Here's the bike I have:
https://www.bikesdirect.com/products/motobecane/mirage_tour.html
Here's the tires I want:
https://www.amazon.com/gp/product/B0B6BXF43T/ref=ox_sc_act_title_2?smid=A3QGN5CVWB8NQ6&psc=1
I recently moved and there's less paved trails and more rail trails. I want a wider tire, but can't figure out if it will fit on the rim, within the fork/frame, or within the brake pads. I measured and 38mm looks like it should fit, but I figured someone else might know more than me. All the products and stats are listed on the site.
Also: I get it, it's not the best bike/tire combo on the market. Cycling is only really a side hobby for me, so don't shame my choices. I love my bike and the price I paid for it.
25mm to 38mm is a pretty big jump. Do you have photos of the part of the frame where the tire is now? With a caliper measurement for scale?
Keep in mind you want some clearance- if the gap is say 40mm, you'll have 1mm of clearance both sides which is not enough. As far as I know, there's no hard and fast rule, but roughly 5mm each side seems to be a rule of thumb minimum.
Take it into your local bike shop. You'll pay a little more for the tyre than if you purchased online, but you'll have advice, different ones to try, and someone to do the hard work for you, with nicer tools. Plus you'll make a good contact and help put food on a local table.
For what it's worth, I run 32mm on my gravel bike, and that's seen some shit. You'd be fine with under 38mm for rail trails (and more).
This is great info too. I would take it into my LBS, but I'd be afraid they would judge me because I didn't buy a $2000 bike for hobby riding.
If they do, at least you'll now know they're a shitty bike shop and you don't want to be friends with them anyway!
They probably won't, though.