Slashed tire - is it still rideable?

Riding park? Sure ride it until it blows and have a spare ready in the car. Riding trail? Wouldn't do it without an insert to make getting back to the car a little less painful. 

That tire tread has seen some abuse after a few hours.  What bike is this on and tube or tubeless?  I've used a tube with a cut like that but used it as a skid tire with about 31 psi.

I'd inquire about a warranty if it was actually only a few hours on a new tire. 

You could try sewing it to give it some reinforcement and throw a patch/boot on the inside to seal it up. I know some people who have gotten out of bike packing trips by sewing the tires. Something like this: https://youtu.be/HTKd1PpOZgQ?si=FzBu7AYCDhhc689r

Dont overthink it, run it till it blows

Apparently dental floss works surprisingly well to sew up a tyre. 

For sure, but if someone needs it (you down the line maybe?) this is a possible solution. And putting a patch over it. 

Sewing and shoe goo glue works awesome 

Second this! Works great and then sealant top off. It's saved my butt on a multi-day enduro before. 

Living in Vegas and riding bootleg a lot meant an absurd amount of tire slashes. I second what has been said. If it’s leaking sew it with dental floss and then throw a little tire patch on the inside of the tire with shoe goo or the glue provided with the patches. You can still run tubeless no problem doing this and the repair will last forever. 

Do you need a special sort of sewing needle to sew through a MTB tire? Especially something thick like a Double Down or DH casing seems like it would be impossible with a normal sewing needle.

I picked up a needle from Tandy Leather. Gets through DH casing with some effort and patience. Use of a thimble highly recommended.

Ignore any post above that mentions sewing: for cut that does not touch the casing, there is ZERO reason to perforate the inside layer of rubber keeping that tire inflated.  Just buy a tube of urethane sealant like Seam Grip for $8, clean the slash, and seal it w/ overlap of 3mm+ on each side.   A quality urethane sealant is far more durable than Shoe Goo, and repairs I've made this way typically outlast the tread life of the tire.  If you re-seal the tube of Seam Grip well in airtight container (e.g. 2x ziplocs) and store cool, it will last for months for future use.  Also works awesome to repair torn saddles, grips, inflatable kayaks, etc.

Yeah similar thing for me. Picked up the thickest needle I could find in the craft section. I use some little pliers or needle nose too

A leather needle is what you want. Just ask the ladies at Joann or hobby lobby.  and dental floss is great for heavy duty applications. I’ve used it to repair backpacks, my earliest repair was 15 years ago and still rock solid even though the pack is breaking down.

but that other guy has a real good point about just using urethane and not penetrating the casing of you don’t have to. I reckon this needs some repair though. 

Everyone should have a toob of shoe goo in their shop. So useful in many situations...

Hi would like to add to this ive just stripped my kenda tyre off my helium bike and wonder if my bike has been slashed by someone when i left my bike tethered in Leeds train sta along my beading and tyre wall while i use my M.T.B. to go out food shopping or small jobs or to ride to steam railway venues or it could be just over inflating of the tyre tube i come back to it in Leeds and noticed it looked like the tyre wall had been gashed or could be badly worn as ive neary worn the tread away off the cycles ive done on the tyre the rubber could have worn due to the damp weather we have in England i do go offroading alot on the helium along canal towpaths and do alot of road riding in Yorkshire as well cos its quite hilly terrain where my area is i live and so far ive had about just 2 years useage out of the tyre from it been new and do alot of mileage during the weeks about 20 to 30 miles sometimes 2 to 3 days a week. the tear on the beading and fabric tyre wall has gone round the tyre over half a way around  the tyre i was thinking if i could get some tyre cement and a strip of the old innertube folded round the tyre wall to hold the beading on the tyre wall by sewing and adhering the old tube aroud the tyre wall would this method hold the tyre wall together?