Hi guys,

I have a bubble in my front passenger right tire that could blow out anytime I hit a pot hole. Currently I have Pirelly P240-Snowsports that are OEM size (225/45/18), so my question is that I'm replacing it with two used tires that are (245/45/18). So it's a finger wider. Since my two rear ones are okay, the guy's throwing the two rears on the front, and replacing it with the two wider 45's on the back. Forgive my ignorance but I have a few questions:

1) Will it cause any damage to my car, or rim?

2) What will I notice? Is this a good setup?

Please let me know ASAP as I'm leaving to get this doen within the hour....sorry for the short notice but I just found out.

Thanks and God bless you guys.

Ben.

 

No...

Many of us run 245's

 

Well my setup will be two Pirelli 240 Snowsports in the front that are 225/45/18's and whatever the brand (used tires) that are 245/45/18's on the back. And also all tires will be installed on the OEM rim. Is that cool? The reason I'm asking is that I dunno if it will be a difference with just the wider tires on the back and the skinnier tires on the front?

 

To be honest, I would recommend using a 245/40R18 tire. That way your front and backs would be nearly identical in height. However, yes, that tire will fit on your OEM rim in the back and fit in the wheelwell nice.

With the 245/45R18 tire you probably wouldn't want to rotate your tires as you would have biggies on the front...might look weird.

 

Cool, thanks man....I was just worried about long-term suspension damage or what not. And I'm taking those sizes cuz it's used and cheaper, plus it was a good deal and I needed a winter tire immediately cuz of the situation with the bubble that I have.

 

Okay I've got the tires on and I noticed that the 245's make the car sit lower (as in two-finger wheel gap compared to the 225's (three finger wheel gap). So the rear is lower than the front....is this alright? Also, does anybody have my setup where they have 225's in the front and 245's in the back?

 

Your looking at it the wrong way. Your front is actually lower than the rear, by 0.65 inches. The 245/45s have a bigger total wheel+tire size filling out the wheel well, which is reducing the well gap as well as actually raising the rear .065 inches.

http://www.wheelsmaster.com/rt_specs.jsp

plug the 225/40/18 18x9 40et into the left box and 245/45/18 18x9 40et into the right. The data below the right box shows the different from stock.

Remember, the /40/ is the percent of the first number. So napkin math, you went from a tire sidewall height of 90mm (40% of 225) to a sidewall height of 110.25 (45% of 245). On the same rim this will lift the axle by that change. The total height of the top of the tire off the road increased by twice that difference.

Of course, tread wear can change these numbers + or - 8mm (10/32 inch), and I honestly don't know if the sidewall height calculation includes the tread or not. It is a decent guide to use though.

 

Oh okay cool....yeah that makes more sense....thanks for the tire size comparison website! I wonder if this setup will affect my gas mileage and if it's recommended?

 

Short Answer: It might, it might not, and if it does or doesn't is REALLY hard to calculate.

Napkin math again:

If you increase your tire circumference by 5%, your axle rotates 5% less every actual mile. Thus your RPMs will be 5% lower, your speedo will show 5% slower, and your odo will tick up 5% slower. This is not easily noticable though, since all your measurements will show you still doing the same speed and covering the same distance, but they are now uncalibrated. In basic theory, this could increase your MPG, however if you still just take trip miles divided by gallons, you will show no increase, because your odo is off. Only real way of noticing is how often you fill up might be 5% less frequent, which is also hardly measurable since your fillups are generally not strictly scheduled.

Don't take this as hard fact though, since the increased amount of rubber is added unsprung weight which will negatively impact MPG, and the new tires might have more rolling resistance which could completely offset the gain. I also don't know if the speedo/odo is measured from front or rear axle, or both. (And if both, when there is a difference, which is the default). If it is the front, which is typical, then you are still on stock revolutions per mile, which would unsync the RPMs from speed/miles, and everything I just did goes out the window.

I.E, it is hard to say, and probably not big enough to measure.

 

you guys are missing the most important thing here! is your car a GT because if it is this will mess up your stability and traction control and possibly your ABS

how do you think the rx8 collects the data for abs/tcs it knows the speed of all 4 wheels. if any wheel is moving to fast or slow it will try to correct this by applying or releasing the brake to the appropriate wheel. i would seriously not recommend doing this if you have any doubts about what im saying call a mazda dealership. more than likely your abs light will come on after a few miles/days and can cause very serious damage over time

 

Mess up? I doubt it. Yes, you are correct that it will change the rotational calculations of each, but only faintly. So it will certainly not be precisely accurate in it's calculation, but that just means it will kick in faintly sooner or faintly later. Yes, that faint amount could mean the difference in an rare accident or not, but it could also kick in soon enough to prevent an accident that stock might not, but it isn't something I would feel concern over.

If it was a cause for concern, everyone that changes their wheels/tires to non-stock sizes has this issue, especially the staggered fitments. And since the difference between full tread and no tread is not insignificant, there has to be some re-calculation metrics within the programming that takes into account the ever-changing rotational distance.

