Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Rolling motion mechanics & ideal economical gearbox ratios

  1. Feb 20, 2015 #1
    Greetings, mechanical engineers of Physicsforums,

    I have been struggling with a particular problem calculating an ideal motor RPM for economical highway cruising.

    The motor I am looking at is the Subaru EE20 Diesel motor. Its torque curve can be found here: https://subdiesel.files.wordpress.com/2011/05/powergraphsubaru.png

    I am considering this engine for a swap in to my car for its great low-RPM grunt and economy. But with the stock 6 speed transmission, in top gear 75 mph comes at 3600 RPM -- just about the peak of the power band, and at around 290 N-m, or 80% peak torque of the motor. I have calculated the peak thrust at the wheels (from 1800-2400 motor RPM) to be around 1000 Newtons, while drag at 75 miles per hour (33.5 m/s) is around 700 Newtons. So clearly the motor running at that RPM for that speed (33.5 m/s) is sufficient to overcome drag on a perfectly flat plain. So why the heck is the overdrive gear set for such a high engine speed? Surely the 110 kW (148 hp) of power at that speed far exceeds what is required to move the car, isn't it? The car I would be swapping it in to weighs around 16.5 kN, or 3700 pounds. This is a rough estimate for my own weight and a bunch of gear in the back. Am I missing something?

    Additional note: The final drive of the car is 4.444:1. I calculated that with a 3:1 final drive, I could travel at 73 mph at 2400 RPM, which is close enough for my needs... or is it? Again, I feel like I am missing something. I know diesel power bands are far narrower than petrol ones, but I have trouble accepting that Subaru would design a transmission to basically never exceed 80 miles per hour. The CVT, which I would much rather have considering the narrow power band, has ratios closely matching the 6MT as well and has the potential to achieve a far greater gear spread, so I am not sure what to think from their perspective.
  2. jcsd
  3. Feb 20, 2015 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    It's not just the transmission, it's the combination of your gearbox and the rather short final drive ratio of 4.444:1. That's drag racer type ratios, intended to make quick starts off the line for short dashes, fuel economy be danged. What's good for pulling stumps may not cut it for long-distance economy cruising on the highway.

    This article discusses some of the trade-offs between different gear-engine-transmission combinations:


    IMO, you want to keep your new Subaru diesel operating in that band of 1800-2400 RPM as much as possible, cuz the torque falls off the cliff rather quickly outside this band.
  4. Feb 21, 2015 #3


    User Avatar
    Science Advisor

    Take a look at the data specifications for the same car fitted with diesel = CI, and gasoline = SI, engines. The final drive ratio is usually lower with CI used than when when SI is used. That compensates for the lower peak RPM of the CI engine. As turbochargers and common rail injection systems have improved, the performance of CI has moved closer to SI.

    Increasing the rolling radius of the tyres can give a significant improvement when converting from SI to CI. But it is safer to lower the final drive ratio than to increase tyre rolling radius, since tyre rolling radius has unfortunate brake performance implications.
  5. Feb 21, 2015 #4
    Right, this is why I was thinking that a quick and easy way to get a big economy boost would be to swap the differential from a 4.444:1 to a 3:1 or lower (which may need to be machined, I need to look into this more). This would keep my highway cruising speed in 6th gear between 55 mph and 72 mph at that 1800-2400 RPM torque plateau, which is what I want. Oddly enough, the final drive on the diesel is actually SHORTER than the final drive on a gas -- 4.444 for the 6MT diesel, 4.111 for the CVT diesel, and 3.900 for the CVT gas.

    I just did some very rough calculations accelerating in 1st gear (3.454:1) and with a final drive of 3:1 based on average torque from 1000-3600 RPM, which is about 265 N-m. It yielded around 7 m/s^2 or about 0.7g with stock wheels. That's still pretty drivable in the city, but I am not sure how badly this would impact off-road performance, namely the ability to get out of mud or even to pull a trailer.

    I wonder if the addition of an electric supercharger would spread the peak torque out more, namely have it come on earlier? The alternator for the diesel is rated at 170 amps, so I'm sure it could put out the juice.

    Another odd thing: VW and Subaru's diesel drivetrains have similar gearing (Subaru is shorter on all but 5th and 6th), so for a comparison of highway cruising RPM I calculated about 3400 RPM highway at 70 mph. But after watching some enthusiast hypermiling videos, they are cruising at closer to 2000 RPM at these speeds in the very same car! I pulled the gear ratios and tire width directly from the VW site for the model that was being tested! What gives?

    Watching 0-60 mph videos for the Subaru diesel, they are in 3rd gear at 60 mph and around 3500 RPM. The gearing is 1.062 with a final drive of 4.444. This calculates to over 5700 RPM. Am I missing something?...
  6. Feb 21, 2015 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    It's still not clear what your ultimate goal is with this car. Do you want to make an economical car for cruising on the highway, or do you want a car which performs well off-road and/or towing something around? You can have one or the other, but I don't think you'll be able to come up with both simultaneously.

    You should post the gear ratios for all the speeds in your 6-speed tranny. Shouldn't this box have an overdrive in 6th gear, maybe even in 5th gear, too?
  7. Feb 21, 2015 #6
    My goal is 45 MPG at 75 MPH, but I will take a hit there if it means the difference between getting across town in a snow storm or not.

    The gear ratios are, for the 6MT:
    1: 3.454
    2: 1.888
    3: 1.062
    4: 0.785
    5: 0.634
    6: 0.557
    R: 3.636
    F: 4.444

    For the CVT it is 3.505:1 to 0.582:1 with a final drive of 4.111. They are geared very similarly and in fact the 6MT is more economical for cruising.

    I don't see anything about an overdrive for either 5th or 6th gears. Assuming these numbers posted are the only ones in the box, the engine should be revving a lot higher than it is in the few videos I can find online. I am very confused.
  8. Feb 21, 2015 #7


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    An overdrive is where the transmission gear ratio is less than 1:1. For the 6MT box, the 4th, 5th, and 6th gears are all overdrives.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook