The 4GS OEM offset is +45 mm.
If I change my wheel offset to +35 mm, then the wheels will protrude +10 mm per wheel, and the tracks will be 20 mm wider.

Is the 4GS OEM setup in negative, zero, or positive scrub radius?
If the OEM setup was zero scrub radius, then a 10 mm offset reduction will result in a 10 mm positive scrub radius.

So how does decreasing the +45 mm offset by 10 mm to +35 mm offset affect driving?

Would the steering be a little heavier?
Would the steering have slightly reduced feel?
Would the agility/turn-in/speed of change of direction be slightly reduced?
Would 10 mm less offset affect the terminal grip?
Would 10 mm less offset affect understeer/oversteer?
With a 20 mm wider track, would the vehicle resist body roll a fraction?
Because the fulcrum and leverage is 10 mm longer per side, would the coil springs compress slightly more easily for a slightly softer ride?
With 10 mm less offset per wheel and a slightly longer fulcrum, would this put slightly more stress on the 5 studs, and more stress on all the suspension components, causing more rapid wear of shock absorbers, springs, bushings and suspension arms?

How do we know when we have too much negative offset by law?

 

That’s not how offset measurements work. If you go from +45, to +35, your wheels are not going to be protruding/creating wider track width. The opposite. Also, that also depends upon wheel width and you need to clarify if that’s constant between the different wheels you are considering.

Wheel spacers will keep things consistent unless you are actively trying to change something.

 

Both rims 8" wide.
Unlike the original OEM +45 mm offset, I am pretty sure the new aftermarket +35 mm offset will stick out 10 mm per side, hence widening the track by +20 mm.
So I wonder what the real world consequences are?

 

That is incorrect. Positive offset is inboard of 0, so the closer you get to zero from positive, the farther outboard your wheels will sit. Going from +45 to +35, assuming the same wheel width, will move the wheel outwards.

If you'd like to see what actual track guys say about lowering offset/widening track width, you could review the following threads.

https://grassrootsmotorsports.com/fo.../104358/page1/
https://forums.pelicanparts.com/pors...-handling.html

 

So far, my on-line research yields something like this.

With zero scrub radius, the center of the tires are rotating around the vertical steering axis.
Apparently, zero scrub radius is not stable.
Hence, FWD often uses negative scrub radius.
While RWD often uses positive scrub radius.

Therefore, when wheels move 10 mm outboards for a 20 mm wider track, the steering has 10 mm positive scrub radius on the RWD vehicle.
With [more] positive scrub radius, rather than the tires rotating about the vertical steering axis, the whole wheel and tire scrubs.
Hence, the power steering is heavier.
The agility/turn-in/speed of change of direction is also obtunded.
There is more tire wear too.
The 10 mm outboard mounting of the wheels does result in more stresses to the suspension components.
Under braking, with weight transfer to the front, positive scrub radius results in more instability under braking.

On the upside, moving the wheels 10 mm outboards for a 20 mm wider track does result in a wider stance for more stability at the limits of adhesion.
Perhaps, lengthening the fulcrum suspension arm compresses the springs more easily for a softer ride?
The positive scrub radius actually tightens the turning circle.

That is all theory above.
However, in real life, you guys who have mounted your wheels more outboards, what have you noticed with your steering, handling and ride?
Is the difference nil, marginal, or large?

 

Paging... everyone who has installed 15mm spacers on their cars.

 

On all cars I've installed an aftermarket wheel and tire setup, it's always been lower offset and often more sporty tires and lighter wheels. Sometimes accommodated by a suspension drop as well, but sometimes those things happen close to each other rather than right away.

With each setup, the steering is slightly heavier, but not noticeably so unless you go back to back immediately after the change. It becomes the new normal almost immediately. The other factors you mentioned, while true, are also usually so negligibly affected that it goes unnoticed after a few drives. Now, I have pushed the envelope a couple times in my youth, with wide stanced small pickups. They definitely feel different, almost twitchy. But 10mm or so is not something to ponder much over. The wheels that I've honed in on for this car will likely be 19x9.5 +40, so interesting thing with that is, yes there's less offset but because it is also a much wider wheel, it should balance out or close.

Oh. And of all the changes I noticed when doing wheels, lighter wheels always made the car more fun to drive. Rotating mass is way more noticable than a few mm of scrub radius deviation, IMO.

 

In the past, using an aluminium hydraulic jack and aluminium jack stands, I have compared my OEM Dunlop SP Sport Maxx 050 @ 235/40 R19 x 8” with +45 mm offset with my minus 1 downsized aftermarket Enkei 18” x 8” with +40 mm offset under similar OEM Dunlop SP Sport Maxx 050 235/45 R18.

The wheel offset diminished by 5 mm per side for 10 mm wider tracks, and the wheels do look a fraction more proud and aggressive.
However, when driving, I could only notice the difference between the OEM 19” rims and the -1 downsized 18” rims.
The downsized 18” rims were less sharp handling, but also less sharp riding, with slightly more comfort and slightly less noise.
The difference between the two was mainly wheel and tire downsizing.
I really could NOT tell the difference between the 5 mm less positive wheel offset!
Perhaps 5 mm is just not enough to matter?

To be able to notice the 5 mm less wheel offset and the 10 mm wider track, I would have to have two sets of tires and two sets of alloy wheels of the exact same dimensions, except for slightly different wheel offset dimension, and since I don’t have this, I have never been able to compare the direct effects of different wheel offsets.

In this youtube video, he has put on 20 mm spacers, for a significant 40 mm wider track.
He thinks it feels more stable mid-corner, and it rides a little better.

You Tube

 

Yep it's just not enough to worry about. Factory offsets always seem too high, no matter the vehicle. I'm certain the engineers pick a certain offset to accommodate and optimize for the reasons explained earlier, but there's a lot of wiggle room for lower offsets (and/or wider wheels) without issue, again for most vehicles.

I will comment, I think going staggered is entirely unnecessary and that is more of a vanity or bragging rights thing from Lexus, rather than a handling thing. In my experience, understeer is not a characteristic I like to add to my cars, and that's what happens with a staggered setup, all else equal.

Lightness, on the other hand...