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Discussion Starter #1
Just been cleaning my car and putting air in the tyres and noticed that my front tyres are:

225/45 R17

And my rear tyres are:

245/45 R17

Isn't this illegal (UK)?

Cheers

Joe

Well having just read the information on my data card and it says:

17" LIGHT ALLOY WHEEL 10-SPOKE DES. W/ MIXED TIRES

Mixed tyres must be ok then.

 

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Registered 2009 SLK55 AMG
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4,915 Posts
Just been cleaning my car and putting air in the tyres and noticed that my front tyres are:

225/45 R17

And my rear tyres are:

245/45 R17

Isn't this illegal (UK)?

Cheers

Joe
Those are the stock sizes. The wheels themselves are also 7.5" wide on the front and 8.5" on the rear, with tires on the rear 2 cm wider than on the front, to match.
 

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As mentioned by dcmulford, should be lower profile on the rear to compensate for the wider tyre, maintaining the side wall height. But yes they should be different widths.
 

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Premium Member 2013 SLK250
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2,339 Posts
Because the rear are wider. To keep the same circumstance the percent of width would be different. Tire size is tread width, the percent of tread width, and rim size. Example (245x40x17) 245 is tread width, 40 is 40% of 245 and the rim is 17. Hope this helps.
 

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Premium Member 2004 SLK200K
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10,745 Posts
Just been cleaning my car and putting air in the tyres and noticed that my front tyres are:

225/45 R17

And my rear tyres are:

245/45 R17

Isn't this illegal (UK)?

Cheers

Joe

Well having just read the information on my data card and it says:

17" LIGHT ALLOY WHEEL 10-SPOKE DES. W/ MIXED TIRES

Mixed tyres must be ok then.


If these sizes are right , 245/45/17 on the rear and not 245/40/17 , you'll more than likely have ABS/ESP and cruise control faults !
 

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Because the rear are wider. To keep the same circumstance the percent of width would be different. Tire size is tread width, the percent of tread width, and rim size. Example (245x40x17) 245 is tread width, 40 is 40% of 245 and the rim is 17. Hope this helps.
Gbarringer100's info is correct.

The wheel/tyre diameter should be the the same front and rear ie. the sidewall height should be the same at 100ish mm.

So for the front the sidewall height is 225/100x40 = 101.25mm, for rear 245/100x40 = 98mm which is near enough. If you have 45's on the rear the sidewall height will increase to 110mm increasing the overall wheel diameter by 20mm compared to the front.

If it could cause ABS/ESP/TPMS errors it's most likely do so if the fronts were worn and the rears were new, this would make for the largest differential in diameters.

Here's the sizes for 16/17/18 and 19 rims for the slk350.

Front : 205/55Z16 | Rear : 225/50Z16
Front : 225/45W17 | Rear : 245/40W17
Front : 225/40Y18 | Rear : 245/35Y18
Front : 225/35Y19 | Rear : 255/30Y19

Personally I wouldn't worry about it unless it did cause ABS type errors which I doubt it will. Handling may feel a bit different as the centre of gravity will be higher on the rear and the tyres will be a little more 'squishy' but it's probably going to be a marginal change.

Roy
 

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Discussion Starter #13
If these sizes are right , 245/45/17 on the rear and not 245/40/17 , you'll more than likely have ABS/ESP and cruise control faults !
I hope not as I've only had the car about 2 months, but yes these are right according to my data card for my car.

Cheers

Joe
 

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Premium Member 2004 SLK200K
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10,745 Posts
When I bought my car they put new tyres on the rear , size 245/45/17 , I then spent all afternoon trying to find out why the cruise control didn't work , when they were changed to 245/40/17 , everything was sorted !
 

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Premium Member 2013 SLK250
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Comparing 225x45x17 to 245x45x17 has a difference in Circumference of 1.2% with in the MB allowance of 3%. You should not have any problems.
 

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Premium Member 2004 SLK200K
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The difference is 2.84% , maybe in my case , new tyres / tyre pressure on the rear , had something to do with it affecting the cruise control , the only thing I know is that when I changed them to 245/40/17 the cruise control worked fine ..

 

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The difference is 2.84% , maybe in my case , new tyres / tyre pressure on the rear , had something to do with it affecting the cruise control , the only thing I know is that when I changed them to 245/40/17 the cruise control worked fine ..

Neat calculator !

Here's the difference between what is and what should be ;)

The difference with 45 profiles on the rear is getting pretty near that 3% differential. If the front tyres were worn down 2mm more than the rears the difference would then be 3.5% so it's not surprising that different outcomes arise on different cars - it all depends front tyre wear.

Gbarringer100 -I was curious as to where you found that info about 3% differential allowance?

Also interested to find how the road speed is measured, is it derived from ABS sensors or propshaft speed?
 

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Premium Member 2013 SLK250
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The 3% was been in many post by other owners on this site over the years. Do not know what MB publication states this percent. Must be true based on owners who exceed the 3% and the cruise stops working.

There are sensors at each wheel that checks the speed of each wheel. In fact this is one way the low pressure tire warning work on some cars. A tire at low pressure will turn at a different speed and the light will come on. These will sensors will turn off cruise control if the speed of one tire is more than the 3% difference.


Here is a site to check the tire sizes. Note this site also allows to put in the rim sizes which can effect the Circumference of the tire. Plus you can see what the off set does in the fit on the car.

http://www.rimsntires.com/specspro.jsp
 

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The 3% was been in many post by other owners on this site over the years. Do not know what MB publication states this percent. Must be true based on owners who exceed the 3% and the cruise stops working.

There are sensors at each wheel that checks the speed of each wheel. In fact this is one way the low pressure tire warning work on some cars. A tire at low pressure will turn at a different speed and the light will come on. These will sensors will turn off cruise control if the speed of one tire is more than the 3% difference.
Here is a site to check the tire sizes. Note this site also allows to put in the rim sizes which can effect the Circumference of the tire. Plus you can see what the off set does in the fit on the car.

http://www.rimsntires.com/specspro.jsp

I've never seen it in an MB document either, but 3% sounds plausible as wear difference between tyres only makes about 1% difference to the correct tyre setup.

My wifes car (mk5 GTi) uses that type of TPMS system, as you say it works on the basis that at low pressure the tyre is smaller in diameter and therefore rotates more [over a given distance]than the other [normal pressure] tyres.


That calculator is amazing - I'm off to play.... :)
 
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