Tyre size swap from 255/40/18 to 245/40/18

[Jitc85]

Hi Guys, 

Would I have any issues if I changed my rear tyres on my IS250 F Sport from 255/40/18 to 245/40/18? 
 

Reason I ask, the prices of the 255’s are silly! 
 

Has anyone done this? How did it effect the handling etc?

 

Thank you 

[scudney]

I have kept mine as they are,however you shouldn't have any problems if you went down to 245.40.18 apart from your speedo reading slightly out if I am not wrong,you could probably go 235.35.18,not sure about cost though 

[Linas.P]

I used run 225/245 on mk3 F-sport wheels... it definitely improves handling as 255 is silly for car like IS250, especially considering that fronts are 225 and just induces "fake-safe" understeer balance. If anything I would go even further for 225/235 or 235/245 to bring that balance back to more neutral. 

In short - it makes car handle better and more like RWD should, but probably still too much grip in rear, too little in the front.

[J Henderson]

Agree with Linas. Been running 245 rears for about 5 years now and it's just a better choice for a number of reasons.

The slightly smaller sidewall doesn't affect the ride quality by any real noticeable amount, and as mentioned, a lot of understeer is dialled out and the car feels more neutral in corners.

Also, the "hopping/skipping" that can happen when steering sharply on cold tyres, is eliminated. I used to experience this all the time when turning out of my driveway with OEM sizes.

Upping the front tyre size to 235/40 (so 235F/255R) also has a similar effect, and I did this on my first IS250, but tyre choice & availability in the 255/40R18 size is very weak.

Never mind the cost, it's actually quite hard to get a decent brand non-runflat tyre that's not an ageing design, or homologated for Mercedes or BMW cars.

As well as cost savings, and more tyre choice, another positive of going with 245s, in my opinion, is looks. There's less disparity between how the front and rear tyres actually sit on the wheels.

A 245 on the 8.5" rear wheel looks very similar to a 225 on the 8" front wheel, in terms of fitment or "stretch". Whereas, a 255 looks a bit much, and a bit balloonish compared to the front. At least for me it does.

Anyway yeah, the only real downside is the speedo accuracy. I don't think it's particularly off by miles though. When driving to work I will quite often speed match and follow an HGV presumed to be doing 56MPH, and my speedo will be reading something like 61/62.

[AFlex]

I also recommend 245s for rears.

I've been running 225/40/18 fronts and 245/40/18 rears for several years now. No problems at all.

I also prefer the way they look as it gives the rear wheels a slightly lower profile (since the '40' means that the sidewall height is 40% of the width, I believe)

Not to mention prices are much cheaper and better availability of tires/brands.

[gji25]

10 hours ago, scudney said:

I have kept mine as they are,however you shouldn't have any problems if you went down to 245.40.18 apart from your speedo reading slightly out if I am not wrong,you could probably go 235.35.18,not sure about cost though 

the speedo reading would only be out if you reduced the profile 40mm to 35mm

[gji25]

1 hour ago, AFlex said:

I also prefer the way they look as it gives the rear wheels a slightly lower profile (since the '40' means that the sidewall height is 40% of the width, I believe)

40% of the width 245 = 147     the 40 means 40mm profile

[Mr Vlad]

No. 245/40/18. The 40 is the aspect ratio of the tyre width. NOT its profile height.

[Hillie]

Pictures help.

[Brian R]

Perhaps this calculator will help, it also shows the miles per hour discrepancy:

Wheel sizes

[H3XME]

You will have no issues, however I recommend 225s all round. Nothing better than a nice balanced setup for balanced handling. 

[Mr Vlad]

I can't remember if the wheels are all the same size, it's just the tyre sizes that differ front and back. 

[ColinBarber]

On 11/2/2022 at 1:08 AM, gji25 said:

40% of the width 245 = 147     the 40 means 40mm profile

In an ideal world the profile figure would be an absolute figure but it isn't. It indicates the ratio of the width so we all have to get our calculators out to work out the actual measurement.

40% of 245 = 98 mm.

 

19 hours ago, Mr Vlad said:

I can't remember if the wheels are all the same size, it's just the tyre sizes that differ front and back. 

stock wheels, both 17" and 18", are 8" width. No difference between front and rear.

Edited November 3, 2022 by ColinBarber

[J Henderson]

17s are the same widths, but the 18" rear wheels are 8.5" wide.

[ColinBarber]

17 hours ago, J Henderson said:

17s are the same widths, but the 18" rear wheels are 8.5" wide.

Thanks John, I corrected my post above, 8" front and 8.5" rear.

[Lex_utor]

I run 225/35 and 255/35 and like the handling. But my wheels are 19". I find stability and handling in the curves really good and I can push the car without problems. I haven't noitced any objectionalble understeering.

I just got a new winter set up, with 225/40/18 all around. Curious how that will do. My prior winter set up was 225/45/17.

[Jitc85]

Thanks for the replies guys. Some very useful information. 
 

I think going to go with the 225/40/18 front and 245/40/18 rear option. 
 

Next question, which brands do you guys have and prefer? 
 

I’ve got Bridgestones on at the moment, however I feel they’re a bit noisy.
 

I’m thinking of Goodyear Eagle F1’s.  

[H3XME]

1 hour ago, Jitc85 said:

Thanks for the replies guys. Some very useful information. 
 

I think going to go with the 225/40/18 front and 245/40/18 rear option. 
 

Next question, which brands do you guys have and prefer? 
 

I’ve got Bridgestones on at the moment, however I feel they’re a bit noisy.
 

I’m thinking of Goodyear Eagle F1’s.  

Brands don’t matter as much as compound. Michelin Primacys are dogsh*te whereas Michelin Pilot Sport 4 & 5 are one of the best road tyres you can buy.. 

[Las Palmas]

1 hour ago, Jitc85 said:

Thanks for the replies guys. Some very useful information. 
 

I think going to go with the 225/40/18 front and 245/40/18 rear option. 
 

Next question, which brands do you guys have and prefer? 
 

I’ve got Bridgestones on at the moment, however I feel they’re a bit noisy.
 

I’m thinking of Goodyear Eagle F1’s.  

I know no better tyres than Continental. Next on our car will be UltraContact, low noise, braking in wet category A, fuel consumption B.

I think that in summer 1 set of wheels and in winter another. Except here were we live summer all year is the right thing.

[J Henderson]

Can't go wrong with Michelin Pilot Sports. They're expensive but top-notch and last a hell of a long time. Maybe not the quietest if that's of importance, but in terms of grip, steering response & wet performance, they're outstanding.

I put PS4s on my car in 2018 and the rears are just about needing replaced only now. The fronts are not even half-worn yet.

If I was in the market for a new set of 4, I'd be looking at either Michelin PS5 or Goodyear F1 Asymmetric 6.

 

[MrTrendizzle]

The first set of numbers is the width of the tyre so 255mm wide. The second set is the thickness from rim to tread so 40mm high and the third set is the rim size.  

So you can fit 225/40/18 up to 265/40/18 without it affecting anything other than handling. If you have a narrow tyre you have less grip or you might find the tyre flexes over a little more than it should. So lower the profile (Second number) if you're to lower the first.  

Just remember the tiny patch of rubber contacting the floor is all there is between you and a wall at 60mph. The less rubber the less grip = more brick sandwiches.  

EDIT: Didn't see the post date. I doubt they still need the question answered lol.

[Vlady]

6 hours ago, MrTrendizzle said:

The first set of numbers is the width of the tyre so 255mm wide. The second set is the thickness from rim to tread so 40mm high and the third set is the rim size.  

So you can fit 225/40/18 up to 265/40/18 without it affecting anything other than handling. If you have a narrow tyre you have less grip or you might find the tyre flexes over a little more than it should. So lower the profile (Second number) if you're to lower the first.  

Just remember the tiny patch of rubber contacting the floor is all there is between you and a wall at 60mph. The less rubber the less grip = more brick sandwiches.  

EDIT: Didn't see the post date. I doubt they still need the question answered lol.

That is not exactly correct, 40 is the percentage of the width, see above from Colin or Mr Vlad. So wider tyre will have a higher wall or what you called it a Thickness from Rim to Thread. 225/40 and 265/40 will have an effect on speedometer reading, therefore your mileage, your car car will be higher with wider tires too.

 

[MrTrendizzle]

11 hours ago, Vlady said:

That is not exactly correct, 40 is the percentage of the width, see above from Colin or Mr Vlad. So wider tyre will have a higher wall or what you called it a Thickness from Rim to Thread. 225/40 and 265/40 will have an effect on speedometer reading, therefore your mileage, your car car will be higher with wider tires too.

 

Just read and i stand corrected. Thank you for the little bit of info i never knew. Thank you.

[Arthurdoller]

I have a square setup of Eagle F1 Asymmetric 6 Tyres: 225/40 R18 92Y 

mainly due to money as you swap them around,but I do have to say these tyres are so so much better than previous dunlops and Pirelli’s in every regard including road noise:)