I’m getting a really good deal on 295/35/21 tires for my Gl and was just wondering how much of difference it will make if at all, right now I have 295/40/21 on the suv.

There's online tire size calculators - they're easy enough to find and that would tell you the actual differences, but... I find it VERY hard to believe that it would cause any issues. My guess is you'd probably drop your ground clearance by about 1/2" and little to no difference in width - certainly nothing that would make a difference. You'll probably see a speed/mileage difference of a few percent.

The only benefit will be a slightly lower ride height. The cons will be a harsher ride, because here is less rubber. A 35 aspect is shorter than a 40. On 21s, that could spell wheel damage and almost certainly increased suspension wear and tear. If you live in an area with crap roads, this might not be the deal you think it is after few months of city driving and your alignment is off for starters. Lower ground clearance is a con. also. Thinner rubber is never a good idea on large heavy vehicles. As the gentleman said, you can do what you want to do.

Its an old thread but my question is, Can I use 285/40/21 instead of 295/40/21 on the GL550.
Same here like the OP, I`ve found a good deal on 285s.
Any input would be appreciated.

10mm narrower and 4mm shorter sidewall. Get yourself a metric ruler and ask yourself if those distances will make a difference.

If you have a bidirectional tool like the Autel MP808 or MB STAR, you can even adjust the speedometer to match the different tire height.

It amounts to one less roll of electrical tape on those 21s…

The tread wears by more than 4mm from new to the wear bars.