AlChemist Posted November 14, 2009 Share Posted November 14, 2009 I think it is fairly well documented that on most modern vehicles the speedometer readings are set around 10% higher than the actual speed. This is certainly the case with my own X250 2.3 based motorhome (when I compare with the TomTom on a straight and level road surface). On the X250 you can amuse yourself by getting a variety of other (useless?) data from the on-board computer system. A couple of questions for those "in the business" or who may have performed their own tests .. * In the old (cable-driven) speedometer days the cable also updated the odometer (mileometer) reading. In which case the recorded mileage would also be some 10% higher than recorded e.g. the 7,700 miles recorded for my vehicle would be nearer a true 7,000 miles? Is this the case now? If so... * My on-board computer also tells me that I have averaged 28.8mpg. Should I be pleased with this - or is it nearer a true 25mpg? * Warranty, servicing intervals (and in my case insurance) are often based on mileage - could I argue that e.g. I don't need a cambelt change until I have covered 123,000 miles (and not the 112,00 as in the service book). Or could I save £40 a year by restricting mileage to 7,000 even though my recorded mileage is 7,700? Tongue firmly in cheek GIGO Al Link to comment Share on other sites More sharing options...
lennyhb Posted November 14, 2009 Share Posted November 14, 2009 On my X250 likewise the speedo reads 10% over, I've complained but Fiat say it's in spec. The law states a speedo's tolerance is -0 +10% for the life of the vehicle, why Fiats are so far out is ridiculous our 2 cars are both better than 2%. The odometer on our X250 appears to be spot on so service intervals will be correct. I checked the mpg readout over 2500 miles & reckon the computer under reads by half mile per gallon e.g. records 28 mpg but I'm getting 28.5. Link to comment Share on other sites More sharing options...
malc d Posted November 14, 2009 Share Posted November 14, 2009 My answer to your question is no. They are only as good as the software they use. The software is only as good as the people who wrote it. :-( Link to comment Share on other sites More sharing options...
Ocsid Posted November 14, 2009 Share Posted November 14, 2009 AlChemist - 2009-11-14 2:38 PM * In the old (cable-driven) speedometer days the cable also updated the odometer (mileometer) reading. In which case the recorded mileage would also be some 10% higher than recorded e.g. the 7,700 miles recorded for my vehicle would be nearer a true 7,000 miles? Is this the case now? If so... * Warranty, servicing intervals (and in my case insurance) are often based on mileage - could I argue that e.g. I don't need a cambelt change until I have covered 123,000 miles (and not the 112,00 as in the service book). Or could I save £40 a year by restricting mileage to 7,000 even though my recorded mileage is 7,700? Tongue firmly in cheek GIGO Al I know its tongue in cheek but your base assumption that the odometer in the good old cable driven days was 10% out is ill founded. Yes the speedo had [and has] not to under read but the odometers were pretty accurate on the right tyres and it certainly does not follow that they ought to over read by your 10%. Comparison between vehicles suggest mine have been within 1%. As for how they now compute mpg, I think I know its based on the summation of what the injector pump is being told to inject as the fuelling part. But the end results seem widely optimistic compared to the good old tank topping technique. Can I use your basis to somehow claim back 10% of my fuel tax!!! Link to comment Share on other sites More sharing options...
Clive Posted November 14, 2009 Share Posted November 14, 2009 the answer to the title question is "We have no option other than to by and large trust them". So much of our daily life is governed by them. Speedometers must by law never indicate slow but are freqently 5% optomistic. To test yours just compare your speedo speed with that on your sat-nav. c. Link to comment Share on other sites More sharing options...
Keith T Posted November 14, 2009 Share Posted November 14, 2009 If not using it to show a route, I tend to use the Satnav for speed accuracy,and find (X250 2.3) it is within about 5% at 60mph. This also tallies with the 'your speed'inidcators as often seen on roadways. For distance, I find that if I follow a planned route on the satbav the actual recorded mileage is pretty accurate. On mpg,I chekc the computer readout, but by using the old 'fill to fill' basis, think the actual mileage obtained is slightly better - in fact up to 5 or 10% than the computer would have you believe. I also find that I get a much improved figure in France, although I am still somewhat dissappointed with the figure against what was heralded in the advertising and selling of the new engine. Link to comment Share on other sites More sharing options...
peterjl Posted November 14, 2009 Share Posted November 14, 2009 Interestingly my current Fiat m/home, my last ford m/home, 3 cars i've tried it on (ford; citroen, renault) have all been between 7 & 10% out BUT hire cars in the states have been almost exactly correct! Peter Link to comment Share on other sites More sharing options...
Guest Tracker Posted November 14, 2009 Share Posted November 14, 2009 On every car that I have ever had the true tank full to tank full mpg has always, without exception, been less than the trip computer's suggested figure. Some have been as much as 10% adrift and Toyota seems the best at less than 5% over optimistic with BMW and Jaguar a little over 5% adrift. Citroen, Peugeot, Skoda, VW, Ford have all been above 5% wrong and some much nearer to 10% adrift - but these were all in the last century! These all related to vehicles partly or fully used for business where detailed and accurate records were required to be kept over prolonged periods for tax relief purposes. Link to comment Share on other sites More sharing options...
lennyhb Posted November 15, 2009 Share Posted November 15, 2009 Keith T - 2009-11-14 8:01 PM I also find that I get a much improved figure in France, although I am still somewhat dissappointed with the figure against what was heralded in the advertising and selling of the new engine. French fuel is no as low sulphur as ours, more like old fashioned diesel so you get better mpg. Link to comment Share on other sites More sharing options...
Guest Tracker Posted November 15, 2009 Share Posted November 15, 2009 Keith T - 2009-11-14 8:01 PM I also find that I get a much improved figure in France, although I am still somewhat dissappointed with the figure against what was heralded in the advertising and selling of the new engine. Maybe it's because there is generally less traffic so less gear changing and more steady speed cruising? Link to comment Share on other sites More sharing options...
starvin marvin Posted November 15, 2009 Share Posted November 15, 2009 I've driven vehicles in the past that have had their speedo's calibrated to be correct. The nearest firm that does this to me is Speedograph in Nottingham. It can be a bit strange at first but brilliant when you get used to it, and its surprising how much faster you can go and still be legal. I'm seriously thinking of getting my replacement van done, its one thing, amongst a few, that I would sooner have than money.....or maybe not on second thoughts. Back to the counting house! Link to comment Share on other sites More sharing options...
Madmaggott Posted November 15, 2009 Share Posted November 15, 2009 Everyone seems to be assuming that the satnav reading is acurate, why should it be any more accurate than any other computer? Link to comment Share on other sites More sharing options...
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