For those tech savvy folk out there

So the xk cones fitted with 3.31 diff ratio and 285-30-20 rears

Now the questions is if I fit up 295-25-20 tyres on the rear the rolling diameter will be reduced from 26.73" to 25.81" will this improve my acceleration as technically I have now shorten my final Ratio

 

Out-the hole acceleration will be quicker, with upshifts occurring at the same engine RPM, which will again help the acceleration.
Thing is, 3.56% isn't a whole heck of a lot of a difference. While it would be measurable with the right equipment, the actual performance won't change much.
If you go with an even smaller diameter tire it would make a larger difference, but how small can it get with wheels big enough to fit the brakes and also have a small tire fit that smaller wheel, all while keeping the car from scraping the ground...
It would basically make your 3.31:1 into a3.42:1 with no differential mechanical work.
Not much change there.

 

Thanks Cee jay I think I'll pull the trigger and get the 295 25 20 any performance is beneficially on a n/a 4.2 !!

 

I forgot to figure in PI, but the ratio would be the same as pi-over-pi is still one.
Hope you get what you need from the purchase.

 

Thanks Cee Jay

Yes anything to make that glorious n/a 4.2 pull that little bit faster is the goal

 

Seriously I would not bother with this, there are better ways to gain a couple of % improvement.

If you're looking at wheels/tyres then you will get a more noticeable benefit from switching to 18" wheels.

 

I'm In need of rear tyres and was thinking that I could kill two birds with the one stone

 

I'd bet you'd lose acceleration, due to less traction.

 

The short answer is no. It will only affect your speedometer which will run faster because of the smaller diameter. Tirerack has a chart that will tell you your speedometer error. The higher the rear gears the quicker off the line or different gears in the transmission for better low end and last more torque will do the trick.

 

Another thing to look into is when changing the diameter dime card then have issues with both the ABS and traction control now that the rear tire still be spinning at a different rato than the fronts... not sure if Jags will encounter that but other cars sometimes do

 

Another thing to look into is when changing the diameter some cars have issues with both the ABS and traction control now that the rear tire still be spinning at a different rato than the fronts... not sure if Jags will encounter that but other cars sometimes do

 

The short answer is actually "Barely".
Plus, all the speedo and ABS stuff.

 

Big Henderson
Bald 285 vs new 295 I don't think traction will be compromised !

 

Remember, on a dry road best traction is from a slic, traction from a tread comes only in adverse conditions.
Regards,

 

Less sidewall isn't it?

 

That's correct the side wall profile goes from a 30 to a 25 however the tread width goes from 285mm to 295mm

 

85.5mm sidewall to a 73.75 sidewall.

 

Right, this is what I was saying. Sidewall height has a lot to do with traction