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My speedo is a liar

Started by adamb, 03 July 2006, 10:19 AM


Coming back home from C to L on Friday night at 1am the motorway was refreshingly empty. I took the opportunity to use the 6.9 for its intended purpose (sorry Mr Policeman) by attempting to get back home in the shortest time possible. Over a period of 30-40 miles the speedo never dropped below 110mph and where visibility permitted I was up to 140. Exhilarating!

However, I don't really believe that I was going that speed. At 140 the tachometer was only showing 4500 rpm and I would expect it to be at least at 5000rpm. Does anybody know what is the true speed on proper tyres per 1000 rpm in top gear? This would allow me to work out my true top speed. My 6.9 wears 205/70/14 which are 2% out from the 215/70/14 which it should be on.

Given how the car was still accelerating at 140 I would not be surprised if I was able to push it another 20 mph on my faulty speedo until I reached the true 140-145 mph maximum.

Also, is there anyway to recalibrate the speedo? I know that it's possible to slow down the mile counter by fitting cogs from a 280 etc but I have no reason for doing that, plus it's illegal.

---zoom 6.9--->


Though I completely understand if a golden opportunity arises to put the pedal to the metal,you should not travel at 140mph if you do not have Z-rated tires..point,blank,period!

aussie 6.9

Acording to my MB service manual  gearing is 44km per 1000 rpm with 2.65 rear axle ratio.Max engine revs vary between versions for different countries. As well as the Euro and US specs I believe there was a third version that went to Australia and Japan. It was between the other two in power and performance .Regards Aussie 6.9.


Quote from: Nutz on 03 July 2006, 01:27 PM should not travel at 140mph if you do not have Z-rated tires..point,blank,period!

Not quite. Z-rated tyres are unnecessary for travelling up to 140 mph. My car wears H rated travel which permits persistent travel up to 130mph. I take your point about having wrong tyres for high speed driving. More importantly replace old or excessively worn tyres. I once had a blow out at 90 mph which was a bit scary. Nowadays my 6.9 wears new Goodyear Eagle NCT3s which are H-rated. (see