Speedometer Accuracy - how to fix it

[ZenArcher]

Just borrowed a mates Navman GPS and went for a drive out on the highway.

I was surprised at how inaccurate the speedo was according to the GPS. 60kph on the speedo was 57 on the GPS,
85kph indicated on the speedo showed an actual 80kph on the GPS, while an indicated 110kph on the speedo was an actual 104kph on the GPS.

I have a import MX5 with the 180kph speedo on it and have 195/50/15 tyres fitted.

Is this much error a normal issue with the imports? or is it a product of fitting the 195/50/15 tyres?

Would fitting 205/50/15 alleviate some of the issue or perhaps fitting an Aussie dash complete with the Aussie speedo be possible?

How much error is everyone else getting? for those that use a GPS or have done this test?

By comparison , my Holden 04 Astra only has a 2 to 3kph error all the way through on the speedo - reading slightly slower.

[StanTheMan]

I have never done a test. But the size tyre would be the closest to stock in size.
stange that you're under speed at 60km/h the above speed at 80 & down again at 110.

we could all be like that. maybe you could swap tyres with somone who does have a 205 size set of tyres. Then it also depends what actual size they are......I believe the Azenis 615's i have fitted are marked 205....but real size is about 208mm which theoretically would make the sidewall even taller. I also have a 4.1 ratio diff which sould make it even taller again......i think?

[JBT]

Is this much error a normal issue with the imports? or is it a product of fitting the 195/50/15 tyres?

Would fitting 205/50/15 alleviate some of the issue or perhaps fitting an Aussie dash complete with the Aussie speedo be possible?

How much error is everyone else getting? for those that use a GPS or have done this test?

This has been covered before but the search function is dead at the moment.

It's a known issue, at least with the NA models. Mine (Australian market NA8) is out by exactly the same amount as yours using 195/50/15 or the original 185/60/14 setup. It's a flaw (deliberate or otherwise) in that the speedo over reads by about 5%. 205/50/15 will improve it a bit but the speedo will still be out by about 3%. You'd have to go to 205/55/15 to correct it completely but those tyres are expensive and will probably spoil the suspension dynamics.

The answer is to live with it and just know that it over reads by 5%.

116 indicated = 110 actual
105 indicated = 100 actual
95 indicated = 90 actual
84 indicated = 80 actual
63 indicated = 60 actual

[rhysk]

Just about every car in the last 25 years has a speedo that reads slightly under. Why? I guess it is for 'cover your arse' purposes (i.e., the manufacturer can't be blamed for speeding tickets). The speedo in my mates 2005 porky boxster is about 5% low, but the trip meter is dead accurate against a GPS.

Don't play with tyres, as your trip meter will then be out.

[ZenArcher]

Nah,

I'm not the sort of person to put up with something like this.

So....I fixed it.

The internet is a wonderful place for information.

After doing some searching on other various MX5 Forums (after I started this thread) I came aross a fix for this that I tried and it works awesome

Check out this link
http://mymiata.paladinmicro.com/MiataOt ... rewallText

Have a look at picture No25 and click on the link.

I followed the basic instructions.

Basically took out the entire main guage console which took all of 5 minutes and then removed the front clear plastic face - easily done as it is only held in by clips. Just be patient and be gentle and it will come off very easy without damage.

Then I carefully pulled the speedo needle off and undid the screws that held on the speedo face and a clear plastic bracket behind the face. I then moved the coil bracket (take a look at the link and you will see what I mean) anti clockwise and put the guage face and needle back on and went for a drive with the GPS again. I didn't need to move it very much, maybe 1 or 1.5mm from the original position in the end.

One thing before you do this, wash your hands to get excess skin oil off your fingers and hands otherwise the guage face will have skin oil smudges all over them. If you get smudges though, they easily wash off with some normal hand wash and water - I used dettol hand wash.

It took a few tries but this works a treat. First time I moved it too much and the wrong way and ended up with reading 10kph faster than my real speed. But then turned it anti-clockwise and after a few more attempts I got it right. You just need to get the speedo needle in the right place again so that at rest it is on the 0kph marker.

My speedo now has the following readings

Speedo 60kph - GPS 61kph
Speedo 80kph - GPS 80kph
Speedo 100kph - GPS 98KPH
Speedo 110kph - GPS 108KPH

This I can happily live with. Though, if I had a little more time, I would have mucked around a bit more to close the readings by another 1kph more. That would have put 60KPH at a real 62 etc but I would be happy with that as it would have closed up the 100 and 110 readings by that little bit more. But I am just picky and as my wife says - at times too much of a perfectionist.

All up it only took me an hour to get it right and that included about 4 adjustments and pulling over to the side of the road to do it. I just re-installed the guage without the coverings etc and went for a drive. This made it easy to just pull over and make adjustments.

Try it out. If you can borrow a GPS or have one, this adjustment really does work and doesn't affect the odometer at all, only the speed reading and will only take you an hour or so to get it right. Just make small adjustments to the coil bracket to start with.

Hope this helps anyone who wants to make their speedo more accurate.

[rhysk]

Cool, a happy ending