Changing the rears from 245/40/17 to 225/45/17 Good idea or not?

[Chrishazle]

To resurrect this thread, I emailed Michelin, turns out the tyre in USA is not a Crossclimate and they have no plans to produce a 245/40 R17 Crossclimate for the UK. Closest Nokian do is 235/45 R17, not sure how suitable that size would be. ATM it seems options are limited - namely 225/45 R17 Crossclimates on the rears, sacrifising 2cm of rear tyre width putting the power on the road, or maybe 235/45 R17 Nokian or 245/45 R17 Crossclimates knowing the speedo will under-read by 1mph at 30 increasing to 2mph at 70.

 

[sonic]

To resurrect this thread, I emailed Michelin, turns out the tyre in USA is not a Crossclimate and they have no plans to produce a 245/40 R17 Crossclimate for the UK. Closest Nokian do is 235/45 R17, not sure how suitable that size would be. ATM it seems options are limited - namely 225/45 R17 Crossclimates on the rears, sacrifising 2cm of rear tyre width putting the power on the road, or maybe 235/45 R17 Nokian or 245/45 R17 Crossclimates knowing the speedo will under-read by 1mph at 30 increasing to 2mph at 70.

I have 245/40R17 winter tyres which are now 8 years old having done 15,000 miles. I am looking to replace those at sometime with Crossclimates, so I might be in the same position as you.

 

[Chrishazle]

I'm in the awkward position of having 2 cars with identical staggered 17" setup - 225/45 front and 245/40 rears.ATM I have Crossclimates on the front and GY Eagle F1 Assyms on the rear of the S204. The SLK has GY Efficientgrip Performance on the front and something like Hankooks on the rear. Both sets of fronts are pretty new, both sets of rears will need replacing in the next year or so - I'm tempted to change the SLK rears earlier as the Hankooks give a too firm ride.

 

[RhodieBill]

To resurrect this thread, I emailed Michelin, turns out the tyre in USA is not a Crossclimate and they have no plans to produce a 245/40 R17 Crossclimate for the UK. Closest Nokian do is 235/45 R17, not sure how suitable that size would be. ATM it seems options are limited - namely 225/45 R17 Crossclimates on the rears, sacrifising 2cm of rear tyre width putting the power on the road, or maybe 235/45 R17 Nokian or 245/45 R17 Crossclimates knowing the speedo will under-read by 1mph at 30 increasing to 2mph at 70.

Hi. I would go for the Nokian if those were my choices.
I've used Nokians on a few cars, albeit Summer Tyres and I found them to be excellent, even in Winter Conditions!

And when I say Winter Conditions it was like this.... (Outside my house and at the Marina - no Gritting!).

I'm presuming the Nokians will be the Weather Proof? They have very good reviews.

 

[Rotorhead500]

+1 for Nokian.

 

[Chrishazle]

Fine, but what's the result of changing the rears from 245/40 to 235/45? Speedometer, rim rotection, ride quality?

 

[Rotorhead500]

Fine, but what's the result of changing the rears from 245/40 to 235/45? Speedometer, rim rotection, ride quality?

-2.23% speedo under-read - so at 30mph, will read 29.33mph... or thereabouts, on a 19" wheel.

Increased tyre wall should increase comfort a little.

Rim protection is a good point, that'll be compromised.

 

[Chrishazle]

Mine are 17", is it the same on that size, 'cos if so it's worth considering. Just checked the history file of the S204, tyres are only 10k miles old and previous set lasted almost 20k, haven't checked the SLK as its still on the rears it had when I bought it a year ago, but do know they've still got a lot of life in them - reason for changing them is ride quality as she's a bit crashy on the rear and the front improved a lot when I changed to GY efficientgrips, reduced the earache from the passenger seat!!

 

[hopeful]

Even if the rear wheels are wider you can still fit a 225 tyre along as the rim isn't wider than 8.5 j

Rim width (inch) Min. tire width (mm) Ideal tire
7,5 205 215 or 225
8,0 215 225 or 235
8,5 225 235 or 245

i do believe the pofile does count . when i got my clk it was on 7.5 all round ,(all blooming wrong , completely) and snatched in high speed changing direction , 8.5 et30 245 40 and 7 .5 et37 i believe on front 225 45 set up stickes like --- now
i wont swear to it ,but changing wheels can upset ecu's
someone may put me right

 

[sonic]

i do believe the pofile does count . when i got my clk it was on 7.5 all round ,(all blooming wrong , completely) and snatched in high speed changing direction , 8.5 et30 245 40 and 7 .5 et37 i believe on front 225 45 set up stickes like --- now
i wont swear to it ,but changing wheels can upset ecu's
someone may put me right

I had 225 front & 245 rear 17" on my CLK270CDI no problems. I also ran 205 16" all around winter tyres, great snow grip being narrow.
Twenty odd years ago I had a BMW320 which the previous owner had fitted massive M3 wheels. The car would wander in a straight line particularly on an uneven road surface.

 

[hopeful]

I had 225 front & 245 rear 17" on my CLK270CDI no problems. I also ran 205 16" all around winter tyres, great snow grip being narrow.
Twenty odd years ago I had a BMW320 which the previous owner had fitted massive M3 wheels. The car would wander in a straight line particularly on an uneven road surface.

where the 16s 8.5 and the front 7.5 on 16s and 17s

 

[sonic]

where the 16s 8.5 and the front 7.5 on 16s and 17s

The 225 front & 245 rear was an option on the 209 CLK, standard was 225 all around.
As best as I remember 225 was 8J, 245 was 8.5J, & the 205 was 7J.
It was 10 years ago.

 

[hopeful]

The 225 front & 245 rear was an option on the 209 CLK, standard was 225 all around.
As best as I remember 225 was 8J, 245 was 8.5J, & the 205 was 7J.
It was 10 years ago.

i had 7s and it was awful, now on staggered as it should be c209

 

[sonic]

i had 7s and it was awful, now on staggered as it should be c209

Never had 225 all around, but the rear 245's on the CLK270CDI only lasted about 5 to 7K miles. The 205 rears would last about 18K miles. I did about 70K miles in it & no matter what 245 rear tyre tyre you used they wouldn't last.

 

[jookomak]

Hi all!
I have the exact same question as the topic starter.
Current setup is the staggered, and I would like to buy michelin cc+ as well, but they don't have the rear size. Is anyone using 4 of the same size? Would I lost some grip or performance? Probably I would still gain some grip changing who knows how old summer tyres to new michelins.
Only problem that I don't know what size of alloys I have currently.
Do you know a good tire shop around Manchester? Best option would be if I go there personally, and have a look at it.

 

[RhodieBill]

Hi all!
I have the exact same question as the topic starter.
Current setup is the staggered, and I would like to buy michelin cc+ as well, but they don't have the rear size. Is anyone using 4 of the same size? Would I lost some grip or performance? Probably I would still gain some grip changing who knows how old summer tyres to new michelins.
Only problem that I don't know what size of alloys I have currently.
Do you know a good tire shop around Manchester? Best option would be if I go there personally, and have a look at it.

That model generally doesn't have a Staggered setup. You can fit the same size that is on the front, I pesume 225/45x17's, on the rear. A least for the duration of our relatively short Winter anyway,

 

[Chrishazle]

To answer my own question, changing from 245/40 r17 to 235/45 R19 increases the rolling radius by 46.75mm (2.47%) so at an indicated 70mph actual speed would be 71.73mph - assuming the speedo takes its reading from the rear wheels I guess, don't know where an MB gets its speedo input from!

https://www.carbibles.com/calculators/

 

[RhodieBill]

To answer my own question, changing from 245/40 r17 to 235/45 R19 increases the rolling radius by 46.75mm (2.47%) so at an indicated 70mph actual speed would be 71.73mph - assuming the speedo takes its reading from the rear wheels I guess, don't know where an MB gets its speedo input from!

https://www.carbibles.com/calculators/

As far as I know and been led to believe is that the Speed Sensor is on the Driver's Side Front Wheel. (RHD Cars).

 

[Chrishazle]

OK, if that is indeed the case then the speedo would be OK as it's the rears being changed slightly. Would it upset the ABS/traction control as the front wheels would be turning slightly faster than the rears? I suspect not - standard setup 225/45 R17 front 245/40 R17 rear gives rolling radius of rears 19.6mm or 1.2% smaller, 225/45 R17 fronts 235/45 R17 rears gives rolling radius of rears 27.15mm or 1.42% bigger, so presumably within the operating tolerances as current setup does not cause error messages, suspect you'd just get an initial TPMS message at first use, tell the machine tyre pressures are correct to reset that part of the system and carry on!

 

[RhodieBill]

OK, if that is indeed the case then the speedo would be OK as it's the rears being changed slightly. Would it upset the ABS/traction control as the front wheels would be turning slightly faster than the rears? I suspect not - standard setup 225/45 R17 front 245/40 R17 rear gives rolling radius of rears 19.6mm or 1.2% smaller, 225/45 R17 fronts 235/45 R17 rears gives rolling radius of rears 27.15mm or 1.42% bigger, so presumably within the operating tolerances as current setup does not cause error messages, suspect you'd just get an initial TPMS message at first use, tell the machine tyre pressures are correct to reset that part of the system and carry on!

No, because you are changing the Aspect Ratio from 40 to 45. It's only because you have the 245's on the Rear that you have to drop the A/R to 40.
The Aspect Ratio is a Percentage of the Width of the Tyre. Hence the wider the Tyre the lower the A/R. This keeps the overall Diameter of the Tyres the same.