Is it ok to have different size tyres?

dellas1979

Went to get 2 new front tyres today.

I rang first and asked if he had 185/65/14R size tyres (needed 2 new front ones).

Anyways, he said he did, but when I went in, he said, "look, I checked the inventory list and it says that I have 2 of those tyres, but I cant find them".

He said that he could offer me 2 185/60/14Rs (5 millimeter difference). He said it wouldnt make a difference that the 2 tyres in the front were 60 and the 2 on the back were 65 (the 2 back ones need to be replaced also in the next 2 weeks or so, but monetary constraints meant that I could only get the front 2 changed today).

So, I said ok, if you are sure go ahead. I paid €100 for the 2 tyres (no tracking - €25 extra and man wasnt there to do it).

I dunno though - was he having me on? I feel slightly taken for a mug. Are 185/60/14R suitable for a Mazda 323 (sporty looking one, 97 reg). Or does it make a difference? My only options now are to get all 4 185/65's (which would mean getting rid of the 2 new ones I just paid for) or getting the back 2 also 185/60's which I am not sure suit the car.

I dunno, maybe its just because I dont understand fully what is the right thing to do. Can anyone shed any light or am I just being over paranoid?

Thanks

L-M

LOTS of cars have different size tyres on the front and back. As long as the two on the front and the same, and the two on the back are the same then ur alright. He sounds like a bit of a time waster though, i wouldn't be giving him ur custom for ur next set. And maybe for the next set get them same as the front? Might level things out lol:D

245

Your speedometer will read differently with the new tyres - it'll read faster than you're actually going as the tyre diameter is lower - difference will be marginal in your case though - maybe 2 mph at 60 mph...

dellas1979

So what should I do?

Would it be better to get the 60's on the back also?

I am more concerned that those 185/60's are not suitable for a Mazda 323. Should I get all replaced to 185/65's?

I dont really understand what you are saying about the speedometer. Can it damage the car?

unkel

As 245 said, your speed will show higher than actual speed. The difference is about 3%

See this handy calculator

L-M

dellas1979 wrote: »

So what should I do?

Would it be better to get the 60's on the back also?

I am more concerned that those 185/60's are not suitable for a Mazda 323. Should I get all replaced to 185/65's?

I dont really understand what you are saying about the speedometer. Can it damage the car?

Ok i am not a mechanic and was givin my two cents, also i;ve never heard anything about different speeds with different tyres. I really don't think the tyre size will make a difference. I had 15 inch wheels on my car with i think 60 tyres. I now have 17" wheels with 40 tyres and there's no difference in speed?... I think ur worrying too much.

listermint

As the guys said it will offset the speedo as you are changing the rolling distance of the wheels, However the difference is negligible dont worry about it.

245

If it was me, I'd put two more of the same on the back just to make life easier if you're swapping them around in the future. Only thing to remember is that if your spare is the old size it may be an issue if that's checked in a future NCT, especially if you've had a puncture in the meantime and left the spare on the car.

MarkR

The tyres won't change your speed, but they will cause your speedometer to be slightly inaccurate. As mentioned in the link below with the sizes you've gives, when your speedo says 60 mph you'll actually be travelling at 58.1 mph. I assume that the speedo gets the speed from the revolution of the front wheels?

Tommy_NDX

The speedo probaly gets the speed from the final drive gear in the gearbox which is directly related to wheel revolution. So smaller tyre means quicker wheel revolution and slight difference in speedo reading

Esel

MarkR wrote: »

The tyres won't change your speed, but they will cause your speedometer to be slightly inaccurate.

So, they will change your speed then!

Tommy_NDX

Basically yes they do. just drive 62mph and youll be doing 60

-Chris-

Between them telling you they had the right tyres and then not having the right ones, to supplying you with the wrong size without explaining the implications, to not having their tracking guy available... I'd call that pretty shoddy service and I'd certainly never go back there again!

Did your tyres definitely need to be changed today? Were they at a stage where they were unroadworthy and couldn't be used, not even until the correct size was ordered in?

