Effects, if any?
 

Hey I'm thinking about changing my tire to a diff. profile from 205/60 to 205/55 on a 15 inch rim. What effect if any will this have on my car? Just wondering b/c the tire dealers dont recommend it!

I ran 225/50's on my 91NA. That's as wide as you can go on the stock wheels.
I had to grind a little off the front strut spring perch to clear.
The springs were shortened just 3/4".
That combination gave really good lateral G results.
The only downside to that wide tread was the tread would shift about 1/2" when turning in. Then it would really grip.
http://208.62.166.66:82/guest/pics/Kuhmo 711 rear.JPG

as far as i know there will be no effects, tire dealers dont recommend it because there computers says different. i had to aruge with one of them one time cuz i wanted to put wider tires on my stock rims/stock car and they was say a whole bunch of bs and looking at what the computer was saying what i should have.

Well with my experience I'd say it's not half as bad as most tire places say it is, I am running 225s on my car now. Just don't try and squeeze something too wide on there like a 235, you can do it but your wear will be awkward and you will have ridiculous stress levels on your sidewalls, but as far as going to a lower profile, I dropped all the way to 45s on my new 16s from 55s. On my old GTU I went down from 205/60/15 to a 215/50/15 so you should have no problems

what's more, dealers typically make more money off of bigger tires, so that naturally says to me that of course they want you to buy bigger tires.

edit: er wait, are you talking about going bigger or smaller? you said 60 to 55, so I figured you were talking about going smaller

this guy jus wants to change tire profile from 60 to 55 on a 16in rim. go for it you wont have no efects. ur not really doing nothing but changing the profile (same 205's) u wont even feel a difference

He did say 15", but no biggie. I didn't see him say if he is actually going to a different sized rim, so I figured he isn't, however with a somewhat smaller circumference tire you'll probably get more torque to the ground, it's what's often called a Polish Gear Change, or something to that effect. Also, your speedometer and odometer will be inaccurate, so you'll need to at the very least calibrate your speedo with somebody (radar or comparison between two cars on the road) to find out what percent to compensate by (or now that I think of it, you could just do the math and take the circumference difference between the 60's and the 55's).

we can speculate all day here but here is something YOU could have found with VERY little effort and the SEARCH FEATURE:
https://www.rx7club.com/suspension-wheels-tires-brakes-20/rim-rubber-sizing-website-resources-160617/ (believe it or not there are plenty of resources on this forum so you don't NEED to post things like this)


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

^^^ fancy

thanks. It took me all of 30 seconds ( not including the time I took to make sure everything was formatted right so he couldn't cry about it)

^ thats some good info.....

now somebody figure how his fuel economy will suffer :P

I just went from a 205/60/15 tire to 205/50/15 Falke Azenis. I haven't noticed any difference yet, but shouldn't my gearing now be a little different? Like at 65MPH my RPMs should be higher on the 50 series than on the 60 right?

Thanks for the info everyone, especially pheonix! That post you link to is very nice but it doesnt specify what effects changing your tire size will do, that just says how much percentage it changes! Any way, thank you for the input everyone!

That percentage is EVERYTHING. Your speedo will be off by that percentage. Your odometer will now add miles skewed by that percentage. Your torque will most likely change by or near that percentage. Furthermore, like I said your fuel economy will probably suffer, but perhaps not real noticeably.

That is correct, however your speedo will still read 65mph at the same RPM as it did before, meaning you're doing less than 65mph when the speedo reads 65.

Your speedo will read about 2mph over what you're doing due to the smaller circumference. Other than that your tires width will be the same, you're just changing the sidewall height by a little less than 1/2 an inch and diameter by a little less that an inch. Not a good change, you should go a little wider if possible to keep the profile the same.

check out the OTHER links in the FIRST link I posted (not the miata link) to find other useful sites where you take other factors into consideration

Yeah, sorry I got it backwards. You cover less distance per rotation, so you're actually going slower than what your speedo reads.

speed traps (25mph zone) say I'm going 26 when my speedo says 30 (4.33 rear end and GTUs tranny so YOURS might be different). I went from stock 205/60/16 to lighter 205/55/16 SSRs. I haven't noticed any MPG differences but then again I've never paid attention to pmg as a result of rim size. I'm sure if we talk through it we can figure it out.

Wow, that's a pretty big difference. Down ~13% from 30mph. Don't forget though, it's not rim size, it's the radius from the center of rotation to road contact.