Michael,
If you have the correct differential and tire sizes and the same gearbox you
had before (only with the OD attached), your speedometer will not need
calibration, unless it was not correct before adding the OD gearbox.
Provisions are made in the transmission so that the same speedometer supports
both OD and non-OD installations.
Regards,
Joe
Michael Hargreave Mawson wrote:
>
> Dear All,
>
> Having recently replaced the gearbox and speedo cable in my non O/D
> 1500, I have noticed that the readings I am getting on the speedo are
> lower for a given engine speed than hitherto. For example, at 3000 rpm
> in top gear I used to see 51 mph on the speedo; now I see 46 mph.
>
> I went straight to the Owner's Handbook and whilst it does not give a
> table of figures, it does give the following:
>
> Engine rpm at 10 mph in 4th - 555
> Road speed at 1000 rpm in 4th - 18.0
>
> If I assume that the relationship between road speed and engine speed is
> linear for a given gear (and, ignoring air resistance, I can't think why
> it shouldn't be), that means that the car should give 54 mph at 3000
> rpm, regardless of which of these figures is used for the calculation.
> Of course, it also means that the car should give 108 mph at 6000 rpm,
> and we know that the correct figure is actually only a fraction over 100
> mph, so there are performance losses of about 7.4% at maximum revs, and
> presumably similar or lesser losses at lower revs.
>
> Now, it would be rather handy to know whether I am doing 46 mph, 51 mph
> or 54 mph at 3000 rpm (or rather, to know by what percentage my speedo
> is reading low at a given speed). Does anyone have a formula to
> convert rpm into road speed for a UK-spec 1500?
>
> A corollary to the above question is, "Is there anything I can do to
> calibrate my speedo?"
>
> Any help, as always, will be most gratefully received.
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