I have been working with a client and as a bonus he has a new set of high end Pirelli Tires he wants to give me. The are size 235-55-17. Mine are currently 235-50-17. I know the speeds will shift slightly ie: 40 becomes 41.4 and 60 become 62.11 and so forth. I also understand that the rmp of the tire will change and consequently has a minor impart on the transmission shift points. Has anyone made this type of switch and is there any negative downside or does it play with the ecm or tcm in any strange way?

Thanks

Tom in Dallas/Plano

2005 S-Type 30.0 91k

 

The diameter difference is 3.33%. It is absolutely not recommended - possibly even illegal - to go beyond 2%.

 

>The diameter difference is 3.33%. It is absolutely not recommended - possibly even illegal - to go beyond 2%.

Foo...

I did, effectively, that exact same thing. I replaced the 18" wheels on my car with a set of stock XF 18" wheels and found that XFs used a slightly higher profile tire (don't have the numbers, but it was very close to that).

My speedo now reads low, compared to the original, but my actual speed, as measured with a Garmin GPS, is now much closer to the posted speeds.

Stock speedos read high, higher profile tire, within limits..., will only make them closer to actual speeds.
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2015 JCNA National Slalom Champion Class M
Current: '08 Jaguar S-Type 4.2 "Satin Edition" (250.06 whp / 259.67 torque)
Past: '05 X-Type 3.0/auto Jaguar Racing Green

 

Oh, the XF wheels I got came with stock XF tires already installed.
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You dreamed of a big star -
He played a mean guitar -
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So welcome to the machine

 

Can you explain why? What is the downside?

Thanks

Tom

 

I am not an engineer. I suggest you do some search for wheel diameter change and read the articles you find. Decide as you wish.