Not a bad deal, but I would love to see 45 mpg at 75 AND reduce engine wear.


#9

In the real world highway speeds of 75-80 MPH are common, so it indeed makes plenty of sense design cars to perform well at those speeds. Granted the MINI is optimized to be something of an around town runabout, but many other cars are built to cruise at 75 MPH and faster all day long. The Crown Vic, and GM H-Body cars are good examples of this. Even sports cars like the Corvette and Viper will only be turning 1500 RPM or so at 70 MPH in top gear. The Corvette has been known to get nearly 30 MPH on the highway even with a 6.2L engine, due to it’s very tall 6th gear.


#10

Final drive ratios are chosen to balance fuel mileage, overall driveability and acceleration performance. It’s important how the car drives during magazine road tests, especially cars like the Mini. A V8 can pull smoothly at 2000 RPM but not a four cylinder car…


#11

You’re not going to get 45 mpg @ 75, no matter how slow the engine spins. Maybe 39 mpg if your engine spins slower. At that kind of speed, you need to burn fuel at that rate to overcome air resistance. If you want that kind of mileage, you need to go at 55 mph or redesign the body of the car.

Edit: I can’t remember where I heard this. During EPA mileage test, the car has to achieve a certain rate of acceleration at highway speed. If the car can’t manage that in top gear, then the tester has to drop a gear, leadinng to a worse mileage rating. That’s why small cars with small engines are geared so short for the highway.