I've been looking at tire options for our '08 RDX with stock 18" wheels and have been fairly frustrated.

It doesn't seem that we have any high performance summer, all season or aggressive tread in this size (28.2" diameter).

I played around and these were the other sizes with approximately the same diameter:

225/60-18 28.6"
235/55-18 28.2" <<< OEM
245/55-18 28.6"
255/50-18 28.0"
265/50-18 28.4"

No luck in those sizes as well. I noticed a few people report going to slightly different diameters, such as:

255/45-18 27.0"

This provides a lot more options. For example, in the ultra high performance all-season category, I see:

- Goodyear Eagale F1 All Season
- Michelin Pilot Sport A/S Plus
- Yokohama ADVAN S4

Can anyone comment on the consequence of switching to a tire with a slightly different diameter? Is an inch too much change? Will it confuse the car computer?

Also, I'd be interested in what alternate sizes people have used and what their experience has been.

Thanks,

-john

 

Oh, as a follow on, does anyone have a source for *smaller* wheels?

I'd actually be interested in running an A/T tire in the winter, but in that case, smaller is better.

For example, the Bridgestone Revo A/T is available in 235/65-17 (28.5").

This would be pretty optimal if I could find a 17" wheel, but places like tirerack.com only seem to list 18" and larger options.

-john


Edit:

Does anyone know if the car computer can be adjusted to accommodate different sizes? Our Jeep could be programmed for a range of sizes.

 

Try 245/50/18's, which 1.9% less in circumference (minor IMO).

 

Thanks.

That looks like 27.6" diam.

But note that Tirerack.com only shows a *single* option in the ultra high performance all-season, high performance all-season, and performance all-season categories.

-john

 

I didn't see any options at tirerack.com and wrote them an email. It turns out they *do* have 17" options, but you have to look at the winter category to see them.

From the front page:

"The Upgrade Garage"
Winter --> {select car} --> "View winter / snow tires for your {car} --> "Build Your Own Package - 17" Packages" --> "Don't need tires? Skip to wheels".

They have some 17x7, 17x7.5 and one 17x8 selection.

Weights vary, but it looks like some workable options.

-john

 

Check out this web site- it might help with this issue. I used it to finalize the snow tire size to buy using 17" wheels.

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

 

Thanks. I have been using the miata.net tirecalc to calculate the diameters of the various options.

-john

 

Well, what you will have is a discrepancy between the miles that the car has traveled Vs what's in the odometer. Also, speed will be different and therefore all the calculations (MPG, DTE, etc) will be inaccurate....By how much? I don't really know.....gas mileage decreases with a bigger tire, but with a smaller one you will be driving at higher RPMs to keep up with traffic (the speedo will show a higher speed too)

Don't know if I confused you more...

 

Update: I put a query into the service department of my dealer, and they indicate they researched the question and found the tire size cannot be adjusted.

So I guess the question comes back to; how bad is it if the size is off?

In looking around, it looks like to get real good all season tires, the diameter is going to be off between .5" and 1".

OEM = 28.2"

Michelin Pilot Sport A/S = 27.9" (245/40ZR20) off by .3"
Goodyear Eagle F1 All Season = 27.7" (245/45R19) off by .5"
Michelin Pilot Sport A/S = 27.2" (235/50ZR18) off by 1"
Bridgestone Potenza RE960AS Pole Position = 27" (255/35ZR20) off by 1.2"

My feeling is the top two would probably be reasonable. I'm kinda thinking anything nearing an inch off seems a bit excessive.

It does make some of the truck options look interesting tho.

Bridgestone Dueler A/T Revo = 28.5" (235/65R17) off by .3"
Pirelli Scorpion ATR = 28.5" (225/65R17) off by .3"

On various vehicles, I've run the Pilot Sport A/S, summer versions of the Bridgestone, the Goodyear F1 A/Ss, the Pirelli Scorpion A/T (slightly more agressive than the ATR) and the Revo A/Ts and they are all great tires IMO.

I like the Goodyear F1 A/S slightly better than the others in that category (the Pilots are lighter tho, but susceptible to wear issues), and the Revo A/Ts better than the Scorpion A/T, although it sounds like the ATR might do about as well, but have better cornering attributes.

YMMV.

-john


Note1: All sizes taken from the "specs" page on tirerack.com. Basically I picked the closest diameter(s) available in these tires that could fit on the RDX.
Note2: Passenger car A/Ss driven on Infiniti G35C 6speed, "truck" tires on Jeep GC (WJ) with Quadradrive.

 

dude, here are my two cents, don't get smaller, if anything get them bigger. If I were you I would stick with the stock, but I understand that there are not that many options available. As for the difference, you may get a speeding ticket for going faster than what your speedo tells you, but you may not......I just bought a set of Yoko's and they work just fine....not sure what exactly you are looking for...

 

Those are the maximum sizes available in the passenger tires. Nothing bigger available.

The truck tires do have larger sizes however.

-john

 

I'm curious to your reasoning.

It would seem that with larger tires my speedo would read lower than my real speed (possible tickets) and the additional diameter require more power to spin the tires since the weight would be distributed farther "outside" (inertia).

This would suggest more sluggish performance, lower gas mileage and possible tickets.

On the other hand, it would probably smooth out the ride somewhat.

-john

 

What you are saying is correct.......but if you think about it, lets say that you are on the HWY and everyone else is driving at 75mph, in me meantime you are driving at 85mph (the proportion may not be exact) and you are not passing anybody, you are simply doing 85 to keep up....your engine is spinning at higher rmps (more wear and tear)......your speedo read will be higher than your actual speed....I think it's a matter of preference on your part....that's why I said, that I would get the OEM size.....

 

OK, I see.

Well, I'd like to get really good tires as close to the OEM size as possible.

Unfortunately, that seems to be harder than I'd like.

-john

 

A couple percent in either direction will really affect nothing other than a very slight speedo error. An inch difference in diameter is less than 4% error. It's on the high end of what I'd consider acceptable. I'd try and get +/- 2% error to keep the speedo close.

It will have a neligable effect on your engine wear or fuel economy.

 

FWIW, the Pilot Sport A/S are on closeout on tirerack.com ($125ea). This is a great price, consider the Goodyears are $211 ea.

OTOH, it looks like they are being replaced with the Pilot Sport A/S Plus which is fairly expensive in this size $374.

The other gotcha is this size appears to pretty much lock you into the Pilot Sport A/S Plus since it, the Pilot Sport A/S and the Kumho Ecsta ASX are the only tires in the ultra performance all season category.

I'm thinking the 245/45R19 size is a safer bet where you have both the Pilot Sport A/S Plus, and the Goodyear F1 A/S as well as some others like the Yokohama ADVAN S.4., etc.

-john

 

so, if you have something bigger than OEM, when you do 100mph, you will have either a 2 or 4mph difference? is that how is translated?

 

Yes, exactly.

The miata tire calculator tells you the percent difference.

 

Anything less than 2% is reasonable. Ideally less than 1%. A perfect match is 1/2% or less.

I have the miata calculator bookmarked. It's the a very useful page when you need it.

 

BTW, the miata calc gave slightly different sizing than the specs on the OEM tire. Probably best to check the specs on the specific model tire as well.

-john