Upgrade Tire Size Forums › Mountain Bike Forum › Upgrade Tire Size Tweet Add to Flipboard Magazine. Tagged: tire, Tire Size, tire width, tires, tyre, tyres Viewing 2 reply threads Author Posts April 24, 2017 at 10:05 #213622 dwirtz I want to upgrade the tire size on my Trek. I have 29″ with 2.20. What is the biggest I can go without change the fork. 2.40? April 24, 2017 at 10:10 #213623 Jeff Barber Chances are your fork has plenty of clearance for 2.4 and maybe even bigger. Most do. Which Trek are you riding? If you can post a photo of the rear clearance we might be able to get a better idea. April 24, 2017 at 12:03 #213653 dwirtz I have a Trek Mamba. I looked at taking a picture of the rear tire and it is already tight on the lower fork already. Not sure if anything bigger will fit on the rear. I am going to order a 2.40 for the front Any thoughts on a brand? Author Posts Viewing 2 reply threads You must be logged in to reply to this topic.