My recently purchased 1996 Auto Disco (3.9i) indicates 2700 rpm at 120Kph/75Mph. I understand it should read 2500rpm at this speed.

I am located in Saudi Arabia and seem to remember having the same problem on my last Disco (1995 model) after the dealer carried out the 'Gulf Cooling Mod'. This mod was also called the 50 yr mod as it replaced the front grill with a commemorative '50 Year' badged version as well uprating the cooling fan and alternator. After noticing the change in rpm at 120Kph I returned the car to the dealer who fixed the problem. However, the dealer now has no record of what cured the problem on my other vehicle.

Is there anybody who can throw some light on what is causing the rpm/speed mismatch?

Wayne
I have a 1997 300tdi Auto. My Speedometer over reads by 4 mph when compared to my GPS. MPH is calculate by using the formulae distance x time. My GPS is infinitely more accurate with distance than a device that relies on pulses from a RPM counter and a tyre with an assumed circumference.
You could be expected to believe that your RPM indicator is reading true therefore you must consider the indicated speed is not reading true (assuming your formulae is correct). Any deviation in tyre circumference will give a different pulse rate which will give a different Speedo reading. This could explain the wide difference you are experiencing. A dealer could adjust the speedo to match your RPM indicator however there is still the possibility that your Auto is slipping.

'Donboon' thanks for the advice, it did get me thinking and I have just been out to run a few tests. First, I have checked the tyres, which are all new and the standard Michelin 235/70 R16, also, the problem was exactly the same with the old set.

Second, I did a test drive alondside my neighbour, who has a 1997 Disco (3.9i Auto). Both vehicles indicated exactly the same speed, 120Kph, but his rpm reading was 2500 where mine was 2700. At 2500rpm my Disco indicates 112Kph. There is the remote possibility that the speedometer on both vehicles under read by the same amount, but this is highly unlikely. However, I will test this out with a GPS tomorrow.

I should of also mentioned in my earlier posting that the dealer has checked out the automatic gearbox ok.

Is there anything that could of gone amiss when the alternator was replaced with a higher rated part during the cooling modification carried out 6 years ago? Where is the 'sender/sensor' for the rpm gauge? I would appreciate any further advice anyone can provide.

Wayne Jones said:

I would appreciate any further advice anyone can provide.

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stop lookin at yer tacho and have a go of slobs bong. "chill out mate"

parp!! keep looking at yer rev counter you might find that when this golf mod was did they changed the pulley size

Could be a different diff

nice thought , change the diffs while doing a cooling mod..

Maybe its the new badge?