J2K

I've read several threads where people are increasing the cross section from 225 to 245 on the front and 255 to 275 on the rear tires while decreasing the aspect ratio to 35 and 30 respectively. Has anyone gone with a wider tire while keeping the aspect ratio at 40 on the front and 35 on the rear?

I'm concerned about ride quality if I go any smaller on the wall of the tire.

lobuxracer

iTrader: (2)

No one would do that unless they wanted to fundamentally change the car. The sidewall height is the same with the smaller aspect ratios because the tire is wider. Ride quality does not change because of the sidewall height if the overall diameter is the same.

flowrider

iTrader: (9)

In other words, the tire height with a particular aspect ratio is not a constant. It is a percentage of the width of the tire and changes (goes up or down) as the width of the tire changes. Thus the width of a 285 or 295 tire with an aspect ratio of 30 is roughly the same height as a 255 tire with an aspect ratio of 35. If we go look at the specs of the Michelin PS2 tire for instance - the OD, Overall Diameter, of a 255/35/19 tire is 26". The 295/30/19 tire is also 26" or exactly the same. Since the stock rear wheel of an F is 10" wide, the 295 tire will not fit (it's too wide). However a 285/30/19 will fit, and the OD of that tire is 25.8" or only .2" shorter than the stock 255 tire.

BTW, I'm running The Continental DW in sizes 245/35/19 in front and 285/30/19 in back with great results.

Lou

airvtec

iTrader: (1)

Change in ride quality might be minute. But you'll definitely throw off your speedo and mileage a bit.

lobuxracer

iTrader: (2)

And roll center, roll axis, instant center, scrub radius, center of gravity...

I wouldn't do it personally, I value the stock performance too much.

flowrider

iTrader: (9)

No it wont. Not if you pay attention to tire height. My 285/30 is only .2" in shorter than the 255/35 stock size or less than 1%. There is a thread somewhere here that compared our MPH numbers at a constant 2K RPM. I was off about .5MPH from folks here with stock tires.

Lou

lobuxracer

iTrader: (2)

0.5 fast at 60 mph? Calculated, you should see 60.68 mph. It's a 1.18% decrease, so your odometer will show more miles than actual with smaller tires and get worse as the tires wear.

Stock rear is not 10". I wish it were. It's only 9.0 inches which is severely limiting for wider tires.

flowrider

iTrader: (9)

Oops - I meant 9" And my difference in tire height is .77%, as I said less than 1%. We have had this discussion before. The tread thickness is much thicker than the less than 1% difference in tire height. The wear difference will be so insignificant, as to be negligible.

Lou

spekterg35

At my last autocross there was a guy who was running 305/30 on the rear w/ 255/35 on the front. Stock wheels. They looked squeezed on, but he ran strong. No rubbing. The tires were Hankook Ventus V12 Evo K110.

Pics

AAM6906

wow haha.......

mugenh22a4

iTrader: (9)

Call me crazy, but I like the beefy tires look.....Stretched tires to me is like a fat chick that needs to squeeze herself into shoes that are 2 sizes too small.

bnizzle87

i thought about upsizing with the stock wheels, but 305 in the rear might be too much.

lobuxracer

iTrader: (2)

Sorry Lou, but your calcs are wrong. Tire Rack shows the DW turning 811 revolutions per mile and the Bridgestone RE050A for the IS F turning 801. This is a 1.24% difference - even more than the calculated difference using the sizing. So it's more than you think. If resale value is any concern, you're giving away mileage by running a smaller tire. If anything, you'd want slightly larger tires. At least the odometer won't be showing more than actual mileage.

And I did figure out the speedometer/odometer uses only the rear wheels for speed calculations, not the fronts, so front size does not impact speed or distance traveled.

J2K

I definitely don't want to fundamentally change the car. I really just wanted to go with a wider tire while keeping the same OD as my current stock setup. I wasn't aware that the sidewall height varies as the width changes. Thanks for the information. I appreciate it.

Joe Z

iTrader: (10)

Right...

When my Speedo shows me doing 80 mph, I know I'm only going around 77 mph..

I verified two ways..
http://www.miata.net/garage/tirecalcold.html

and

Also opened up the tech menu in the nav & watched the km/mph data from the vehicle speed sensor...

~ Joe Z