My 2010 RX450 is currently running 235/55/19 101/W tyres and is ready for a new set (at least on the front).
My driving style is occasionally hard and I notice that when pushed the front appears to be scrabbling for grip.
Replacement tyres at this spec are £193 fitted each.

So, having done a little investigation I've found that;

If I change the tyre and profile to 255x50x19 it will give me a rolling radius difference of 0.5% i.e. indicated speed will be 0.5% more than actual speed (232.82cm becomes 231.72cm).
I understand this to be an acceptable threshold when considering a tyre uplift.

Additionally, this tyre puts you into land rover territory which means that the tyres are much cheaper £160 fitted for Yokos, and if you're that way inclined you can get a budget tyre for £110 (I would imagine that the budget has a grip rating very similar to teflon so I'm not wanting those).
I do acknowledge that there would be additional resistance from the road that could affect how quickly the electric is operating for but for the size of the car I'm thinking that the tolerances would be less than noticable.

The added bonus is that it would give me a little more rubber contact on the road and potentially assist with traction under my graceful driving.

As I've scoured the internet and cannot find anyone that has done this before, does anyone know if the 255 will even fit in the wheel arch and does anyone know of any other pitfalls that I've not considered here?

Thanks in advance,
Steve.

Well, I've spoken to Lexus service manager. Naturally non-commital and stated that whilst the additional width will probably fit, he'd not recommend it for many reasons, but could only name insurance. I'll check further.

I've been quite reassured about the width in the wheel arch as the following sites confirm;
wheelfire.com (yanky site) state that the 2010 RX450h can be fitted with a 275x45x20 or a 265x35x22.
performancealloys.com (UK site) also support the 2010 RX450h up to 275 on a 20"
Think I'm set at that.

Will check insurance tomorrow which just leaves the question of whether the 19" rim will accept the tyre.
The main issue is that at this point I'm not sure of the rim width on the current 19"? Anyone?

OK, I've now found reference of someone on clublexus.com forums who has already done it however they've gone 275x55x19 (3% difference to speedo reading) on standard 19" rims.
Think I'm happy with the choice, just need to confirm insurance tomorrow.

This is e really good site for working out the difference in rolling radius for different sized tyres http://www.rimsntires.com/specs.jsp
I used it when calculationg what sized winter tyres I could fit on my RX400h

Right, checked with insurance and all is well.

However to qualify that I've completely thought about this, I've tried another size site (as per the last thread (many thanks Lydiamight!), which shows less of a difference between the two tyres (7mm instead of 10mm) which even further re-inforces my thoughts.

The final thought that occured to me was that changing sizes would be fine on a 2WD however those keen observers out there may have noticed that I actually only need front tyres. This means that rear drive operating at the same speed as front drive will work against each other (rear running too quick).

I have additionally thought about this and think that this is still acceptable for one reason.
At present my front tyres are down to the wear indicators (At least the wear indicator is still flat with the tread so it must be still borderline )
Now that point aside, the rear tyres still have good tread left and my contention is that the lost tread is probably tantamount to the same if not more than 7 or even 10mm rolling radius that the change in tyres will provide.

Clearly this is only a short term issue until the rears wear down and are changed too.

Anyway, not necessarily recommending this way of working, but at least its out there for consumption and its short term.

Ok so new Avon coopers ordered for tomorrow @170 fitted. These tyres were recommended by the garage have had good reviews including clarkson for topgear on a rangerover and whilst I don't rate his technical abilities any higher than my hamster, I reckon he can probably tell a decent tyre. The garage have convinced me to switch the old rears to the front and to have the new wider tyres on the back. Works for me.

[quote name='Hayabusa' timestamp='1312911190' post='760892']
Well, I've spoken to Lexus service manager. Naturally non-commital and stated that whilst the additional width will probably fit, he'd not recommend it for many reasons, but could only name insurance. I'll check further.

I've been quite reassured about the width in the wheel arch as the following sites confirm;
wheelfire.com (yanky site) state that the 2010 RX450h can be fitted with a 275x45x20 or a 265x35x22.
performancealloys.com (UK site) also support the 2010 RX450h up to 275 on a 20"
Think I'm set at that.

Will check insurance tomorrow which just leaves the question of whether the 19" rim will accept the tyre.
The main issue is that at this point I'm not sure of the rim width on the current 19"? Anyone?
[/quote]
Hi all,
In France, this change is very regulated (technical features of the car are mentioned on our car card or what we call "[i]carte grise[/i]" which is a card given by the constructor, we pay a lot of money for luxury cars but a reduced price or nothing in some french areas due to low emission or just because it is a hybrid car) and when the difference between 2 tire styles is above 3%, this requires expert and insurance agreements but, when I've questioned Lexus France (Toyota services) and my Lexus/Toyota dealer for mounting 235/55/18 (winter tires coming from my former 400h) to my 450h which is equipped with 235/55/19 but which accepts 235/60/18 (the first price model), the difference does not match with Lexus France acceptance and they recommend not to do this... conclusion: I've bought winter tires only in 235/55/19 which I will change around end of October once temperatures will decrease to 5-7°c .

Note: in case you have an accident, experts are very prudent with tires which are not exactly the same than the constructor requirements... insurance tend to show the victim that your cars is not compliant with initial technical data and therefore claim that this is not mentioned in your insurance policy, bla bla bla.
I do not know how much are your tires in your country/area but in France it is quite expensive. Best prices are obtained via Internet and the ones bought yesterday (Vredestein Wintrac 4 xtreme 235/55 R19 105V XL) 219,80 € each or total: EUR 852.40 including transport to the nearest tire specialist for changing tires).

See my latest consumption of my rx 450h techno-dynamic pack and president pack here http://www.spritmonitor.de/en/detail/455893.html
Al