I'm about to purchase a 2008 4Runner SR5 which has 20" rims. The rims are really sweet (high end), but a local mechanic told me that this 4Runner is designed for only 17", and that it is too risky to go with 20" rims. I called another mechanic and he agreed. Seeing as this 4runner wasn't designed for them, I should ask the dealership to replace them with 17" rims even if they aren't as nice. Do you agree?

It'll handle differently.

The tire is a designed part of the suspension... changing the tire size [aspect ratio] changes the way the suspension works.

We're only talking 1.5" of sidewall... a bit, but not substantial.

It may be more prone to skidding and a bit more rough in some situations, but it's no different from putting 35" tires on it, from a "risk" stand point.

Mechanics tend to be "risk-adverse" from anything they don't fully understand. I would prefer the opinion of an automotive engineer or a mechanical engineer over that of a mechanic for something like this.

Honestly, the 20's will be great until you need new tires... I'd bet tires are not cheap for them.

If I remember right @superman3043 has 20" wheels. You can check out his build. His truck is sexy!
Superman3043's Build Thread

Not sure what that mechanic was talking about? I have the factory 20's from the 5th gen on mine and it rides like a dream and handles and corners very well too. +++ it looks sweet as hell!!! IMO anyway. There is no way I would tell a dealership to take some really high end wheels and tires off my truck and "downgrade" me.

I have 20" on mine 275/55/20. If it is good 4runner and you like how it rides and don't have any mechanical issue, i think you should get it. Good luck!

Alex, is your 4runner a 2008 SR5? How long have you had 20" rims on there?

Mine is 2005 SR5, i had my 20' since October 2011. I have no issues, i love it looks, now i just wish i would get 18 and more rubber, because i'm getting in offroading more, but for DD 20" looks awesome, check out my album. My 4runner also has daystar lift 2.5/1.5 to fit those tires.

Keros pretty much nailed it. These mechanics that you're talking to are just flat out wrong. These trucks were not "designed" for a specific size wheel. An overall tire circumference maybe, but not wheel size. And this has to do with things like speedometer calibration, tire clearance and things like that. They come from the factory with 16", 17", & 18" wheels, and if you're in the Southeast region like me, there is even a port installed package called the X-SP, that comes with 20" wheels. Also as calikev pointed out, the 5th gen Limiteds come with 20" wheels.

You'll notice a different road feel because you're swapping out rubber for metal. This equates to a rougher ride, but you'll also get improved steering response because a shorter side wall will have less tendency to deflect. Back in the day, the tires were a lot bigger factor in the suspension design but the suspensions and tire technology is a lot more advanced now.

Also a great point Keros brought up was how 20" rubber hit's you in the wallet. Trust me, I know. But look at every truck line out right now (or even car for that matter). You'd be hard pressed to find a model that doesn't offer 20" wheels at least as an option.

So in summation; I have big wheels because I like the way they look. These are not the best option if you plan on wheeling the rig. Rubber is expensive. Bottom line, if you want the 20's, then keep them. From a mechanical engineers' point of view. Hope that helps.

I run 22s with 285 45 22 tires, ride is more comfy then when i had 285 40 22, also saved about 200 back in my wallet when i went bigger, i only run yokohoma parada spec x. Best tires so far, had well over 65-70k on the 285 40 22 before i need to change. I did feel more road imperfection with 40s

You'll be fine