Mismatched tyres

[rf065]

Hello,

At the risk of offering an egg sucking lesson,tyre sizes are as follows,using one of mine as an example.

195/70/R14

Width of the tread(mm)/cross section of the tyre(mm)ie the distance from the rim to the tread/rim diameter(inches)

Regards,
Nick.

Sorry, 195 is indeed mm & the width of the tread.

70 stands for 70% of the width, i.e not 70mm but 70% of 195mm, which is 135.6mm I think & 14 is the diameter in inches.

Russ

 

[Number_Cruncher]

It would appear that a space saver will fail an MOT if fitted to the car, how can it be legal to drive on the road then?

Russ

Although there is quite a lot of overlap, there is no direct correlation between MOT requirements and Construction and Use regulations.

It's entirely possible to have a motor car which correctly passes a properly carried out MOT which is then subject to prosecution via Construction & Use regs if used on the road.

It is unusual to find something where the MOT would be correctly failed and not also contravene C&U regs, however, I thing that the get out is that space saver tyres are allowed under strictly controlled conditions of being below a maximum speed, and for temporary emergency use only - presenting the vehicle for an MOT can't really be considered an emergency!

 

[brandwooddixon]

Hello,

Although the tyres are mismatched,the difference is only the width,not the circumference,so there will have been no effect at all on the differential.

The whole purpose of a differential is to allow the driven wheels to rotate at different speeds,so even if there had been a difference in the circumference,it would have coped with it.

At the risk of offering an egg sucking lesson,tyre sizes are as follows,using one of mine as an example.

195/70/R14

Width of the tread(mm)/cross section of the tyre(mm)ie the distance from the rim to the tread/rim diameter(inches)

Regards,
Nick.

May I point out that they are in fact a different different diameter and therefore a different circumference. The profile is an indication of the "thickness" of tyre from rim to tread as a proportion of the thickness.

Width 225 profile 45 = 101.5
width 255 profile 45 = 114.75

Theoretically thats almost an inch difference in height.

Or put it another way about 87 mm larger circumference.

 

[television]

May I point out that they are in fact a different different diameter and therefore a different circumference. The profile is an indication of the "thickness" of tyre from rim to tread as a proportion of the thickness.

Width 225 profile 45 = 101.5
width 255 profile 45 = 114.75

Theoretically thats almost an inch difference in height.

Or put it another way about 87 mm larger circumference.

I am running wheels and tyres with the same over size with no ill effect, the speedo is OK and the only thing that I find is the MPG indicates less by about 2 MPG over 100 miles, this was going from 18" to 19" my reason for doing it was to stop the ride from being too hard and to protect the wheels, the tyres were also £100 each cheaper one you get away from AMG sizes

 

[Number_Cruncher]

Rr_225=(17*25.4/2)+(225*45/100)

Rr_225 =

317.1500

>> Rr_255=(17*25.4/2)+(255*45/100)

Rr_255 =

330.6500

>> difference=Rr_255-Rr_225

difference =

13.5000

So, a 13.5mm difference on radius, which is over an inch on diameter.

 

[Enigma AMG]

MY SL500 (2003 registration, 42,000) is currently being serviced by an MB authorised repairer. They have called to say that the o/s rear tyre is showing signs of uneven wear and proposed to fix the steering geometry. In more or less the same breath they told me that the n/s rear tyre is the wrong size
(225/45, should be 255/45). I reminded them that they had fitted the wrong tyre and suggested that was the cause of the wear on the opposite tyre. I also suspect that the diff could have suffered accelerated wear due to the mismatch.

I would be grateful to learn if that analysis sounds right.

I'm proposing to ask for two replacement tyres (not surprisingly) and geometry re-alignment free of change and a promise to fix the diff if it fails in the next two years or so.

I would suggest that your analysis is correct. As two others have already stated the middle figure ie. 45 is a % or ratio of the width to the wall of the tyre so your tyres have a different rolling radius. Search the web for tyre size calculator for info on different sizes and compatability. Those tyres should NOT occupy the same axle as it is potentialy dangerous.
I have owned many cars and bought alot of tyres and have noticed that the stated width ie. the first figure eg. 255 actually varies significantly + or - 5mm between manufactures and tyre types so despite the info on the tyre the footprint can be different. This is another reason why tyres should always be replaced in like pairs on the same axle.

 

[ncooper]

Sorry, 195 is indeed mm & the width of the tread.

70 stands for 70% of the width, i.e not 70mm but 70% of 195mm, which is 135.6mm I think & 14 is the diameter in inches.

Russ

Thank you for that,I stand corrected and as every day, I have learnt something new.

Regards,
Nick.