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RPM vs MPH

Discussion in 'Motorcycle Tech Talk' started by DaveFxst, May 17, 2004.

  1. DaveFxst

    DaveFxst New Member

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    Has Harley ever published a chart indicating rpm in correlation to bike speed for the 88TCB motor? Specifically, for those who do not have a tach on their Softtail, what RPM would you be turning at 65MPH in fifth gear? This would seem valuable since the factory does not recommend pushing the stock motor over 5500 RPM for an extended time. From my limited understanding of engineering, that RPM in top gear would equate to a speed that would make the bike aerodynamically unstable. It would seem they should provide a maximum road speed in each gear to prevent overrevving the motor. Or am I overthinking this?
  2. wildvan

    wildvan New Member

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    Hope this helps, I noticed no else gave you a answer, I found this posted back a few days..
  3. SISK

    SISK New Member

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    RPM MPH correlation

    DaveFXst,

    I recently had my bike dyno tuned at an H-D dealer. The bike is an '03 heritage FLSTCI with stock engine, V&H pipes, high flow a/c and power commander (all purchased from Direct Parts, Inc. of course! :) ) The tech let me watch and ask questions while he was doing the dyno, and he was very knowledgeable and helpful in helping me better understand the RPM MPH relation. I have the speedo / tach combo gauge on my bike, so I can see my RPM, but even so, I was not using the bike to it's potential.
    The tech told me that the "sweet spot" for the 88TCB engine is in the 3200 to 4200 range. After modifying my driving / shifting to take advantage of that RPM range, I've noticed much more available power.
    65 MPH in 5th gear is only going to be turning your engine about 2800 RPMs.
    I've got my dyno print out for 4th gear which is as follows: 45MPH = 2400 rpm, 55MPH = 3000RPM, 65M = 35R, 75M = 40, 85M = 45R, 95MPH = 5.0RPM. The rev limiter kicks in at about 5800 RPM.

    I don't have the print out for the other gears, but as you can see, the engine can easily does 75 MPH in 4th gear at only 4000 RPM, and I'm generating 80 ft. lbs of torque at that speed, so I can easily roll on it and get around the car in front of me.
    Now I rarely get into 5th gear unless I'm cruising at 70 or faster (only if the posted speed limit permits of course :D ).
    If you're interested, let me know, and I'll watch my tach make some notes of the corresponding MPH next time I'm out riding.

    SISK
  4. DaveFxst

    DaveFxst New Member

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    Many thanks for the reply. Your info is dynamite stuff. If you want to make a note of rpms in your travels, there's probably a whole bunch of riders who would like to know. I have the carbureted version with the hi flow setup but all gearing is stock so the data should be the same. Based on your dyno data, I'm gonna modify shifting patterns and because it's obvious I'm not using anything near the potential of this puppy.
  5. SISK

    SISK New Member

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    RPM vs MPH Chart

    DaveFXst,

    I went out riding this weekend and made some notes of my speedo / tach correlation (I strongly recommend stopping the bike before taking any notes, at 70 mph the wind really whips the paper and pen around). :D

    As you recall, my bike is an '03 heritage FLSTCI with the stock 88TCB engine, V&H pipes, high flow a/c and power commander.

    Disclaimer: The following numbers are for reference only and are not official data published by Harley-Davidson. The numbers were taken from my speedo/tach combo while riding, and therefore may vary by 2 or 3 miles and by 100 to 200 RPM, but it should give you an idea of where the bike is running.


    1. 1st gear 3500 = 25 mph / 4500 = 35 MPH
    2. 2nd gear 3500 = 37 MPH / 4500 = 50 MPH
    3. 3rd gear 3500 = 55 MPH / 4500 = 70 MPH
    4. 4th gear 3500 = 70 MPH / 4500 = 90 MPH
    5. 5th gear 3500 = 80 MPH / 4500 = Speeding Ticket

    Rev it up!!!

    SISK
  6. SISK

    SISK New Member

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    RPM Red Line

    Hey, I forgot to note in my prior post that the tachometer redline is at 5400 RPM, and the rev limiter kicks in at around 5700 5800 rpm.

    SISK
  7. DaveFxst

    DaveFxst New Member

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    Speeding tickets suck. Sorry - that wasn't supposed to happen. I would imagine the ticket happened around 105 or so. A little math exercise indicates 44 rpm yields 1 mph in 5th gear, meaning at 5400 rpm you would be motoring at around 125 mph. 5700 rpm would yield around 130 mph. Think I'll skip that. By the way, forget the disclaimer - no lawyers are allowed on this site. Thanks a million - this is great stuff. Judging from the number of hits on this thread, it's an interesting subject.
  8. SISK

    SISK New Member

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    Speeding tickets

    DaveFxst,

    Don't worry, I didn't get a speeding ticket. I was just indicating that if you intended to cruise around in 5th gear at 4500 you would most likely end up with a speeding ticket (but only if they can catch up to you :D ).

    Disclaimer: I always ride within the posted speed limit (when law enforcement is present).


    Shiney side up!

    SISK
  9. hotroadking

    hotroadking Super Moderator Staff Member

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    LMAO too funny!

    The rpm to speed ratio is also affected by the primary gear ration and trans sprocket to rear sprocket belt setup.

    Most EFI HD's are setup with a final drive ratio of 3.37 although they changed it a few years.

    Softails are setup with 3.15 to offer a lower cruising RPM and less vibration in fifth gear (or any gear I guess)

    I changed my primary gear with the 95 build so my motor would have higher RPMs and better acceleration. at 70 MPH in 5th it's about 3000 RPM
    at 60 it's around 2500, give or take a few rpms.

    With the 3.15's 70 MPH would be about 2700 RPM.

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