I was looking at replacing my 7-1/2 year old tyres in stages, say one axle now and the other axle in a few months time to spread the cost. Checking on AI (I know but I did) it told me this was not a good idea. The current tread depth is 5mm and the new would be 8mm so different circumferences. I'm loath to scrap tyres with plenty of tread left but thinking about the age deterioration. If I'd bothered to read the Owners Manual it apparently would have told me it's not a good idea - but who reads the OM from cover to cover - besides I'd had an Impreza before and not worried about it. Below is what AI said - anyone got any comments or experience about this?

For an automatic Subaru Impreza, the recommendation to put new tyres on the rear axle still applies for safety, but being an automatic (usually equipped with a Transfer Clutch or Multi-Plate Transfer system) makes your choice of tyres even more critical than a manual. 

1. The Automatic AWD Conflict 

Unlike some AWD cars that only engage when slipping, your Subaru is likely always sending power to both axles. 

• Clutch Pack Damage: In an automatic Subaru, the centre differential uses a clutch pack to distribute power. If the front and rear tyres are different diameters (even by a few millimetres), the clutch must constantly "slip" to compensate for the different rotational speeds.

• Overheating: This constant friction generates heat, which can cook the transmission fluid and eventually lead to a failure that feels like a "binding" or "shuddering" when you turn corners. 

2. The "2mm" Tolerance Rule

Subaru and many specialist workshops strictly enforce a 2mm (or 2/32") tread depth tolerance between all four tyres. 

• If your old tyres have significantly less tread than the new ones (e.g., they are at 3mm and your new ones are 8mm), you are well outside this safety zone.

3. Recommendations for Your Impreza

If you are set on only buying two tyres, consider these steps to protect your transmission:

• Measure First: Use a tread depth gauge on your remaining tyres. If they are within 1.5mm–2mm of the new tyre's depth, you are likely safe.

• New Tyres on Rear: As mentioned, put the new pair on the rear axle to maintain vehicle stability and prevent oversteer in wet UK weather.

• Match the Specs: The new tyres must be the exact same brand and model as the ones staying on the car. Different brands can have different rolling circumferences even if the size listed on the sidewall is identical. 

I certainly agree with the statement that AWD / 4WD vehicles need to have exactly (as near as possible) the same rolling diameter for all four wheels.....as does any vehicle that has ABS and Traction Control essentially.

I would be tempted to keep the other tyres if there is still loads of wear left in them and plant them on the car when the others reach a certain level or are damaged in some way

I know what you're saying but I didn't keep any of the tyres - too much stuff round the house already !!

Went for 4 x Bridgestone Turanza 6 and they feel OK (especially after I corrected the tyre pressures put in by the fitters - 41psi and 35psi on the front axle?) Seems to have good straightline stability. Price of tyres seems to have increased 25% in the last 6 years . . .

Left the existing TPMS sensors on the front (noticed they didn't bother to change the valve stems though) as sensors were OK still. The same place fitted a non-OEM sensor on one of the back wheels and it reprogrammed fine.

Should be good for a while, fingers crossed . . .

Good result - apart from the odd 'human error' glitches by the fitters.
I would as a precaution run a torque wrench around all the wheel bolts to ensure they are not too loose or especially too tight where they used the nutgun

Good thinking - the place I usually go to always finish with a torque wrench (hopefully set correctly) but this new place which is more convenient - the jury is out on. I did check all the tyres were fitted with the 'outside' actually outside. I don't think they changed the tyres around so the TPMS sensors are in different positions either.

Well, checked the wheel nuts all round - torques were all over the place from too tight to too loose - reset all to 120 NM as recommended by Subaru.

Also found out that the TPMS sensor they fitted and is recognised by the system was a Hamaton Hybrid EU , not sure which model but maybe Hamaton EU-PRO Hybrid 3,5 HTS-A68BA which looks suitable?

On 4/26/2026 at 11:33 PM, norfolkRX said:

I was looking at replacing my 7-1/2 year old tyres in stages, say one axle now and the other axle in a few months time to spread the cost. Checking on AI (I know but I did) it told me this was not a good idea. The current tread depth is 5mm and the new would be 8mm so different circumferences. I'm loath to scrap tyres with plenty of tread left but thinking about the age deterioration. If I'd bothered to read the Owners Manual it apparently would have told me it's not a good idea - but who reads the OM from cover to cover - besides I'd had an Impreza before and not worried about it. Below is what AI said - anyone got any comments or experience about this?

For an automatic Subaru Impreza, the recommendation to put new tyres on the rear axle still applies for safety, but being an automatic (usually equipped with a Transfer Clutch or Multi-Plate Transfer system) makes your choice of tyres even more critical than a manual. 

1. The Automatic AWD Conflict 

Unlike some AWD cars that only engage when slipping, your Subaru is likely always sending power to both axles. 

• Clutch Pack Damage: In an automatic Subaru, the centre differential uses a clutch pack to distribute power. If the front and rear tyres are different diameters (even by a few millimetres), the clutch must constantly "slip" to compensate for the different rotational speeds.

• Overheating: This constant friction generates heat, which can cook the transmission fluid and eventually lead to a failure that feels like a "binding" or "shuddering" when you turn corners. 

2. The "2mm" Tolerance Rule

Subaru and many specialist workshops strictly enforce a 2mm (or 2/32") tread depth tolerance between all four tyres. 

• If your old tyres have significantly less tread than the new ones (e.g., they are at 3mm and your new ones are 8mm), you are well outside this safety zone.

3. Recommendations for Your Impreza

If you are set on only buying two tyres, consider these steps to protect your transmission:

• Measure First: Use a tread depth gauge on your remaining tyres. If they are within 1.5mm–2mm of the new tyre's depth, you are likely safe.

• New Tyres on Rear: As mentioned, put the new pair on the rear axle to maintain vehicle stability and prevent oversteer in wet UK weather.

• Match the Specs: The new tyres must be the exact same brand and model as the ones staying on the car. Different brands can have different rolling circumferences even if the size listed on the sidewall is identical. 

Good choice to replace all tyres so the tread is essentially equal across all 4. Also, same brand and model too, is important. I know it's a right pain if you get a puncture on a only part worn tyre that picks up an unrepairable puncture, but that's the cost for having a capable permanent all wheel drive motor. This is especially true if you're thinking of keeping the car for years, or you're worried about hidden long term damage the next owner might encounter.

When I bought my first OB, I did consider doing a 5 wheel rotation plan, adding the spare into the mix every rotation, just in case I got one of those punctures. But honestly, as for half of the year my car has a set on directional winter tyres it wasn't really a practical choice.

I think tyres, are one of the most important safety parts on any motor vehicle. Best not to skimp ever.