Speedo accuracy and 17" wheels

[Anonymous]

Changing to 17" wheels, and reading all the stuff about tyre sizes with relish. I have checked out the tyre size calculator as suggested, ( m http://www.miata.net/garage/tirecalc.html m ), but as I already find my Jspec speedo is quite optimistic this might be a good way to put matters right.
On the face of it replacing rears 205/50x15 with 225/40x17 will make my speedo slower - i.e. better by 4.4%, as according to my GPS speed on OriginB2 that would be about right. 225/35x17 will be about the same as now.
But if the front drives the speedo have to muck about with front sizes!
Anybody happen to know from which end (front? back?) the speedo gets it's reading from?

[mph]

I get consistently different speed readings - speedo vs. GPS - when changing tyres on 15s, let alone changing spec with 17s!

Though I can't quite recall which way was what, I think my A539s made the speedo read high just as an example. Given you've got GPS device, I suggest not worrying too much, do some tests (eg. 30/50/70mph on speedo equals X/X/X on GPS) and just mentally adjust the speedo from there on.

[Anonymous]

Mentally adjust?
Well I've been doing that naturally ever since I got the car in October last year, having fitted the GPS device the day I got it I found out how bad the speedo reading is. Car being a Jap import, you rely on the converter thingy they fit on the speedo to be accurate enough. I have tried to find an adjustable converter, without success.
I rely more on the GPS device than the speedo which is one of the most inaccurate of any I've ever had, it is 7mph fast at an indicated 110. I think only the Saxo VTS was more optimistic (and on stock tyres).
Don't know if the UK version speedo is any better?
The idea of upping the tyre diameter a bit, as I began in this thread, would go towards correcting that.
Perhaps the normal petrol heads who discuss everything to such detail ad nauseum are away on holiday.
My question was where does the drive for the speedo come from, front or back wheels?

I'm just going to do it.

[SteveJ]

Perhaps the normal petrol heads who discuss everything to such detail ad nauseum are away on holiday.

Comments like that really aren't going to endear you to the people you are seeking advice and help from  s:evil: :evil: s:evil:

My question was where does the drive for the speedo come from, front or back wheels?

REAR

The idea of upping the tyre diameter a bit, as I began in this thread, would go towards correcting that.

This wont work either - you may or may have not noticed - the error is not a fixed percentage across the range - the speedo is far from linear, and only gives an approximation of the speed at any given moment (always well within the 10% accuracy required and always high unless you mess with the tyre sizes)

[Anonymous]

Cheers. I'll see how it goes then.
I know speedos are never more than a compromise. But these days with cameras at every change of speed limit it pays to know how fast you are actually travelling at all times. I find the readout for speed on my Origin B2 a little small to read and I have glasses for distance, which doesn't help much, so the speedo is more visible.
Try the A34 between Stoke on Trent and Stone, for instance, about 10 cameras over 15 miles, speed limits changing all the time between 30, 40, 50, 30, 50, 60 etc., I've got 6 points already, and you'll know where I'm coming from.
Years ago I  went through many weeks of timing and measurement over a measured 1/2 mile outside Hull, and then made up my own custom speedo face to reflect where every 10mph should be, for an RS1600 Escort.
A good sense of humour is needed for visitors to this site, I think.

[Anonymous]

I have tried to find an adjustable converter, without success.

I doubt if such a thing exists, because the ratio of km to miles isn't variable   s:wink: :wink: s:wink:

[GSB]

So, at an indicated 110mph (no wonder you've got 6 points!  s:wink: :wink: s:wink:  ) You're actually doing 103mph according to the GPS decoder on the Origin Blue thing...

Assuming that the origin blue-eyed thing is 100 accurate (which it isnt - trust me), then you have an error of +6.3%

Assuming that the GPS thing reads in whole mph and not to 1 decimal place, then the error could be as much as +7.3%, or indeed as little as +5.3%. For an indicator which is subject to a large amount of parralax error, and has considerable variation in the quality of its input signal (Tyre wear and pressure can have a huge impact on the reading), this would appear to be achieving a very good accuracy.

Just to put this in perspective. The current (and long standing) guidlines issued to traffic officers etc. with regards to speeding are basically anything thats moving at 10% plus 2mph over the posted limit is a nick. So you have to be;

16.6% over a 30 limit
15% over a 40 limit
14% over a 50 limit
13.3% over a 60 limit
12.9% over a 70 limit

Personally I'd stop worrying about it and watch the road instead, because regardless of weather you get caught at 103mph or 110mph, you're still fucked...

[Anonymous]

Grant

Which Moderator missed the last word in your post? I've always assumed that the system threw out naughty words over the forum.
Who moderates the Moderators? LOL LOL LOL

Regards
  s:D s:D    s:D s:D    s:D s:D

[markiii]

erm the auto censor feature shoudl, hang on a mo.

[Anonymous]

Not very good pics, from my Sony Ericcson camera phone, but will get some better at weekend and replace this.
The speedometer is spectacularly accurate now - it agrees completely now with my GPS at all speeds 30, 40, 50, 60, 70, 80, 90mph.
Couldn't be sure because not enough time to check thoroughly last night but at indicated 100 the GPS may have said 99mph.
To be sure, I don't care about that little inaccuracy.
I drive at at cruise speed of 85 and have set a bleeper to come on by the Origin B2 at that speed. I am now about 250rpm down on previous at that speed, so should see some change to my fuel economy.
Wheels fit ok and no signs of rubbing anywhere.
The tyres are Falken - rears 215/40x17, fronts 205/40x17.

Glad for a number of reasons I have done this.
I like the black chrome effect, it is really different, and the car looks hugely better.
Only thing is the ride is a bit hard compared to the Goodyear Eagles on 15".
Got my tyre pressures set at 2bar, dealer said. I don't relate to bar, only psi. Have to sort out what exactly pressures I have got in psi, and exactly which pressures to use, whether softer is better, etc.
Wheels from Auto Image on Gt Western Road, Glasgow, £695 all up including msw locking wheelnuts

[scottee]

gotta love those wheels