 

Everyone doesn't put different height tires on an end of their car. I do agree that the DSC probably isn't sensitive enough to care though it might at higher speeds where the rotation difference is greater. We'll wait for the thread asking about why DSC is cutting in during regular driving.

 

alright well time will tell. ben you seem like a honest person and if/when your abs light turns on please post up a pic or something. i hope im wrong but im doubting i am

 

Wow guys, I really feel worried now....so half you guys are stating that the setup I currently have with the 245's on the rear and the 225's on the front will mess up my DSC/TSC and ABS (I have a GT)? Does anybody have my setup and been using it for a while?

 

Dude, I have 275/35zr18 on all 4 corners and it doesn't noticeably change anything BESIDES better grip.

DSC/TCS is fine. ABS is fine.

Do not panic! :D

 

Thanks bro! Just one question tho....you have all four tires (even though they are larger) of the same size....if you split it up - stock front tires and larger rear tires, would that have any effect, or no?

 

Very minor. It would not make your car less reliable. Many cars have stock setups that provide fatter tires in the rear. Having different DIAMETERS may make a small difference, but nothing you will notice. The only exception to this is in American cars if you go from, say, 225/35/18 to 225/55/18.. Messes up gear ratios and all sorts of good stuff...

You will be safe, much safer than having a bubble in your tire .

If you notice anything really messed up happening hop back onto this thread and report it, but I severely doubt you will.

Best of luck!

 

Thanks again dude....God bless!

 

Yeah, nothing to worry about. We were talking strictly on the technical side. Yes, it technically does change the calculation impact, but it isn't enough to change the real-world impact of how they work. Keep in mind that there is a possible shift on stock tires if you go from completely bald back to brand new. This difference is enough to change your actual speed vs speedo by up to ~3 mph. And this does not negatively affect ABS/TCS. The difference in diameter probably has to get significantly different, probably 5+ inches different, in order to mess up ABS/TCS, and I am 100% sure that if it messes it up, the TCS light would be flashing, and simply not be on.

Not just speculation. If you disconnect the battery for long enough to clear the internal computer settings, it clears TCS data as well, and TCS will be off until it has the data it needs again to function. Which means that the TCS has the ability to cut itself out if it feels it doesn't have the correct data to function safely.

Nothing to worry about. Just technical talk.

 

If the difference is significant, the car will constantly be thinking that the rears are sliding. Constantly. Again IF THE DIFFERENCE IS SIGNIFICANT. THe bad thing is, if the difference is enough to cause the car to think things are sliding, you will eventually go into limpmode. I have done this myself, so I speak from experience.

 

Here is a good tool as well http://www.miata.net/garage/tirecalc.html

I use it for all my tire sizing.

That calculator sez you are 3% larger in circumference in the rear. This means that for every 100 revs in the front, you only go around 97 times in the rear. While I dont agree with one of the above posts that you will break anything, I have done two things to make me think that this will not be healthy. First, playing in the snow and forgetting to turn off the dsc eventually put my car into a kind of limpmode. All the lights on the dash were on and the car would not rev, until I restarted.

A very similar thing happened with one stock wheel and three aftermarket tires. THe dash lit up until I restarted, this happened just driving down the street and around a circle. I think the differences in tire size were around 5%. YMMV.

Try it out; if it works, great, if not, call me out on my BS. I dont think you are going to break anything.

 

i currently run 255/35/18's on all 4 wheels the significant part is all 4 wheels running tires with different overall diameters front to back will cause issues i cant believe people would think other wise. how do you think the tcs works? by calculating the exact speed of all 4 wheels compared to each other. i will say 3% difference is barely noticeable but noticeable to the cars ecu none the less

 

Thanks again guys....so nate340, do you recommend switching all four tires to 245's instead of the two rears if I go larger? In my case, I had no choice, pus the 225's on the front have really good tread so why waste it? Also, the front's are Pirelly 240's and the two larger rears are Pirelli 210's.

 

yes its important to have all 4 tires the same height.

the ecu the computer in the car measures the wheel speed using a special ring located on the hub. it monitors all 4 wheels and lets say you start applying the brakes it notices that the right front wheel locks up it notices instantly then realses the brakes on that wheel until it starts rotating at the same rate as the other 3. if the back right wheel starts spinning faster than the other 3 it will apply brakes to that wheel to slow it down until its running the same speed as the other 3 wheels.

so if your rear tires are constantly moving slower under normal driving conditions the ecu has to be some what confused and aware of it. you may even notice that your able to spin your tires longer before the tsc system kicks in. iv'e seen a brand new ford ranger at a dealership because they had larger tires on the rear its abs light would keep flicking on and disabling the abs.

this is also why your supposed to run the same brand of tire on all four wheels as well since every manufacture even though they are the same size 225/45/18 there overall height is different for each manufacture

if you have any doubts about whats being said in this thread call any Mazda dealership and explain to them your tire set up and see what they have to say.

 

I have 7000 miles on staggered tires; and no DCS TSC problems... 245 rear and 225 front - both 35 sidewalls (which make the rear a bit bigger)... so I wouldn't fret over it.

It is not ideal to stagger on the RX8 since normally we don't want to promote understeer. But my car isn't exactly normal.