Gear Ratio Calculator

Discussion in 'Transmission / Drivetrain' started by jpilot, Nov 20, 2007.

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  1. jpilot

    jpilot Guest

    I wrote a little gear ratio calculator that some might find handy. The program can be downloaded from Just right click on the file gearratio.exe and select "save link as" and put it somewhere you can find it again. Once it's saved, double click on the gearratio.exe file and it will run.

    The program allows for up to three ratios in series. If you have fewer just leave the unused ratios set to 1:1.

    The program will also calculate speed based on engine RPM, drive ratio,and wheel diameter. Be sure to measure the outside diameter of your wheel for accurate results.

    If you have any questions post them in this thread.

    Have fun!

  2. Alaskavan

    Alaskavan Guest

    Thanks. That looks like it will be handy.
  3. ocscully

    ocscully Member

    I was just playing with your calculator and using numbers for the Grubee 4-stroke set up I get a top speed of 28.9 mph for the 56t final drive sprocket @ 8000rpm. But yet members here have been reporting top speeds 10mph higher with the same set up. Why would the math and the actual mph differ so much? The Grubee Site indicates that their gear box provides a 4.20:1 reduction and then you have an 11t to a 56t final drive. Don't take these questions as a complaint. I've been punching in the numbers for various belt drive pulleys I've been considering for my current project and your program has been very helpful to compare various set ups.

    Last edited: Nov 21, 2007
  4. jpilot

    jpilot Guest

    I'm glad you found the program useful.

    I can't explain the difference between calculated and actual speeds. I'm pretty sure my math is correct. Have the people that reported the higher speeds actually used a tachometer to measure engine revs or are they guessing?
  5. gone_fishin

    gone_fishin Guest

    i think peeps feel like they're going faster, so they report faster speeds...

    i know i analogue speedo said 40, i was clocked at 30, but it felt like 50 :lol:

    btw-thank you for a great contribution :)
  6. ocscully

    ocscully Member

    Claimed MPH vs Calculator At this link two members are claiming 40mph or better. likemyspoiler says 44mph and HoughMade is reporting 40+. perhaps they can offer how these readings were determined? I have no reason to doubt them, I just find it interesting that the math says something different is all, and am curious as to why?

  7. srdavo

    srdavo Active Member

    Excellent Calculator

    Thanks!!!!:arrow:---------> :D

    the old Spitfire engine site had one....but not this cool!!
  8. jpilot

    jpilot Guest

    I didn't find any mention of engine RPM in the thread you referenced. Where did you get the 8000 RPM number? likemyspoiler said in the thread that his speed was verified by GPS but, again, nobody mentioned a value for engine revs at the stated speeds.

    The numbers don't lie. If you enter accurate data for ratios and wheel size in my little program, you can adjust the value for RPM until you get the stated speed. That will be very close to the revs the engine was turning at the given speed. That, of course, assumes no slippage in the drive train.
  9. I have to agree with Augi's post....sometimes I think these little bikes feel faster than they actually are....I have a bike outfitted with a digital tach and speedo and with stock gearing I really don't wanna go 30 MPH for fear of throwing a rod due to excessive rpm's....It feels like she may even blow at 27 MPH....I think it was pushing 5900 RPM but I'll have to double check cause memory isn't as good as it used to be!....YOu are right...if the formulas are correct hard to beat calculations.
  10. ocscully

    ocscully Member


    I pulled the 8000 rpm from the power curve tables provided by Honda USA . They provide a power curve and torque chart as part of their specs on the GXH 50, 8000 rpm is at the high end of their recommended range for operating this engine. HoughMade has mentioned in other posts that he has disconnected the governer on his motor so perhaps you are right and the RPM's may be higher.

    I'd like to again say that your calculator is really great. I've used it several times to day to compare various drive trains I've been considering. Thanks for a great tool.

  11. jpilot

    jpilot Guest

    Thanks for the props guys. I'm glad you find it useful.
  12. iRide Customs

    iRide Customs Member

    Got one for MAC? HAHA!
  13. jpilot

    jpilot Guest

    Sorry, no Mac version.
  14. Hey Guys,

    As promised I finally hooked up my Tach (Sendec) and did some speed VS engine RPM measurements....Sometimes it is hard to try to read both gauges and remember the respective numbers while trying not to crash into anything but I did get some numbers you may find interesting....

    At 6000 Engine RPM's the bike was going 31.5 MPH....Of course this was assuming I had the proper circumference programmed into the speedo.....I was using a 26 " wheel mountain bike with a 36 T rear sprocket.....Dax 70 engine

    The bike tends to buck and surge at low speeds (approx 13 MPH with low throttle) but
    smooths out quickly once 15 MPH is reached....Maybe a boost Bottle will help....(I will have it hooked up soon).

    Max RPM I have pulled thus far is 6340.....

  15. Dockspa1

    Dockspa1 Guest

    So if we are running a 4:1 reduction at the gearbox and another 4:1 at the sprockets/belts, would that be a total of 8:1 over all?
    I am using your very handy calculator to try and figure a belt drive need straight from the engine to the rear pulley and need to know what the best setup is for taking off w/o killing the engine. Like an old belt driven garden tractor.
  16. jpilot

    jpilot Guest

    Two 4:1 reduction stages would yeild a total ratio of 16:1. You can enter known ratios into the calculator. For example, if you know your first reduction is 4:1 put a 1 in the drive gear/sprocket field and a 4 in the driven gear/sprocket field. Any ratio will work as long as the drive and driven gear sizes are entered in the same units.

    I'm glad you're finding the calculator handy. It sure beats banging on a calculator and writing down intermediate results.

  17. Dockspa1

    Dockspa1 Guest

    You got that right!
    Thanks again.
  18. Egor

    Egor Guest

    Jpilot - Thank You, that is the most handy thing I have seen here. I am just getting ready to design a new type of drive for my happy time bike and this will come in handy. Have fun, Dave
  19. BoltsMissing

    BoltsMissing Active Member

    Thanks for this calculator, will help muchly to determine a ratio for my next builds.