Is the speedo still accurate for 15 in wheels?

[jamesdav]

My recently acquired 1990 MX5 has 15 in wheels. I believe the original wheels were 14 in. Does this mean the speedo now over-reads by a factor of 15/14 (about 7%)?

[Angreal]

My recently acquired 1990 MX5 has 15 in wheels. I believe the original wheels were 14 in. Does this mean the speedo now over-reads by a factor of 15/14 (about 7%)?

If they are 195/50/15 then they should read approximately the same.

[WideOpenThrottle]

Check the calculator here

http://www.miata.net/garage/tirecalc.html

[Mr. Shine]

To explain more fully what others have already said, as long as the tyre profile has been changed appropriately (so from 185/60R14 tyres to 195/50R15 tyres, for example) then the speedometer will read correctly (which is to say, out by only something like 0.3%)

[RMS]

Short answer yes, long answer - with my 195/50-R15's my speedo matches the speed on my gps perfectly while with the stock 14" size it reads a little low.
I trust that the GPS is more accurate so consider that an improvement.

[MrGrey]

I have a similar issue and I thought I would pop it in this thread instead of starting a new one.

My car currently has 185/60/14's on it at the moment and the speedo reads low. to test this I drove past the speed camera sign and found that when I am doing 50kmph my speedo is reading 58kmph.
Now I have to get new tires for my warrant next month so I was wondering a few things.

1) would getting 195/60/14's instead of 185's help rectify the problem or will it make it worse?

2) as far as I can tell, 185/60/14's are the stock size, so why would the speedo be reading differently?

thanks in advance, I tried using the calculator but just made myself a bit more confused.

[RMS]

getting 195's will make the problem worse.

the 195/60 tyre will have a slightly larger diameter than the 185/60 tyre so the circumference of the tyre will also be greater.

Your speedo measures the number of times that your wheels turn and effectively multiplies it by the circumference to give you your speed. So larger circumference = more speed for the same reading on your speedo.

I'm not sure why your speedo would be out by so much because that would be approximately 16% under which is quite a lot. One possibility is that the diff ma have been changed to a different ratio diff without changing the speedo drive gears? If the speedo drive was for a 4.3:1 diff and you have a 3.9:1 diff for instance then the speedo would read low, but 16% still seems really high. Could just be an inaccurate speedo or maybe you're not reading it right?

From what I've read it seems that it would be more common to have the opposite problem - the car is travelling slower than the speedo indicates, which as certainly true in my case.

[warrior]

the width of the tyre will make very little to no diffrence, MOST cars speedos are diffrent from there real speed.

P.S dont trust the road side speed camera things, borrow someones satnav and check it that way.

[siren676]

Those signs are hardly ever accurate, use a gps, or borrow one and check your speed that way.
By going up to a 195/60-14 you will be going around 2% quicker
e.g in your scenario you would be going 51kph while the speedo shows 58kph

[HK_Fooey]

I have a similar issue and I thought I would pop it in this thread instead of starting a new one.

My car currently has 185/60/14's on it at the moment and the speedo reads low. to test this I drove past the speed camera sign and found that when I am doing 50kmph my speedo is reading 58kmph.
Now I have to get new tires for my warrant next month so I was wondering a few things.

1) would getting 195/60/14's instead of 185's help rectify the problem or will it make it worse?

2) as far as I can tell, 185/60/14's are the stock size, so why would the speedo be reading differently?

thanks in advance, I tried using the calculator but just made myself a bit more confused.

16% over reading sounds like a lot, have you checked the speedo against a GPS?

1) It will help, but only very slightly. Difference is about 2%, so 1kmh at 50kmh.

2) Altered diff ratio, worn speedo internals, Mazda build quality

[MrGrey]

thanks for the info siren at least that's a little better than nothing i guess
next year I plan on getting some 15's anyhow so I guess 195/60/15's or 205/50/15's would be in order

-=edit=- I have previously borrowed a gps and had the same results

[siren676]

195/60-15's??? You building a monster truck?

Its wierd for a speedo to over read unless its got too few teeth on the speedo cable

[MrGrey]

would you suggest a replacement speedo, speedo cable or replace both? If I was to replace the speedo, due to the odometer, would I have to get it done by a mazda dealer?

and no, I am not trying to make my car look like this

[HK_Fooey]

195 60 15 will probably rub everywhere...and look terrible. 195 55 15 would be the go, although they will still only get you 3% 'correction'

By worn speedo I meant the unit in the gauge cluster, lower resistance in the spring thingy.

When you checked it against a GPS was it the same % out at 100kmh? Or does it appear to be a consistent kmh out? Maybe your needle has jumped or something weird?!

[MrGrey]

it seems to be a % based discrepancy, using a GPS when doing 100kmph the gauge was reading about 115kmph

[chris]

MAKE IT LIKE THAT! DO ITTTT

[Angreal]

MAKE IT LIKE THAT! DO ITTTT

The back of my car looks like that at 355mm. Need to drop it another 30

[authentic]

By now the NA's analog speedo must be reading slightly off, wouldn't it? I wouldn't get too worried about the accuracy. Its easy enough to figure out with that tool now anyway.

I just calculated the speedo reading for the 17's I used to have on there and at 100km on the speedo I would have been doing 103.2kmh. Add to that the fact that I usually sit at about 5kmh over thinking that the cops wouldn't mind too much and I would have been travelling at 108.36kmh, which is faster than I what I would be comfortable at . I can recall driving past police cars at the 105 reading. Haven't been pulled up once. They must be pretty laid back down here.