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Can any one else with Michelin PS4 tyres (225/40) confirm if the accuracy of your speedometer has changed since fitting?

In my last polo the speed always used to read about 3mph faster than what gps used to read when doing 70mph. As such I always got into the habit of cruising at 72mph to make up for the discrepency and not worry about breaking the speed limit.

Yesterday I noticed that speedometer on my GTi is matched bang on to what my GPS shows so no leeway this time. I'm assuming this is down to the slightly bigger tyres that is eating into the safety net. Can anyone else confirm with these tyres that their speed accuracy has changed?

Also as a bonus question what psi tyre pressures are people running with these tyres? Drivers side door sticky note would say to put 41psi fronts and 38psi rear, these numbers are for two passengers which i very rarely carry more than two. That's a lot higher than my older polo and wanted to check if these numbers are correct for these tyres.

The rolling radius difference between the 2 tyres is 1.26%. By my calculation this would mean an indicated 72mph on 215s would become just over 71 (71.09) with 225s.

I think VW recommended tyre pressures are on the high side following the introduction of WLTP to reduce the tyres’ rolling resistance, which should then have a small positive impact on fuel consumption and emissions.

By way of comparison, VW’s recommended tyre pressures for my previous generation Polo GTI on 17” wheels with two occupants in the car are 38psi front and 35psi rear.

I just drove mine on the motorway (has standard 17 inch alloys with the OEM tyres) and noticed that at my "normal cruising" speed the speedo showed only 1mph more than Waze said I was actually going, in previous cars it's always been a good 3mph out.

TurboTrev wrote: Sat Jun 29, 2019 11:32 am The rolling radius difference between the 2 tyres is 1.26%. By my calculation this would mean an indicated 72mph on 215s would become just over 71 (71.09) with 225s.

You're right. I had considered the increase in diameter when deciding to change mine and considered it well within tolerances.

Of course, the effect is variable, with tyre wear.

The Bridgestones are 7.2mm tread when new and 1.6mm at legal limit. Over the life of the tyre, the rolling diameter is going to decrease 11.2mm, or almost 2%.

A fully worn set of Muchelins is going to have a marginally smaller diameter than a brand new set of Bridgestones, that's how little there is between the 2, and that discrepancy between indicated and actual speed will change with wear too.

As for tyre pressures, I'm going with 33psi unmounted, which is about 36psi mounted and the car's weight bearing down on them. 36-37psi is around the recommended tyre pressures for a pre-WLTP Golf GTI on 225/40 R18 tyres. As the Polo GTI+is only about 40kg lighter than the Golf, I'm happy to stick with that.

WLTP compliant tyre pressures are higher for the sake of maximising mpg, but not particularly good for even tyre wear, noise or comfort.

Do you guys trust the Air machine readings at petrol stations?

Seeing this thread made me want to check the current tyre pressure levels, which I haven't done since they were fitted (MPS4's). Each tyre was reading at around 32-33psi. Seeing the reply from monkeyhanger I put them all up to 36psi.

My father told me to be careful as the machines aren't always accurate so I was planning on buying a digital pressure gauge to double check. Hoping I haven't over inflated them now.

S_94 wrote: Sun Jun 30, 2019 4:26 pm Do you guys trust the Air machine readings at petrol stations?

Seeing this thread made me want to check the current tyre pressure levels, which I haven't done since they were fitted (MPS4's). Each tyre was reading at around 32-33psi. Seeing the reply from monkeyhanger I put them all up to 36psi.

My father told me to be careful as the machines aren't always accurate so I was planning on buying a digital pressure gauge to double check. Hoping I haven't over inflated them now.

I never use the petrol station air machines. I have my own tyre pressure gauge, and a small compressor that I keep in the boot to top up the pressures if necessary.

SRGTD wrote: Sun Jun 30, 2019 4:58 pm

S_94 wrote: Sun Jun 30, 2019 4:26 pm Do you guys trust the Air machine readings at petrol stations?

Seeing this thread made me want to check the current tyre pressure levels, which I haven't done since they were fitted (MPS4's). Each tyre was reading at around 32-33psi. Seeing the reply from monkeyhanger I put them all up to 36psi.

My father told me to be careful as the machines aren't always accurate so I was planning on buying a digital pressure gauge to double check. Hoping I haven't over inflated them now.

I never use the petrol station air machines. I have my own tyre pressure gauge, and a small compressor that I keep in the boot to top up the pressures if necessary.

Yeah that's what I should have done first! Ordered a kit on Amazon for future use.

S_94 wrote: Sun Jun 30, 2019 5:08 pm

Yeah that's what I should have done first! Ordered a kit on Amazon for future use.

Your GTi should have one standard in the boot.

fazzy wrote: Sun Jul 07, 2019 10:02 am

S_94 wrote: Sun Jun 30, 2019 5:08 pm

Yeah that's what I should have done first! Ordered a kit on Amazon for future use.

Your GTi should have one standard in the boot.

The UK GTI / GTI+ gets a space saver spare wheel, not the tyre inflation kit / compressor that some other non-UK markets get.

SRGTD wrote: Sun Jul 07, 2019 10:29 am
The UK GTI / GTI+ gets a space saver spare wheel, not the tyre inflation kit / compressor that some other non-UK markets get.

Here you could choose a tire or kit.
Mine came with spare tire.