Dropping tyre size from 265 to 255 on Automatic

[L_A]

I have been told from my local tyre place that dropping the tyre size from 265/35/18 to 255/35/18 on e320 cdi auto will cause my car to pull off in second gear is this true. I am dropping tyre size as i can get very good part worn tyres with no puntures in 255/35/18 however find it impossible to get part worn 265s and the new ones are mega bucks not unless they are budget brands?

 

[Rory]

Will all 4 tyres be the same size?

 

[Gareth]

I doubt it, fronts are probably 245/40/18. Whilst its not something I'd do, I can't see it causing the car to start from a different gear.

 

[Rovinghawk]

I have been told from my local tyre place that dropping the tyre size from 265/35/18 to 255/35/18 on e320 cdi auto will cause my car to pull off in second gear is this true.

Less than 4% difference in circumference. I think they're talking crap so they can sell you more expensive tyres; call me cynical.

RH

 

[Mactech]

They are only 3.5mm difference in radius. The electronics allow at least the amount you will wear the tyres by, which for most will be 6mm.
The rolling radius will be affected by the tyre pressure and again the electronics have a good tolerance for this.
So my opinion? They are talking rubbish!
Oh! and I've got very different tyres on mine than I'm 'supposed' to have, and the gearbox works just fine!

 

[Rory]

There are stories of cruise control not working with small differences in circumference.

Sounds unlikely this would affect the autobox, but I wouldn't be totally astonished if it did, bearing in mind how all these systems are inter-linked.

 

[Dazzler]

You cant go below what the manufacturer states or you will void your insurance

 

[L_A]

i was planning to drop the front from 235/40/18 to 225/40/18 and the rear from 265/35/18 to 255/35/18.

I had this same set up on my last w210 and it was fine however that was a manual box this is why i am a bit unsure.

I also try to explain to the tyre guy that there are many people that change thier wheels to aftermarket get different sized tyres do they not get problems and he toild me that he has came across this problem once before.

 

[Rory]

You cant go below what the manufacturer states or you will void your insurance

Strictly speaking you're right of course, but has that ever happened in practice?

It's certainly been held that you can use lower speed rating tyres and cars these days come with such a wide variety of wheels and tyrer, who would know if they were wrong? You could easily buy a used car with the wrong size tyres and you'd never know.

 

[timskemp]

You cant go below what the manufacturer states or you will void your insurance

cite?

As far as I know the only thing that is safety related is ensuring the load rating is adequate for the vehicle and the speed rating not exceeded by the driver, and the tyres are in good servicable and legal condition. Insurance shouldn't care if you've got 225/55/16 or 265/35/18 on there.

 

[Shude]

You cant go below what the manufacturer states or you will void your insurance

I guess anyone with a space saver fitted just crosses their fingers then and hopes no-one finds out in the event of a claim!

 

[EDZ649]

The OP says he was going to drop the size of the tyre width, not the profile, therefore the rolling radius is not affected, still 18" diameter rim with 35/40 profile respectively.

All he will do is risk kerbing the wheels more.

 

[Dieselman]

i was planning to drop the front from 235/40/18 to 225/40/18 and the rear from 265/35/18 to 255/35/18.

The smaller sizes will be closer tolerance than standard tyres.
Standard (F:R):= 645.2mm and 642.7mm circumference. Variation 2.5mm
Smaller (F:R): = 637.2mm and 635.7mm circumference. Variation 1.5mm

If the tyres were significantly different it could cause the gearbox to select 'limp home' and hold the last good gear, which would probably be 3rd before the amount of wind up became too much for the ECU to allow.

 

[Shude]

The OP says he was going to drop the size of the tyre width, not the profile, therefore the rolling radius is not affected, still 18" diameter rim with 35/40 profile respectively.

Not true, think about it
225/45/17 tyre and 245/40/17 tyre has approx the same height sidewall, and we know the wheel is the same diameter so why is the profile different? It's a ratio rather than an absolute.

 

[EDZ649]

Not true, think about it
225/45/17 tyre and 245/40/17 tyre has approx the same height sidewall, and we know the wheel is the same diameter so why is the profile different? It's a ratio rather than an absolute.

See above, 265/35/18 to 255/35/18 side wall size is the same, only the width has changed.
Where does he mention a difference in side wall height???

 

[timskemp]

See above, 265/35/18 to 255/35/18 side wall size is the same, only the width has changed.
Where does he mention a difference in side wall height???

the 35 is the ratio of sidewall height to cross section width, so a 255/35/18 has a smaller sidewall, therefore a smaller diameter (635mm) than a 265/35/18 (643mm)

 

[V12]

Agreed, changing by this small percentage will probably not effect the rolling radius enough to cause you problems.

HOWEVER, while i was waiting for my 275/30/19 tyres to come through, i thought i'd temporarily put some 225/40/18's on that i had sitting in the garage. I bolted the 18's on the back, left my 235/35/19's on the front and thought i'd test this theory of how much the ESP / gearbox etc are effected by different sizes.

Drove it out the drive fine, as soon as i hit about 10mph it shut the throttle, with the esp light flashing, slowed to 5 mph and then the throttle picked up again to about 20mph and then did the same. I then turned the esp off and it drove perfectly.

Fairly evident that the car (ESP) thinks the back wheels were spinning as they were rotating faster due to the smaller wheels that what's on the front.

Once i took the front 19's off and put the other 225/40/18's on the front the car drove perfectly for a couple of weeks until the tyres finally came and i had enough time off work to go and get them fitted!

This probably shows that the esp only measures the speed of each of the wheels and doesn't use any reading from the gearbox otherwise the problem would not have been cured by changing the front wheels to smaller ones as well

For the maths geeks out there.

Rolling circumference
225/40/18 - 1943mm

235/35/19 on front - 1974mm

275/30/19 on back - 1975mm

Does that all make sence?

 

[Rory]

See above, 265/35/18 to 255/35/18 side wall size is the same, only the width has changed.
Where does he mention a difference in side wall height???

The profile (the 35 number) is a percentage of the width.

So 35% of 255 is a bit less than 35% of 265. Don't forget that it's x2 for the diameter (as there is, if you like, a top and bottom sidewall in the diameter) then you have to calculate the circumference.

You can compare here: Tire size calculator

 

[Shude]

This probably shows that the esp only measures the speed of each of the wheels and doesn't use any reading from the gearbox otherwise the problem would not have been cured by changing the front wheels to smaller ones as well

Plus the gearbox on 90% of models has no connection to the front wheels at all!

 

[timskemp]

Plus the gearbox on 90% of models has no connection to the front wheels at all!

Does the A / B / ML / GL etc make up only 10% of MB sales