In terms of your tyres, the 185/65/14R designation means that the tyres are 185 millimeters wide across the thread and that they're made to fit a 14" wheel. The 65 is the profile of the tyre (how high the side wall is) and it's a percentage of the width of the tyre.
A 185/65/14R will have a sidewall that's 120.25mm tall (185mm x 65%).
A 185/60/14R will have a sidewall that's 111mm tall (185mm x 60%).
This change in sidewall height results in a change in the rolling circumference of the wheel, necessitating a recalibration of the speedo.

My preference would be that the front and rear tyres had the correct size (and generally the same size) as the manufacturer specified. If I were in your situation and looking to change two tyres, I'd be more focussed on returning the front tyres to the correct size rather than changing the rear tyres so that all your tyres are "wrong".

Mr.David

The amount of time and engineering that goes into selecting a tyre for a new car to market is staggering. Even the differences between brands held in equally high esteem is huge, with different tyres suiting different versions of the same model better (i.e. mainly as a result of varied mass from different powertrains). To get back to the point I would generally try to fit the OE spec tyres to my car (the same brand) and most certainly the same size! However seen as your car is not new the suspension characteristics will have changed to a degree to negate everything I have said above. Hmm......:rolleyes:

crosstownk

Get the right tyres. It's really as simple as that. They are last connection between you and the ground. If one place doesn't have them then try another. If your car is not designed to have different sized tyres, then don't mix the sizes.

maidhc

If your car has abs the computer should throw a hissy and the abs light should come on! And also I guess abs would be disabled.

Balfa

maidhc wrote: »

If your car has abs the computer should throw a hissy and the abs light should come on! And also I guess abs would be disabled.

maidhc, what you say makes sense in theory (as it would for stability control, etc.) but are you aware of this actually happening in reality? I wouldn't be surprised if the computer wasn't able to sense the difference through normal driving and adjust accordingly, or maybe it doesn't even care if two wheels are rotating only 3% quicker than the others?


To the OP, There are going to be lots of technical differences with your tyres, but when it comes down to it, I have two points to make.

1st: If I was stuck with a car in the same situation, I wouldn't have any quams owning, driving and maintaining it. It's just no big deal.

2nd: The technician misinformed you. He may not understand how tyre sizes work, but it's not just a 5mm difference, it's a total wheel diameter difference of almost 20mm. As others have said, it'll mean your speedo is overestimating your speed by 3%, but not only that, your odomoter will be clocking 3% more km on your car than before. So when you go to sell it when it reads 200,000km, it'll really only have done under 194,000. Not a big difference, maybe. But the mechanic has effectively marginally reduced the value of your car, and he should have made you aware of this. If you only got them today, I'd call him tomorrow and complain and ask that he put the right size on.

maidhc

Balfa wrote: »

maidhc, what you say makes sense in theory (as it would for stability control, etc.) but are you aware of this actually happening in reality? I wouldn't be surprised if the computer wasn't able to sense the difference through normal driving and adjust accordingly, or maybe it doesn't even care if two wheels are rotating only 3% quicker than the others?

I had 195/55s on the front of a car a while back and 195/65s on the back (bigger difference, granted), and that is what happened! Once i put the correct tyres on all 4 wheels peace broke out again.

Mr.David

What type of ABS do you have? is it a casselated ring that is on the driveshaft. If so the ABS would operate independently of tyre size and would not detect a fault but would not work correctly either as it would work on a speed calculation that is out by 3%.

-Chris-

Mr.David wrote: »

What type of ABS do you have? is it a casselated ring that is on the driveshaft. If so the ABS would operate independently of tyre size and would not detect a fault but would not work correctly either as it would work on a speed calculation that is out by 3%.

Would the fact that the front wheels are continuously rotating faster than the rear wheels leave the ABS system to presume the rear wheels are partially locking up or the traction control system to presume the front wheels are continuously losing traction? The ABS compares all 4 wheels.

