Tyre size question

woodhead722 · Post by **woodhead722** » 22 Apr 2005, 23:28

I have a 405 diesel. In the owner's handbook the two size options are 175/70 R14 or 185/65 R14.
I have the 175/75s.
Does anyone know if I could put 2 185/70 R14 on the same axle (rear?)on my existing steel wheels instead as my friend has 2 going cheap!

hill · Post by **hill** » 22 Apr 2005, 23:57

Quite a few cars run different size tyres front and rear as standard. (It obviously doesn't affect the handling of Ferraris and Porsches too much does it) As long as it's not four wheel drive it won't affect anything. What you should not do is mix sizes on the same axle. However, on that front, think what the difference in circumference between a brand new tyre and a part worn one is when you have a punctured tyre replaced. Nobody ever worries about that do they! Or what about variations in tyre pressures. As you're very rarely driving in a completely straight line your wheels are almost always rotating at different speeds. Grab your bargain wheels but do remember that if you alter the size on the driving axle your speedo accuracy may be affected.

flux · Post by **flux** » 23 Apr 2005, 03:46

are you going to carry two different size spares?

woodhead722 · Post by **woodhead722** » 23 Apr 2005, 04:20

Thanks to you both.
I know I should carry 2 spares, but I hardly do any miles and will carry some tyre weld just in case.
I only know the relevance of 14 and not the 185 and 70. It was expained years ago but I cannot remember.

Homer · Post by **Homer** » 23 Apr 2005, 04:41

My first BX was written off. My second needed a couple of new tyres but they were different sizes. I put the wider ones on the front. It made the car a bit oversteery but no other ill effects.
I wouldn't worry about the spare, it's still a lot better than the spacesavers they stick in many new cars.
As for what the numbers mean. the 175 (185) is the tread width, the 60 is the tyre wall height (profile) as a percentage of the tread width. So the 175/70 tyres are 175mm wide and the profile is 70% of that (122mm). The 185/65 tyres are 185mm wide and the profile is 65% of that (120mm). Which is close enough to not affect your speedometer reading noticably.

np · Post by np » 23 Apr 2005, 05:20

As Homer says,i wouldn`t worry about the spare.I had an Astra with 195/60/14`s on,but the spare was 165/70/13.A sierra with 195/60/14,spare was 185/70/13,& my Xantia has 205/60/15,with a spare of 185/65/15.Never had any probs with handling when using any of the spares(i have a lot of punctures!).Although i`ve always took it steady just in case.
Think about most modern cars with their tiny thin space saver wheels.Although speed & distance is limited.

Paulmi16 · Post by **Paulmi16** » 23 Apr 2005, 14:35

You might have a problem with insurance, if the car was ever inspected by an engineer, and also the possibility of the larger tyres fouling the bodywork, especially with a full or heavy load in the boot.
Paul.

Homer · Post by **Homer** » 24 Apr 2005, 01:00

I doubt it will be a problem on either count since the bigger tyres were available as an option on the vehicle as new.
The difference in actual tyre width between a 175 and 185 tyre is negligable, they fit the same rims.
The only downside I had from running the slightly wider tyres on my BX was the steering was a bit heavy (no PAS).
For safety I would put them on the rear, then when you need a new set of fronts check the price of 185s, if the 185s are more common you might find they are cheaper than 175s.[;)]

Homer · Post by **Homer** » 24 Apr 2005, 01:16

A few handy tyre size calculators.
Here,
here,
and
here.

woodhead722 · Post by **woodhead722** » 24 Apr 2005, 04:02

Interesting comments and references, got tyres now thanks.