Tyre sizes

Morning all,

Been trawling the pages and struggling to find a definitive answer... Looking like I need to get a pair of replacement rear tyres, but after just looking at the sidewall markings its confusing the hell out of me... every other car i've owned have been the same size on all 4 corners...

current front : 185/55R15
current rear: 205/50R15

I believe my car is a standard 2003 TF135, can anyone confirm if these sizes are indeed correct?

Thanks in advance.

Andy

Hi Andy,
I can understand your query, it's unusual to have different sizes on a car, but my 04 TF is the same, except for having 16" wheels with different widths front and rear.

Hi Pharg55,

Yeah it is weird.... I can understand different widths, but different profile depths has thrown me a bit and making me question what is correct...

Thanks again

Andy

My understanding of ex-factory sizes:

Front: 185/55R15
Rear: 205/50R15

The radius is slightly different front and back whether fitting 15" or 16". I know this as when I set up my race logic traction control, I had to input wheel sizes and there is a small radius difference between front and back when you key in the above tyre sizes.

OK.......A tyre that is 185/55R15 is 23.01 inches in diameter, has a circumference of 72,29 inches and will do 876.42 Revs to the mile.
and a tyre that is 205/50R15 is 23.07 inches in diameter, has a circumference of 72.48 inches and will do 874.18 Revs to the mile.

The 1st figure of the tyre size is the width, the next is the aspect ratio and the last is the wheel diameter.
The aspect ratio is the sidewall height relative to the width......... in this case 55% of 185mm compared to 50% of 205mm
So as the rear tyre is wider it has a lower aspect ratio to keep the overall diameter about the same as the narrower front tyre with the higher Aspect ratio.
In MG's infinite wisdom, they decided to have different widths front to rear (not unusual on mid engined cars) they used different aspect ratios to keep the same overall wheel/tyre diameter............as clear as mud!

OK, I'm kinda getting there!!!

So in "non tyre buff" terms, these sizes are correct?

Would have been so much easier if the previous owner had just kept the bloody handbook! :lol:

Cheers

Andy

What Cobber said!

It is completely correct for the TF to have wider tyres at the back than the front.

The definitive guide, on sizes at least, can be found on David Aiketgate's post here .

The tyre information is a bit out of date but suffices to say that the TF is allegedly very fussy about its tyres. Current favourites are the Falken Ziex ZE310 or Toyo Proxes TR1. Other tyres have been reported as working well.

Cue lots of people citing their own particular brand of tyres...

I personally have Falken ZE914 on the front and Event Ditchfinders on the back which have served perfectly well for my style of driving but this does not constitute a recommendation!

Glad that's sorted, now can someone explain why virtually the whole world drives on tyres measured in metric for width but imperial for wheel diameter!!!!!!!!
I know BMW did a metric diameter wheel at one time but I don't think it caught on, have any others tried?

Yes, lots. Michelin TRX tyres were used on Citroen CXs and Saab900s and I think that there was also a metric wheel fit for the Metro, long ago. Mixing imperial and metric happens everywhere. In aviation, runway lengths are given in metres but height, altitude and flight level are always in feet.

I have since read Ferrari wheels are metric, now all I have to sort out is why cocaine is sold in grams and weed in ounces must be a bugger for the dealers having to lug two sets of scales around

Yeah Ford Australia tried metric wheels on their LTD luxo-barge as did Jaguar on the XJ40 and Renault on the Feugo to name some that I can recall
Apart from Michelin I don't think anyone else bothered making tyres in metric.
The problem is that no one wanted to have to carry a whole lot more inventory, in order to supply metric sizing........ and really who needs it, the system works well enough

Having different size tires front and rear is normal for a mid or rear engine car, it has to do with the handling characteristics of a mid engine or rear engine car, which tend to oversteer and the tire size choice on the F/TF promotes understeer (safer). Yes, it was wisdom on MG's parts.