Mr.David

AudiChris wrote: »

Would the fact that the front wheels are continuously rotating faster than the rear wheels leave the ABS system to presume the rear wheels are partially locking up or the traction control system to presume the front wheels are continuously losing traction? The ABS compares all 4 wheels.

Yes you're right, my comment wasnt thought out enough! The other issue is that you have changed the front ride height which will in turn change the roll centre etc and the balance of roll/yaw/pitch. And the front suspension linkage will of course have been tuned to the correct tyre size. If it were me I would insist on him changing back to the correct tyres.

-Chris-

Mr.David wrote: »

If it were me I would insist on him changing back to the correct tyres.

+1,000

Make the call first thing tomorrow.

samih

OP, worry not. Your new 60 profile tyres have a greater diameter than your old worn out 65's. New tyres generally have approx 8 mm of thread so the new tires are 16 mm taller than fully worn out ones. If the previous posters are correct about difference of 5mm between 60 and 65, your new tyres are actually much bigger than the old ones.

The same about ABS. It has to have some leniency towards tyres in different state of wear. About speedo. Many cars show at least 5 km/h more in speedo compared to the real speed. Just dial in +5 mph when you drive and you'll be grand.

For your peace of mind please get 60's for your new rear tyres.

Balfa

samih wrote: »

OP, worry not. Your new 60 profile tyres have a greater diameter than your old worn out 65's. New tyres generally have approx 8 mm of thread so the new tires are 16 mm taller than fully worn out ones. If the previous posters are correct about difference of 5mm between 60 and 65, your new tyres are actually much bigger than the old ones.

Crazy talk. The only person who said it was 5mm was the person who sold the tyres. As I said, the true figure is actually almost 20mm, which is a bigger difference than your old-new compensation. And your compensation assumes the OP drove the old tyres until they were completely bald, rather than changing them with maybe 1 to 1.5mm left on them. And what happens when these tyres wear and lose those ~14mm anyway?

OP, get the right size fitted.

Run_to_da_hills

It is highly illegl to mix up cross ply and radial tyres on a car. Cross ply is still manufactured and is very popular with larger commercial and classic vehicles.

Cyrus

Limerick-man wrote: »

LOTS of cars have different size tyres on the front and back. As long as the two on the front and the same, and the two on the back are the same then ur alright. He sounds like a bit of a time waster though, i wouldn't be giving him ur custom for ur next set. And maybe for the next set get them same as the front? Might level things out lol:D

generally cars with wider rears not same size wheels all round

Dirty_Diesel

IT may not have any effect on the mazda in question but a friend of mine had 205/45/17 and 225/45/17 on the front/back of his PD150 golf and it was downright dangerous to drive. Then abs didn't know what to do when he braked and the esp system didn't know what to do when cornering hard :eek:

Esel

Run_to_da_hills wrote: »

It is highly illegl to mix up cross ply and radial tyres on a car. Cross ply is still manufactured and is very popular with larger commercial and classic vehicles.

That's a bit of a red herring in this thread though!

VH

AudiChris wrote: »

A 185/65/14R will have a sidewall that's 120.25mm tall (185mm x 65%).
A 185/60/14R will have a sidewall that's 111mm tall (185mm x 60%).

only when both tyres are new - old tyres had probably worn down 5mm

op - dont worry about it - the difference is so small that it wont make any difference

nct dont care as lots of cars have space saver tyres now

VH

Mr.David wrote: »

What type of ABS do you have? is it a casselated ring that is on the driveshaft. If so the ABS would operate independently of tyre size and would not detect a fault but would not work correctly either as it would work on a speed calculation that is out by 3%.

has the op got abs?

colly10

Limerick-man wrote: »

Ok i am not a mechanic and was givin my two cents, also i;ve never heard anything about different speeds with different tyres. I really don't think the tyre size will make a difference. I had 15 inch wheels on my car with i think 60 tyres. I now have 17" wheels with 40 tyres and there's no difference in speed?... I think ur worrying too much.

Of course it makes a difference, the difference is generally small enough though and the speedo is wrong with the stock tyres anyway, chaning to smaller tyres will probably make the clock speed more acurate