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Road speed at Tach readings 700 1991

posted by  ghas on  Thu Oct 22 09:46 CST 2009
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last visit: Wed Oct 15 12:27 CST 2014
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I would like to temporarily use the tach on my 1991 740, automatic, non-turbo to measure mph while driving in overdrive. Does anyone have an engine speed comparison for example at 40, 50, 60, 70 mph.








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    Road speed at Tach readings 700 1991

    posted by  289 on  Fri Oct 23 00:02 CST 2009
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    last visit: Mon Feb 10 14:26 CST 2014
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    My '92 740 wagon with an aw70L does 2500 rpm at 60, 2000 rpm at 50, and about 1700 rpm at 40. 185 70 15 tires

    Hope this helps, but various final drive/trans/tire size combos will make a difference.








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    Road speed at Tach readings 700 1991

    posted by  Erling on  Thu Oct 22 19:33 CST 2009
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    I strongly support Aleekat's advice regarding using a GPS. A standard, handheld GPS will typically have 0.1 mph accuracy on flat ground. Other drivers' experiences with their cars will vary with tire circumference, speedometer accuracy etc. Be aware that circumference may vary with +- 5 pct from manufacturer to manufacturer for the same dimension tire.

    Erling.
    --







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    Road speed at Tach readings 700 1991

    posted by  Smitty on  Thu Oct 22 19:32 CST 2009
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    If you tell me what your trans, final drive and tire size are, I can give you theoretical rpm and road speed numbers. (Theoretical is usually within 1-2% of actual.)
    --
    In God We Trust. Everyone else, keep your hands where I can see them.








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      Road speed at Tach readings 700 1991

      posted by  ghas on  Fri Oct 23 06:09 CST 2009
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      last visit: Wed Oct 15 12:27 CST 2014
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      The trans is a AW70L. Tires 185/65x15. Final drive 0.69:1 and I think the rear axle is 4.10:1








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        Road speed at Tach readings 700 1991

        posted by  Smitty on  Fri Oct 23 09:43 CST 2009
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        last visit: Sat Aug 9 22:44 CST 2014
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        Using an average value of 849 rev/mile for a 185/65-15 tire, 4.10 final drive and 0.69 OD, here are engine speeds for 3rd and OD:

        40 mph 2257 1557

        50 mph 3385 2335

        60 mph 3482 2402

        70 mph 4062 2802


        Now for the disclaimers! From my quick look at various tire mfr's data, revs/mile for the same size can vary by +/- 2%; wheel width and inflation pressure (which varies with tire temp) affect that number. Speedo error of 1-3% is also fairly common.

        If you guessed wrong and your final drive is really a 3.73, just correct the above number by (373/410).

        If you get substantially different numbers, post back and we can sort out the data.
        --
        In God We Trust. Everyone else, keep your hands where I can see them.








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          Road speed at Tach readings 700 1991

          posted by  ghas on  Fri Oct 23 11:06 CST 2009
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          last visit: Wed Oct 15 12:27 CST 2014
          [RELATED]

          Thanks so much for all your help. My daughter will be happy that she can drive her Volvo safely without a working speedometer. How many 28 year olds are in love with an 18 year old car?







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