245/40/18 on 8.5" Wheel - Too Much Tire for Stock 8?
 

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I am undecided on which way to turn for my new rubber.
I have 18x8.5" wheels with the stock 225/45 RE040's currently mounted.

I have been contemplating the seemingly logical 245/40 size as a replacement.

My speedo is optomistic by about 4%, so going with a smaller 235 size will make this even worse.

The problem I have with the whole BIGGER is BETTER argument is there must be a reason why manufacturers choose certain tire sizes.

Take for instance the 8. A relatively light car at under 1400kg - comes with 225/45 all around.

There are plenty of similar HP, similar weight cars with this size of tire.

Take much heavier, much more powerful cars and the front tires are usually no larger than 245 width. These are found on all sorts of cars with different layouts - FF, FR, MR, etc.

Need some examples - From issue 103 of UK EVO magazine

• Holden Monaro - 493bhp, 1677kg 245/45/19 all around
• 911 997 GT3 - 409 bhp, 1425kg, 235/35/19 front, 305/30/19 rear (38/62% dist)
• Aston Rally GT - 425bhp, 1400kg, 245/645 18 front, 265/645/18 rear
• Lambo Gallardo Superleggera - 522bhp, 1420kg, 235/35/19 front, 295/30ZR19 rear.

So, is going big necessarily better? If is was, then why wouldn't we see supercars with huge steamroller tires on the fronts?

The above examples have much higher power/weight ratios than the 8, yet don't use above 245 width tires.

So what exactly are the merits of going 245 vs. 225 on the 8 especially if you don't intend on going with any kind of FI?

As I see it - 225's offer:

• Lighter by around 2-3 lbs.
• Better steering feel
• Less tramlining than 245's.
• Cheaper

Comments from people who have made the switch?
I would like to participate in the occasional track day with the street tires.
Suspension mods will be limited to bars at most, as a drop would compromise winter driving clearance.

Get 245s and call it a day. I ran my stockers with 245/40 before I got rid of them. Hell I have 265/35s all the way around ... no FI. All I have in terms of power mods is intake and exhaust. Grips nicely at the track.

I'm very interested in this as well. Any experts in here that can tell me what I gain or lose by going with the 245's? I do no track stuff. No kind of alterations to the car at all.

Of course there are, but those reasons also include cost and liability.
You want better performance, you want wider, stickier rubber. TeamRX8 has 275s mounted on 18x10.5 wheels on all four corners for a NA car and gets better performance at autocrosses.
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VAPIR OXYGEN

No, I ran 275's on mine on a 9.5" wheel A OK.

Every single one you listed had 245 or larger width tires.

There is a downside with running a wider tire. You increase rolling resistance and wind drag. I liked the 245/40x18's I am running on the stock rims. Speedo reads 2 MPH high (measured by GPS and speed radar sign)

doing 245/40 will mean you're adding an extra 1% more miles to your odometer.. so driving around for 40,000 miles on that tire = 400 extra miles you didn't drive.

I currently have 245/35/19 and I'm off by 0.84%, so I've added around 300 miles to my odo that it should have read.

Next tire is going to be 255/40/18, that will be off by -0.23%, so I'll slowly get back some of those miles :)

Actually, the Superleggera has 235 fronts.
I quoted both front and rears - no car has larger than 245's up front.

All I am trying to state is the obvious. Supercars with 3-4x more power than the 8, have 235, or 245 sized fronts.

Is there a reason they didn't go higher than 245?
In the case of the Superleggera - a car that I can only assume has higher cornering load capacity than an RX-8 went with 235's up front.

The fact is 235's and 245's will have to do all of the braking and turning - so you would think wider is better.

Remember - the size of the contact patch doesn't really change as that is a load factor - it is the shape of the contact patch.

My guess is going too wide on the front really compromises steering response. So are you compromising steering feel for the sake of steady state cornering grip?

I know a lot of people have used 245/40's for the 8, and claim to like it.

But the question is - is this too much tire for a stock 8?

I just thought of this, would it be possible to get a speedometer fooler to fool your car into thinking that you didnt drive all the miles you really did for warranty purposes?

yes, but since the variations are so small, it's really not worth it.

also by going to a bigger tire, you're going to get less power out of your car since your ratios are changing. Going to a smaller tire will give you more power.

they're not too much for the car the driver may be a different story

and yes, 9.5" was A-OK, but 10.5" is AAAAA-OK :)

I have 245's and my 1/4 mile times were the same. I have not tested my speed but I will tomorrow.

No such thing as free power.

Going with a smaller diameter tire will effectively change your gearing - in this case effectively shorter.
This will lead to FASTER acceleration, but power is the one thing that stays constant.

10.5" on a 175hp car?

Yeah, but im talking about MAJOR fooling, like telling your car its going 1 mph all the time and just using a gps dashboard mounted for your real speed.

I have 245/40 on my 9.5 rims, and the steering response is almost laserlike. MUCH better than 245s on 8" rim.

If your worried about oversizing that much, then just get 235s and call it a day.

the amount of power you have is insigificant to how much speed you can carry through a given turn, but in my case add 1/3 more rwhp and drop 200# off the car weight ;)

Wow 230+rwhp in STU trim? I didn't think all the boltons added together netted that much.

200# came from where?
Get rid of A/C
Replace front Seats with Racing Seats?
What else did you have to give up to realize those weight savings?

230 rwhp with a 2800# car would be nice.

I wouldn't say power is insignificant, as you need the power to develop the speed at corner entry in order to carry the speed through said corner - right?

This is especially true in auto-x as things like short term acceleration and throttle response are so important.

200 pounds off a base/sport is more like 2600 and change not 2800. The idea is not to develop speed for corner ENTRY but rather ASAP on exit, slow in fast out.

not necessarily, I watched an older BMI vid last night where a car had triple the power of the car it was "racing" in the togue, and the smaller, lighter, way underpowered car destroyed it, the only time it was even remotely close was in the straight away where the power probably helped, but then the turns would come and the lesser powered car would just annihilate the more powerful one. The Speedsource cars don't have the same power as their competition (right? or?) and seem to be doing well so maybe it's not insignificant, but it's definitely not always the advantage that people think it is. :dunno: :)

I think the Speedsource cars had more power and were lighter at the beginning of the season then they got some weight added and haven't been doing so hot lately. Last race the rear was sliding all over the place and a pack of porsches went right by.

I'm in the low 27xx range, 200lbs decrease is a reference figure. It could be less or more than that depending on which options you have, an early Base model should be just under 2900 lbs, I've heard the '06+ is heavier but don't have any proof

I never said extra power was insignificant, rather I'm saying that just because you have low power doesn't mean there aren't advantages to running super wide wheels. I can still smoke 1st off the line and heavily bark 2nd when running down near sea level. What can't be shown on paper is the instantaneous response my engine has, it's incredible for it's power level, especially since the flywheeel, clutch etc are all OE ...

wouldn't know, haven't been able to watch them race this year :sad:

that's the size i run and I love it