I just got myself a small upgrade for my 2nd Gen, I threw away the 14" hubbies and replaced them with 15" TH Advance Alloys..

Anyway, my problem is.. What tyre width should i be using to have no change in speedo reading? I got myself a set of 205/50/15's because i assumed--- I increased the wheel rim size, so to compensate i should decrease the tyre width??

I took em to TTF and they said they cant do it because its not the correct load rating :confused: And that 65's are the only ones they can put on.

The tyres that are on the new wheels are currently 65's..

Are TTF talkin sh*t or are they right? What tyres should i be putting on? And if i use the 50's is my speedo going to be off by alot??

ta

To retain correct rolling diameter you need to run 205/65/R15 in at least a 92 load rating. Running 205/50/R15 is going to put your speedo WAY out, and such a change is unroadworthy.

I think you don't understand the figures you are working with because when you increase wheel diameter you decrease tyre profile, not the tyre width.

205 is the width in millimetres, 50 is the tyre sidewall height as a percentage of the tyre width, and R15 is the wheel diameter. Wheel diameter + 2(sidewall height) is your rolling diameter and the figure that matters, when changing wheels and tyres you can't change this figure by more than 15mm or the car becomes unroadworthy. Load rating is how much weight the tyre can hold safely, you need at least 92 for your car.

I just got myself a small upgrade for my 2nd Gen, I threw away the 14" hubbies and replaced them with 15" TH Advance Alloys..

Anyway, my problem is.. What tyre width should i be using to have no change in speedo reading? I got myself a set of 205/50/15's because i assumed--- I increased the wheel rim size, so to compensate i should decrease the tyre width??

I took em to TTF and they said they cant do it because its not the correct load rating :confused: And that 65's are the only ones they can put on.

The tyres that are on the new wheels are currently 65's..

Are TTF talkin sh*t or are they right? What tyres should i be putting on? And if i use the 50's is my speedo going to be off by alot??

taGenerally one you increase the rim size (diameter) you increase the tyre width and decrease
the tyre profile to match the overall diameter of the old setup.
I presume your stock ones are 185 75 R14? If so, then going to 15" wheel, you'll need 205 65
R15 which means the new overall diameter is 2.3% bigger which means you'll be doing 61.4
km/hr when the speedo says 60km/hr. You can go 195 65 R15 which is only 0.2% bigger than
stock meaning that you'll be doing 60.1km/hr when the speedo says 60km/hr.

Check out http://www.miata.net/garage/tirecalc.html for more info. The minimum tyre load
rating for you car is 92 so actually, 195 65 R15 is also bad as it has a rating of 91. You may
find a tyre brand that has a 195 65 R15 with a load rating of 92 or a 205 60 R15 with a load
rating of 92 (they also are normally 91 rated).

What about a 215/60x15, alot closer!

I think you don't understand the figures you are working with because when you increase wheel diameter you decrease tyre profile, not the tyre width.



When i said tyre width, i meant tyre profile, My bad there.

I get what your all saying.

And yea i originally had 185/70/14.. I used a tyre calculator and put in that figure along with 205/50/15 and it came up with a -4.7% difference.
Does that mean ill be going 4.7% faster than the speedo says?

Looks like im gonna have to get different tyres...

i just compared 185/75-14 tyres against 205/65-15 on the tyre calc (original magna sizes). There is a 2.3% difference (i.e speedo under reads 2.3%) so now i'm wondering do magnas that were fitted with 15 inch wheels as standard have different speedo drive gears to magnas that were fitted with 14 inch wheels or did Mistubishi consider this difference "within tollerence"?

i just compared 185/75-14 tyres against 205/65-15 on the tyre calc (original magna sizes). There is a 2.3% difference (i.e speedo under reads 2.3%) so now i'm wondering do magnas that were fitted with 15 inch wheels as standard have different speedo drive gears to magnas that were fitted with 14 inch wheels or did Mistubishi consider this difference "within tollerence"?
I think 5-10% is typically considered to be within tolerance, so I'd expect them all to have the same speedo drive gears.

Cheers,
Martin.

i think the magnas that came with 14's have a speedo to 200kph where as the ones that came with 15's have a speedo to 220