Hello new here and to the mountain biking world. Got my first bike after 22 years of not being anything on two wheels. Anyways I get it home and it has 29's on it and i inflate it to the max pressure on the side wall which is 40psi. I'm 6'4 250 and when I get on they look and feel under inflated?!? It handles like poo and takes bumps the same! Got off checked it and still 40 psi... Help!

40psi is very low as a MAX pressure. How wide are they? What are they.
For a tire about 2.2", 40 psi is about right for your weight off road. Yes, it would sag a bit, but handling should be excellent. Something sounds odd, and not necessarily tires.

I don't think this is a tire or bike issue. Its you needing to get acclimated to riding a mountain bike. If anything you have overinflated the tires.. Are your tires set up tubeless? Even if you are running tubes, for your weight, 40PSI is really high for a mountain bike.

Sounds like your not loose over the bike, sounds like your hanging on tight..
Elbows always bent a little, knees never locked,,
Get your rear end OFF that saddle unless your fire road riding to get to the single tracks or on a long grinder climb..
Your bike cannot deal with you while pinned to it,, you can,,fold a wheel.... yeah not kidding at all ~~~~


The Base formula new riders should use is:

Rider weight IN FULL GEAR,,,
divide by 7,,,
add 2 psi for the rear tire
subtract 1 psi for the front,

Carry TWO spare tubes as you work your way down below these numbers and start about 3 psi above If your a big guy,,
And You are. Learn to,'Ride Light' and help your bike by using your arms and legs as suspension...

I'm tubeless now but when tubed my numbers looked like this (My trails are not sharp rocks).
My numbers:
pack and all, full ride gear 185 pounds.
26.43 psi PffFFtt 26 psi
26 +2 28 rear psi
26-1 ,,, 25 psi front. So adding 3 as a rookie I would be at 31 rear and 29 front.
This is a general target psi for and standard mtb tire running tubes In the 2.2 to 2.35 width range

NOT Cyclocross tires..

Lower psi off road lets the tire deform around and over bumps adding traction and LOWERING rolling resistance,,
Higher psi makes the tire bounce off objects and those impacts eat watts.
I run 25 rear and 24 front TUBELESS because lower gave me too much sidewall flex and a vague feeling.
These numbers would snake bite a tube under me,, I got that T-shirt.

You,,, start at:
39 rear,
36 front,

Go tubeless if your rims will seal and then when tubeless you should wind up at about:
35 rear 33 front. Get a good tire guage, a digital. Mine is the TOPEAK Smartgauge D2
Accurate scale to 1/10th psi and btw I can feel a single psi change in my front tire...

Most cheap gauges are off by 2 to 5 psi,,
The floor pump gauges,,, a real gamble,, one day mine is spot on,,the next 3 off..

Low quality tires found on walmart bikes and big box sporting goods stores can vary greatly In construction quality.
Just sayin,,,,,

Thanks for all the responses they are 29x1.5...they have like a WD on the side... I just noticed the max load is 198 lbs.... would that be my problem?

Wait hold on... 29x1.5 tires?

What kind of bike is this?

What type of riding are you doing? Paved roads, dirt roads, actual mountain bike trails?

Never seen a mountain bike with less than 2.1 Inch wide tires...
My 27.5 X 2.35 wide's have a max psi of 50, so does my 2.25 tires,,,...

1.5's and your weight,,,50 psi would be near your lower limit I would think....

29 x 1.5 sounds more like a hybrid bike tire than a mountain bike tire.

Lol sorry, it's 29x1.95... I missed a number typing haha. It's a Mongoose Impasse HD. It's a big box store bike, but it's what I could do right now. I haven't been on a bike in forever and around here there aren't too many trails, so initially it would be more road time right now until I get comfortable on one again and then maybe try venturing out onto a trail...

My Epic WC is a XC race bike and comes with 1.95s. I ride at about 190 with a pack and tubeless I run them at 26-28.

I think what you are feeling/experiencing is a combination of being off a bike for a very long time and the bike’s weight. I looked around and several sources list that bike as being 43 pounds. Many hardtail MTB’s are in the 26 to 28lbs range… a 17lb difference on a bike is huge.