correct ride height for wheel change

[Shuie]

Ive currently got a set of 16x7/8 wheels with 205/55 and 225/50 tires on them. I think the ride height of the car is close to where its supposed to be with these wheels and tires.

If I want to swap between the 16s and a 15x7/8 set of wheels, what profile tires do I need to use on the 15s to avoid screwing up my ride height?

Is it a common practice to swap between wheel sizes like this for street/track/whatever, or are the smaller wheels going to make the car look like a dune buggy if it is already setup for 16s?.

Any help is appreciated

[jmohn]

Shuie,

You'd need to go to something similar to a 195/65x15 to keep the same over-all tire/wheel diameter. Here's a site that has a handy tire size calculator http://www.miata.net/garage/tirecalc.html that allows you to try several sizes to find the optimum. (Yeah, I know it's a Miata site, but it's a great calculator, and Miatas are sportscars.)

Jerry M
'78 SC

[Russ in VA]

I beleive the 195/65/15 fronts would be appropriately paired with 205/60/15's rear ..... but I'd go with 225/50/15 all the way around .... or you could pair 225/50's rear with 205/55/15's on the front if you prefer to maintain some understeer/saftey-net. The smaller diameter will give you better track gearing (at a 3.3% change it's probably inconsequential), will lower the car less than 1/2 inch (lower CG could actually be benifitial) and most importantly the tire will be more responsive (shorter sidewall) and have more grip (wider contact patch). Of course reducing your unsprung rolling weight offers significant increases in all aspects of the car's performance envelope. Your fender gap will also be less than 1/2 inch (.413 in. in the rear, with a similar change in the front) larger, but it will still look OK and performance is what matters for track/play-time wheels/tires.

To answer your question, I'd say its a common practice to make a swap down in wheel diameter before hitting the track. Lots of people have pretty boy wheels for the street and smaller (and more importantly LIGHTER) wheels for track days where it is performance that matters.

Have fun!

[Russ in VA]

By the way, though the calculator on the miata page works great, if you want to build your own calculater on a spreadsheet OR if you just want to play with this on a calculator whenever a different thought strikes you (I can't be the only one that does this?) you can figure it out as follows

using a 205/55/15 for example:

tread width in MM (205)
times aspect ratio as a decimal (.55 for 55%)
divided by 25.4 (to convert mm to inches)
times 2 (accounts for both the top and bottom sidewall)
+ diameter (15)
yeilds total tire height

((205 x .55) / 25.4) x 2 + 15 = 23.878 inches

Height is proportional to circumference and inversly proportional to number of revs per mile so you don't really need to go there, you can just compare the diameter numbers of the sizes in question to get a percentage change for gearing and speedo error