My car is 91 import 1.6 V-Spec, but the instruments have been replaced for st1:country-regionst1:placeUK</st1:place></st1:country-region> ones so it should all read in MPH.These were changed by the previous owner who said they were very accurate.Well whilst driving today and using my Sat Nav (TomTom) I noticed that my Sat Nav was reading a higher mph than my speedo (in the Peugeot and Subaru we have the Sat Nav usually reads about 2mph slow e.g 70mph speedo = 68 Sat Nav).
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My Speedo readings are:
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Speedo Sat Nav
20mph.24mph.
30 “33“
40“43“
50“52“
60“62“
70“72“
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The wheels on my car are 15” and the tyres are 195/50/15, if you check out the tyre size from the original fitment to the ones fitted to my car it shouldn’t really make that much of a difference. http://www.miata.net/garage/tirecalc.html<o:p></o:p>
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I’ve always understood that the Sat Nav is not 100% accurate and is usually running slow, so if this is the case then my speedo readings are way out.Is their more than one st1:country-regionst1:placeUK</st1:place></st1:country-region> speedo, could I have the instruments from a car with a different final drive, if so how can I tell?
Hi Sucram, for the record My sat nav (Tom Tom) in both my Vectra and 5 always reads 2-3 MPH slower on the sat nav than on the speedo.
SO it is as you say a little slower on the Sat Nav but it is the same in both cars.
I agree that 15" wheels shouldnt make a difference - For the record I am runnig the standard 14"s on 186/65/14
I have always accepted that a mile or 2 here or there between the Sat Nav and the speedo isnt really a bother. (be careful at the speed cameras though [A])
I would suggest that your readings are pretty good. I have checked both my MK1 and MK2.5 and both are way out when compared with Tomtom speeds. In fact on the MK1 if the speedo reads justy under 80mph thats 70 on the tomtom !!! The other 5 is not a whole lot better certianly more than 2mph under reading. On the other hand my Audi is about 1-2mph below tomtom speed. By law a speedo must not under read… and as is evident in my experience they all give you a few mph in hand at the speed cameras if you are on the limit.
Of course lots can effect speedo and tomtom readings, tyre wear must have a small effext on a speedo and hills chuck out a satnav.
As long as you’re travelling in a pretty straight line, and more or less level, the speed reading on a satnav is going to be pretty accurate.
Usually, a speedo will read a little more (a few percent) than your actual road speed - set that way to make sure the manufacturer won’t be troubled by any threat of legal bother should you be prosecuted for exceeding the speed limit and attempt to excuse it on the grounds that “I might’ve been doing 75 but my speedo was only reading 70, officer”. Yours, on the other hand, is reading BELOW the road speed - which is most unusual. Also, going by your figures the difference decreases as your speed increases, which is exactly the opposite of what it should do (i.e. the greater the speed, the greater should be the error).
I wonder if the previous owner replaced the speedo dial rather than the whole speedo. If this is the case then it’s quite likely that the speedo needle was taken off and refitted, quite probably slightly inaccurately. This would account not only fot the error but also for the fact that the error decreases as the speed goes up.
Thank you for the many replies its very appreciated. Can someone do me a favour and take a picture of a dash showing the speedo so I can see if the needle on mine is at the same place at rest. Thinking about it I’m sure the needle on the speedo didn’t start indicating any speed until my Sat Nav reached about 5 to 8 mph?!!
A UK MX5 Speedo is calibrated 0-140 mph. A replacement MPH speedo face for an import is calibrated 0-110 or 0-120 mph, depending on the source (Roadster speedo in 0-180 kph). The accuracy of these faces depends on which cheapo printer was used to run them off with.
As far as I know, the instruments from Mk1s are all interchangeable, with the possible exception of the temperature guage (1.8 resistors are a bit different from 1.6, but I suppose it might still work).
I reckon the theory posted above, about the pointer having been refitted badly, is the most likely explanation - it would explain the under-reading, the error decreasing as the speed increases, and the lack of any movement of the speedo pointer until almost 10mph on the satnav.
I’ve now got the original instrument cluster back that was originaly fitted in my car and it looks like the original speedo has been removed and fitted into the UK spec instrument cluster so using the MPH face of the speedo only. So maybe the needle has not been refitted correctly after all. I could do with seeing another to compare where abouts the needle rests.
Here you are, hope this helps. I had to use a flash as it’s a bit dark in my garage.
Yours is in a different position to mine, that’s for sure. I noticed that you had the engine running when you took your photo, can’t imagine that this would effect it though.
Hi Marcus, I have just zoomed right in on the picture in photoshop and there is no pin stop on the speedometer pointer. So I double checked on the speedo in the car too. There is one on the Rev counter though. I wonder if mine or yours may have not had the dials positioned correctly when they were changed over to read MPH from KPH. Here is a picture to show you the pin stop.
Yes my speedo is the same with no pin stop, but has one on the rev counter. I’m going to reposition the needle on the speedo so at least it will read a little faster than what I’m actually doing.
Quote SUCRAM " it looks like the original speedo has been removed and fitted into the UK spec instrument cluster so using the MPH face of the speedo only"
I didn’t think that would work, isn’t that why the replacement speedo face for the Eunos speedo only goes to 120mph? Just a thought
