Results 1 to 13 of 13

Thread: Diff ratio calculations for speed/rpm

  1. #1
    Registered User darwinschops's Avatar
    Join Date
    Dec 2010
    Location
    Terra
    Posts
    3,365

    Diff ratio calculations for speed/rpm

    Tried a bit of a search but didn't come up with much. Likewise google has some calculators but couldn't find a straight forward one that seemed accurate with what I thought it should be (gear ratio 0.5 in sixth, 2.92 axle ratio, 19s and rpm@100 odd of about 1200rpm kept coming up higher when entered?)

    Essentially, swapped the car over to manual, still has the auto diff ratios in it (2.92 iirc). It's a low boost TT setup with twin 3071s so full boost is there by c.2500rpm anyway so don't need or want anything too steep (3.9) and street car that'll pbly never see the drag strip. Plus I'd like to keep highway revs as low as possible because I do a bit of that.

    Being the trutrac is going in figured a reasonably thing to chuck some 3.45s, maybe 3.7 though as above and don't want to make 1st useless.

    So either experience or a good calculator would be appreciated.

    Cheers.

  2. #2
    Banned
    Join Date
    Nov 2017
    Location
    Australia
    Posts
    486
    3.75 or 4 is from memory what the VZ hsv's ran and that was a good ratio for the 6 speed. Been in a 4.11 6 speed from canberra to sydeny it wasn't too bad.

  3. #3
    Opens
    Join Date
    Nov 2001
    Location
    One street away from Guido's den of sin
    Posts
    24,302
    I use this one:

    http://www.apexgarage.com/tech/gear_ratios.shtml

    Is accurate for my car.

  4. #4
    Registered User lukevl's Avatar
    Join Date
    Oct 2003
    Location
    Brisvegas
    Posts
    3,270
    PM me your email and Iíll send you the one I did for my car

  5. #5
    Littering and....?
    Join Date
    May 2003
    Location
    Sydney
    Posts
    1,609
    Ive got a turbo 5.7 6 speed with a 3.46 and its too low. You have to go back to 1st around round abouts, it does 130km/h in 2nd etc. It's a pain.
    Im looking to go to a 4.11 or similar.

  6. #6
    Banned
    Join Date
    Nov 2017
    Location
    Australia
    Posts
    486
    Also is the wheel size the wheel and tyre diameter or just wheel diameter as your 19" wheels might have a 21-22" overall diameter that might be where you are getting higher figures.

  7. #7
    no mods, leave it stock Jack Nicholson's Avatar
    Join Date
    Aug 2012
    Location
    B̶e̶v̶e̶r̶l̶y̶ ̶H̶i̶l̶l̶s̶,̶ ̶C̶A̶ Baulkham Hills, NSW
    Posts
    5,722
    Jaguar XJR, Freelander 2 HSE, Jaguar XKR

    Quote Originally Posted by Marv View Post
    Look, just because you drive a couple of imported luxo Pommy cars doesn't mean you need to act like an Imperialistic Lapdog and be the last bastion of defence for Straya The Beaut's mid-20th-Century ideals
    Quote Originally Posted by nutttr View Post
    People must assume you are some sort of drug dealer with all these nice cars turning up to a fibro home

  8. #8
    Registered User darwinschops's Avatar
    Join Date
    Dec 2010
    Location
    Terra
    Posts
    3,365
    Quote Originally Posted by psi999 View Post
    I use this one:

    http://www.apexgarage.com/tech/gear_ratios.shtml

    Is accurate for my car.
    Cheers was one I was fiddling with but as per below...

    Quote Originally Posted by S View Post
    Also is the wheel size the wheel and tyre diameter or just wheel diameter as your 19" wheels might have a 21-22" overall diameter that might be where you are getting higher figures.
    Yeh, obviously wasn't looking at it close enough, banged the 19" in thinking rim not rim+tyre

    Quote Originally Posted by lukevl View Post
    PM me your email and Iíll send you the one I did for my car
    Will send pm, cheers dude.

    Quote Originally Posted by Rhys View Post
    Ive got a turbo 5.7 6 speed with a 3.46 and its too low. You have to go back to 1st around round abouts, it does 130km/h in 2nd etc. It's a pain.
    Im looking to go to a 4.11 or similar.
    Huh, really, maybe the l98 has a bit more in the bottom end, typically hold mine in second? Good to know though, will run the calcs after work but starting to think the 3.7 might be a good balance.

  9. #9
    Look behind you... Milkman Don's Avatar
    Join Date
    Feb 2013
    Location
    look behind you
    Posts
    2,929
    3.74 if you keen but 3.4s are good for boosted LS unless you want to blow the tires worse than you are already. They donít mind being made to work so taller isnít a bad thing
    Quote Originally Posted by Faux Forg View Post
    I agree with Rdyno

  10. #10
    Registered User darwinschops's Avatar
    Join Date
    Dec 2010
    Location
    Terra
    Posts
    3,365
    Yeh, still torn between the two, 3.45 or 3.7, was almost inclined to look into keeping the 2.92 but some slightly steeper wouldn't go astray.

    Will look up the ratios for the box in my car and have a bit of a play around with the calc above and the one lukevl was nice enough to offer. Main concern is not making first useless and still having the thing turning under 2000rpm at 110-120.

  11. #11
    Opens
    Join Date
    Nov 2001
    Location
    One street away from Guido's den of sin
    Posts
    24,302
    Calculator i posted is bang on. Feed it the right data and happy dayz

    For my car with calculated 4.8% converter slip utilising real track data the apex garage calculator gives a theoretical top speed of 126.35mph.

    My 3rd run last week was 126.22mph. Solid as.

  12. #12
    Registered User darwinschops's Avatar
    Join Date
    Dec 2010
    Location
    Terra
    Posts
    3,365
    Heh, yeh, garbage in garbage out as it turns out!

    And nice one mate, that'd have to be mid 11s then, the bluebird or the new rig?

  13. #13
    Registered User darwinschops's Avatar
    Join Date
    Dec 2010
    Location
    Terra
    Posts
    3,365
    And thanks for the link Jack, will reset long neglected dropbox password and check it out though spoilt for choice now!

    In other news, I have need of your Euro expertise, see euro travel thread and let me know if Oktoberfest, Vienna, Budapest and Prague > Switzerland!

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •