Correct Tyre size

[Dunx]

Does anyone know what the correct tyre size should be for my '97 E300TD Estate Avantgarde? The car has 8Jx18 alloys fitted all round but the tyres currently on the car are 245/40 on the rear and 235/40 on the front.
Is there any reason why the car would have different Tyre widths front to rear for the same wheel size?

TIA,

Duncan

[KLP 92]

Should be 235 40 18 on a 8J wheel all round.

You can go for a wider tire at the rear but not really recommended, unless its on a 9J wheel whena 265-35-18 tyre can be used.

As you are using 235 40 18 at the front and the rear is 245 and the same profile you may experience problems with the cruise control malfunctioning due to different speeds being detected by the wheel sensors due to different rolling radius.

[Dunx]

Originally Posted by KLP 92

Should be 235 40 18 on a 8J wheel all round.

You can go for a wider tire at the rear but not really recommended, unless its on a 9J wheel whena 265-35-18 tyre can be used.

As you are using 235 40 18 at the front and the rear is 245 and the same profile you may experience problems with the cruise control malfunctioning due to different speeds being detected by the wheel sensors due to different rolling radius.

Not experienced any issues with cruise, but I've not really used it for any great distance. Pity I didn't check this a few months ago as I just put new tyres on the rear (same size as the old tyres; 245/40). I just presumed the rim sizes were different. However I had the wheels off today and checked the sizes stamped on the back and found out they were the same.

[Benzowner]

My 1997 E300TD has always had 215 55 W16 on and still do oops didn't see the 18" sorry, ignore me I will get my coat

[grober]

SEE http://www.carbibles.com/tyre_bible.html for the tyre and rim calculator down the page a bit. You will have to know the original wheel / tyre setup for your car if you want to retain the same gearing and speedo reading. Maybe the fronts are slightly smaller to avoid fouling the wheel arches. but I,m just guessing

[DieselE]

Originally Posted by KLP 92

As you are using 235 40 18 at the front and the rear is 245 and the same profile you may experience problems with the cruise control malfunctioning due to different speeds being detected by the wheel sensors due to different rolling radius.

Showing my ignorance maybe but why? The profiles and wheel diameters are the same, the only difference is the tread width. I would not have thought this would affect rolling radius.

[Dunx]

Originally Posted by DieselE

Showing my ignorance maybe but why? The profiles and wheel diameters are the same, the only difference is the tread width. I would not have thought this would affect rolling radius.

The profile height or aspect ratio is given as a percentage of the width. So a 245/40 Tyre has a profile height of 98mm (40% of 245mm). A 235/40 tyre has a profile height of 94mm. On an 18" rim, the rolling radius for the 245 tyre is 326.6mm and 322.6mm for the 235 tyre. If I can remember back to high school geometry, the distance travelled by the 235 tyre over one complete revolution would be 2027mm. For the 245 tyre this would be 2052mm.
Perhaps a little ****, but I hope this helps!

[DieselE]

I did say I might be showing my ignorance.

Thanks for the explanation.