Differant size tyres on rear axle

One side is 175/65 and the other is 165/70.
I know tyres shoud be the same size but just how dangerous is the size differance between these two?

dougie-lampkin

You have a 3.5mm difference in tyre height between the two tyres, which is (I would say) insignificant. Assuming they're on the rear of a FWD car, I wouldn't be bothered. I was driving a Micra for many months with two marginally different sized tyres on the back after I had to source a new rim, didn't notice any difference in handling or braking.

I'd be more concerned about the thread pattern and wear differences between the two.

shogunpower

if your car is rwd, it will wreck the diff in no time.

dougie-lampkin

shogunpower wrote: »

if your car is rwd, it will wreck the diff in no time.

Why would it wreck the diff? Whether it's an open or locked it won't make a difference. That's like saying driving with both wheels on two different surfaces or having slightly different tyre pressures will wreck the diff

shogunpower

slightly different tyres pressures would also have an effect, as there would be a different resistence on each wheel and in the op's case the diameter of each wheel is different, so the diff has to work over time all the time even on a straight road. it wouldn't wreck it over night but in about 3-6 months the diff would be hammerd, ive seen it first hand with a car that had a 16inch wheel on one side and a 17inch wheel on the other,

RobAMerc

Its vital to have the same thread pattern and size tyres on the one axle. I once bought a R 19 16v which on day one lost the back in a big way at a fairly low speed. I immediately got very worried I had bought a ringer so brought it to a panel beater friend who checked her out and said nope - car is fine. So I rechecked the tyres and bizarrely for the first time noticed the differing rear tyres ( not like me to miss it before purchase )- new tyres made a huge difference and the car gripped superbly from them on.

dougie-lampkin

shogunpower wrote: »

slightly different tyres pressures would also have an effect, as there would be a different resistence on each wheel and in the op's case the diameter of each wheel is different, so the diff has to work over time all the time even on a straight road. it wouldn't wreck it over night but in about 3-6 months the diff would be hammerd, ive seen it first hand with a car that had a 16inch wheel on one side and a 17inch wheel on the other,

I still don't see how it might affect an open or locked diff. It might screw up an LSD's magickery alright.

Plenty of people drive open diffs off-road, where the two tyres will have different levels of grip all the time, and there's no bother with the diff. I can see how long term (i.e., over many years) running two majorly different sized wheels could have an effect, but 3.5mm difference wouldn't matter. An open diff should be fine, since it's designed to only fully power one wheel at a time anyway, its job is to find that difference in traction. And a locked diff will work the same even if you took off a wheel on one side :pac:

Think about it, look at the sidewall flex in your tyres as an example. Unless you're driving on glass, the overall diameter of the tyres will always be different, due to sidewall flex. While my car is FWD, the diff has the same principle as a RWD car. I run 25psi front and 32psi rear, so you can imagine how much flex my front sidewalls have. I've been running these pressures for 2 years now, never had a problem with the diff, still like new.

mickdw

Are you sure these tyres were not swapped off the front axle. Its one way of correcting a steering pull due to a bent chassis.

Esel

If these tyres would wreck the differential, then so would a pair of identical tyres if one had 1.75 mm less thread than the other, no?

gofaster_s13

shogunpower wrote: »

if your car is rwd, it will wreck the diff in no time.

It would just cause a diff to do what a diff is designed to do.

Darsad

ideally you should have the same size tyre on the same axle but there is actually very little difference between the two tyres as the aspect ratio of 65 on a 175 tyre and 70 on a 165 is negiable assuming the wheel size is the same ie 15" or 16" . It wont damage the diff as said the diff will do what its designed to do ie differential , now were they on different axles on a 4wd or awd car then that a problem.

CiniO

I don't think anyone pointed out, that such setup is just illegal.
You need to have the same size tyres over each axle.

If you have an accident OP, I'm sure insurance company is going to take this fack against you, telling you that car with different tyre sizes over an axle is not roadworthy.

-Corkie-

Darsad wrote: »

ideally you should have the same size tyre on the same axle but there is actually very little difference between the two tyres as the aspect ratio of 65 on a 175 tyre and 70 on a 165 is negiable assuming the wheel size is the same ie 15" or 16" . It wont damage the diff as said the diff will do what its designed to do ie differential , now were they on different axles on a 4wd or awd car then that a problem.

Thats true about the diff. It would impair handling though. I often had different tyre sizes on all wheels and you would notice the car handling badly.

Joe 90

dougie-lampkin wrote: »

I still don't see how it might affect an open or locked diff. It might screw up an LSD's magickery alright.

Plenty of people drive open diffs off-road, where the two tyres will have different levels of grip all the time, and there's no bother with the diff. I can see how long term (i.e., over many years) running two majorly different sized wheels could have an effect, but 3.5mm difference wouldn't matter. An open diff should be fine, since it's designed to only fully power one wheel at a time anyway, its job is to find that difference in traction. And a locked diff will work the same even if you took off a wheel on one side :pac:

Think about it, look at the sidewall flex in your tyres as an example. Unless you're driving on glass, the overall diameter of the tyres will always be different, due to sidewall flex. While my car is FWD, the diff has the same principle as a RWD car. I run 25psi front and 32psi rear, so you can imagine how much flex my front sidewalls have. I've been running these pressures for 2 years now, never had a problem with the diff, still like new.

Is that correct, 25psi front and 32psi rear on an FWD? The other way round would seem more like it:)

dougie-lampkin

Joe 90 wrote: »

Is that correct, 25psi front and 32psi rear on an FWD? The other way round would seem more like it:)

It's to allow for some give in the tyres up front, as the car is lowered. Even at standard height the front tyres should be ran at 26-28 psi to allow for tyre flex as a part of the suspension. It's also to correct the understeer that plagues the Mini if the rear of the car sits level with the front. From the factory the rear sits 1" higher than the front, but this in itself causes snap oversteer at the wrong moments due to there being feck all weight over the back. Nothing more terrifying than feeling the rear of the car trying to overtake you when coming down a sliproad :eek:

When I lowered it I decided to make the car sit level front and rear, and this has transformed the car and made it far less twitchy under cornering and heavy braking. The downside being it can have pretty bad understeer unless the tyre pressures are corrected.

During the winter I run ~25 psi all around though and drive slower. This makes it surprisingly one of the best winter cars going, with the skinny tyres and superb road holding

Darsad wrote: »

now were they on different axles on a 4wd or awd car then that a problem.

What makes you say that? A centre differential relies on the same principles as a front or rear diff. I'd be more worried about having two different tyres on the back of an RWD car tbh.

RobAMerc

RobAMerc wrote: »

Its vital to have the same thread pattern and size tyres on the one axle. I once bought a R 19 16v which on day one lost the back in a big way at a fairly low speed. I immediately got very worried I had bought a ringer so brought it to a panel beater friend who checked her out and said nope - car is fine. So I rechecked the tyres and bizarrely for the first time noticed the differing rear tyres ( not like me to miss it before purchase )- new tyres made a huge difference and the car gripped superbly from them on.

Panel beater must have been some genius not to check the tyres first.

dougie-lampkin

dougie-lampkin wrote: »

What makes you say that? A centre differential relies on the same principles as a front or rear diff. I'd be more worried about having two different tyres on the back of an RWD car tbh.

The sh1tty system in Freelanders was prone to disliking uneven tyre wear so in fairness Darsad does have a point