I'm riding the Allegheny passage on my Co-motion Java in about 6 weeks. I'm looking for tire advice most of our riding is done on bike paths or road with 38C Schwalbe Marathon plus tires. These tires have been great for weekend trips loaded up with gear but they get very sketchy on crushed stone surfaces. Not sure if I'm overthinking things but most of the gravel tires I've been looking at have a weight limit. Want something that will have a little bit of speed and withstand a loaded bike with all our gear over gravel or crushed limestone. I was thinking about 42C to 45C. Any advice will help

I had Marathons before but never rode them on a tandem or the GAP. I have done the gap several times on Schwalbe G-one Allrounds at 37c without issue (loaded, with me at 200# and a 29# bike). Unless it's wet, the gap surface is very good, not very loose, so while more of a road tire I wouldn't expect much of an issue with your Marathons, but you have the experience of how they feel loaded off-pavement.

I have them same tires on my gravel bike and had plans of putting them on a tandem. When I read the max weight it was 200 lb per tire. The combined weight and all the gear would probably put us 100 lb over. I may be overthinking this.

We have ridden the gap 100+ times over many years with tire widths ranging from 32 to 50mm. When loaded with camping gear we now prefer 50, but we have done it loaded with 32s as well. The surface is very good and drains well in rain. The camping at the Outflow in Confluence is awesome. Enjoy the view at the Maryland side of the Savage tunnel! Pack some wine/crackers/cheese. We will do again at the next patch of good weather.

I’d guess you’ll be fine on your 38s. We did it on our Co-Motion with 28 or 32s without any issue. The surface varies quite a bit from section to section, but wasn’t a problem even on the rougher sections. I suppose if you’re very heavily loaded it might be different, but we were staying in B&Bs and had frame bags, so bike handling wasn’t a problem.

The road surface on the GAP is pretty smooth. You could easily do it on 28mm road tires. 38mm Marathons will be way more than adequate.

if you continue on to the C&O, the surface is rougher, but even there the 38 marathons would be adequate.