Tyre dimensions

[Winghui Pang]

Does anybody know why on the 2016 Lexus RC 300h f sport that the tyre profile is different. Why on the back is 35 and the front is 40?

[steve2006]

There are various options for the wheel sizes including both diameters and widths, the rear driven wheels tend to be wider than the fronts and therefore the tyre tends to be wider. To ensure the rolling radius is the same with different tyres on the front and rear the aspect ratio (35/40) is different.

This is known as a “Staggered Wheel” set up and is common amongst more “ sporty” cars.

Looking at the list below 19x8 is a 19” diameter rim 8” wide, the next option is 19 x 9, same diameter but 9” wide.

So the reason why you have different tyres is probably because you have wider wheels on the back, what are the other figures off the tyres, the first 3 give the tyre width in mm. For example 245/40/18.

[Rabbers]

The fatter rear tyres on the 2016 RC F-Sport are a feature carried over to the 2019 model also.  As in all staggered tyre size set-ups, the wider

tread on the drive wheels provides more grip in acceleration and better handling and cornering.  The fatter rears also look better - sportier and

more purposeful -  and I suspect this factor was at least as important in the RC's designers' thinking as the better handling.  I once drove a non-

F-Sport RC with uniform-size 18" wheels for about 25-30km in fluid town traffic before getting back into my own F-Sport immediately afterwards,

and I can confirm that the stability in cornering was decidedly inferior, though it should be kept in mind that the F-Sport is also favoured by the

adaptive suspensions.

[ColinBarber]

8 hours ago, Winghui Pang said:

Does anybody know why on the 2016 Lexus RC 300h f sport that the tyre profile is different. Why on the back is 35 and the front is 40?

You have to remember that the person who came up with the tyre measurement standard was an idiot and made the width figure an absolute measurement but the profile figure a percentage.

On the front you have 235/40 tyres and the 235 means 235 mm width which makes perfect sense. The /40 however, which is the height (profile) figure, doesn't mean 40 mm as most people think; it is 40% of the width of the tyre. So for a 235/40 tyres the profile is actually 94 mm.

Lexus wanted a wider tyre on the rear so went with 265 mm. This is mainly looks, and to match the RC350 (the RC300h doesn't produce enough power to really need that extra width). If they went with 265/40 the profile would be 106 mm and the wheel/tyre would be noticeably larger. So to keep it the same the matching size is 265/35 which has a profile of approx 93 mm.

[NemesisUK]

The lower profile also provides a stiffer sidewall reducing side to side movement or squirm giving a tighter control on that end of the car. Reduced tendancy to oversteer.

[ColinBarber]

9 minutes ago, NemesisUK said:

The lower profile also provides a stiffer sidewall reducing side to side movement or squirm giving a tighter control on that end of the car. Reduced tendancy to oversteer.

But the point is it isn't lower - the front /40 and rear /35 are the same (or as close as you can get with the limited tyre size options).

[NemesisUK]

3 minutes ago, ColinBarber said:

But the point is it isn't lower - the front /40 and rear /35 are the same (or as close as you can get with the limited tyre size options).

And the rolling circumference is the same. If one simply increased the width one would upset that balance and the handling.

[Winghui Pang]

Thanks for all of the replies. It gave me a better understanding of the wheel setup on the RC now.