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Your RPM at 40mph in 5th?

Discussion in 'General Land Rover Forum' started by Disco Moneypit, Aug 2, 2009.

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  1. Disco Moneypit

    Disco Moneypit New Member

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    Just a little curious here chaps.

    1996 Disco 300TDI manual, running on 235/70/16 MT's. Drivetrain, engine and turbocharger otherwise standered as far as I am aware.

    On a pretty much flat road, I'm doing about 1100rpm in 5th. Just wondered how this compares to others, as i'm not 100% certain that my rev counter is accurate.


    Cheers.
     
  2. baggyjim

    baggyjim New Member

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    your speedo might be out as well if your tyres have a differant diameter:doh:
     
  3. Disco Moneypit

    Disco Moneypit New Member

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    According to the GPS it is out by 2-3mph below 60, and over-reads by about 5mph above that.
     
  4. Teflon

    Teflon New Member

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    GPS speed cant be relied on; according to GPS with seven sats in site, t'other day, my house was walking around the block...... would have been convenient if I had wanted a pack of fags, but unfortunately, I didn't ACTUALLY see the tobaconists pass my window......

    Relationship between speed and speedo reading is also a straight liniar one, and they rarely go from reading high to reading low, they will either under read or they will over read, but very, very rarely both.......

    Unless you have a Land-rover, in which case they can do both at the same time, curtecy of the un-keyed speedo drive on the gear-box out-put shaft that can slip and grab, making the needle waver around a nominal that rarely has an co-incidence with that which you are actually travelling at......

    235/70's are, I think the 'standard' size for later Disco's and are an 'equivilence' for the older 205R16, so that shouldn't influence things much.

    Rangie doesn't have a fifth gear, as its an auto, and it wont go that slow in top (4th) becouse the Torque concverter wont lock up until 5000rom, which is 55pmh, but IF you do the maths, that works out at just under 1,500rpm in top for 40mph.

    1,100 RPM sounds a little low, but 4th on the Rangie is 1:1 as it would be in the Disco, which has 5th adding 25%, so it's not THAT farr off what might be expected.

    If you check the manuals, either Haynes or the RAVE, will give the reduction ratios in each gear, and if you multiply them out you can work out how fast you should go at any given rpm in any given gear........

    Maths goes like this:

    Engine RPM x Gearbox Reduction = output shaft speed
    Gearbox output shaft speed x Transfer Box Reduction Ratio = Prop-Shaft speed
    Peop Shaft Speed times Differential Reduction Ratio = Wheel Speed
    Wheel Speed times Circumference of wheel = Road Speed
    Where the Circumference of wheel = Pi (3.142) x Combined Diameter of Wheel = Tyre

    In the case of a 235/70R16, the wheel diameter is 16" or 406mm
    Side wall height is 70% of 235, or 235 x 0.70 = 164.5mm
    But there's a side wall at the top and the bottom of the wheel, so you need to multiply that by two, to get 329mm to add to the wheel diameter, 406mm, to get the roling diameter of the wheel, 735mm. Multiply that by Pi, and you get 2.3m per revolution.

    Which is the hard bit done for you; just remember the ratio's in the boks are reduction ratio's, so the number you get out should be smaller than the number you put in.... if not, you need to divide, rather than times, as the ratio will have been quoted 'backwards'

    Eg: Series Diffs have a ratio often quoted as 4.7 , Coiler Diffs quoted as 3.5, but that's short hand for 4.7:1 or 3.5:1, the bottom number of a fraction. If you multiplied the input revs by aone of those ratio's number you'd get out would be bigger, rather than smaller, so what you have to do is divide, rather than multiply.......
     
  5. Beast o Bodmin

    Beast o Bodmin Banned

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    never did like the welsh.....tom jones is also cra
    get a wireless cateye bike computer.This is accurate.1100rpm is bout right tho....
     
  6. Disco Moneypit

    Disco Moneypit New Member

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    WOW and thanks :bow:
     
  7. shinydiscoball

    shinydiscoball New Member

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    Well I tried this with my 200tdi with standard wheels with brand new tyres and I'm getting 40mph in 5th gear at 1600rpm indicated on the tachometer :confused:
     
  8. halo

    halo Active Member

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    me too in a 300tdi with standard road pattern 235/70-16's.
     
  9. hughesy

    hughesy Well-Known Member

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    Aye 1100 rpm sounds a bit iffy. I'm on 265/75's and I'm showing somewhere around 1500 at 40 in top.
     
  10. shadowster

    shadowster Well-Known Member

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    a would say the same
     
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