Gravel wheelsize

Road, 700 definitely. 

If you're not out for faster times and prioritise comfort on poorly surfaced lanes and bridleways, either 650B x 45 - 50mm or 700 x 40 - they're quite similar on balance. The tyre construction and tread/type of tyre is more important imo.
fwiw I tend to ride my 650B bike mostly for that kind of mix, the bike has had both sizes in the past 8 yrs but I've ended up on 650B full time. They look bigger and slower, they won't win you a sprint or hill climb but over a long ride something like 650B G-One Speeds are very efficient. The comfort makes a difference and they roll along rough lanes really easily. There is some difference between a harder-wearing tyre like the WTBs and a lighter G-One though. The real benefit is I find myself taking tracks and exploring more on the 650s, the added volume makes the difference there.

I run 650x47 Wtb Byways. They work great for me. I like the extra width off road and they still roll pretty good on tarmac (to the point i sold my road bike). I probably am in the minority preferring the smaller wheel size tho.

I've also used both sizes and have settled on 700c.

I'm riding mainly road in the winter with the odd bit of smoothish gravel now and again, on 35mm Schwalbe all rounds. By the time it dries up a bit , I'll go back to 40mm all rounds and be riding 80-90% off road.

Maybe it depends on how rough your off road is?

Maybe it depends on how rough your off road is?

Yeah, also your ability to use lower pressures or tolerance to the softer feel, so how heavy you are or how light you can ride. Part of my preference for 650B is I use the bike for loaded riding or clattering through bad lanes in the dark or wet, yet I'm happy on 700 x 30mm on the same roads for a long Sunday ride.

Yeah, fair point jameso, I'm 70 kg wet through and don't ride loaded up off road on this one.

I don't ride loaded that often, it's just good to have a bike that just works for all the uses I have for it. I'm 75kg, someone here pointed out that 65-70PSI in my 700 x 30s was high so I went down to  a bit under 60, maybe 55. They feel good. Oddly soft at first but good. Was happy on most of the local lanes on them, they feel better now over a long ride and I'm not noticing any more need to be careful around holes. With the 650s it's more like riding an MTB - what holes? A bit ott perhaps but fun.

I was looking at an Orro Terra S Steel but is limited to 40mm tyres. I will probably ride 80% shitty Hampshire lanes and 20% connecting paths, bridleways etc. Is 40mm. enough?  Is it a reasonable bike?

Assuming the 40mm is on 700c wheels, you should be able to go a bit larger on 650's because, (simplistically) the fat bit of the tyre moves a little towards the hub where there's a bit more clearance. 

40mm is quite small for a modern gravel whip though, unless that's with mudguards.  

@ wheelie, I'm riding the same in south Wilts, so probably pretty similar. As said up thread  700 x 40mm does me , but I suppose, why restrict yourself to that ? There are plenty of bikes out there which allow much wider tyres.

Mine had 700x40 and used it for exactly the riding you describe. If I took it down something rough I wished for something bigger but it was still rideable if I took it steady. With 650 I think you’d be putting in higher pressures to make it better on road then you’d be too high for rough off road.

Ultimately I found mine too compromised for both, as I already had road bike with 30c and a 29er with fast rolling 2.1’s.

It’s a really tough choice

700c might be whisker faster

650b is really comfy. I’ve never upped the pressure for the road

I think tread pattern and tyre construction probably matters more. A 45mm 650b slick will stil be pretty quick. Whereas a 2.1mtb tyre will feel a bit slower.

My gravel bike has 700x40/42 which is a half decent compromise. The speed difference isn't that great, I can still keep up with the 16-17-mph average club group on a lunchtime ride as long as I'm feeling fit.

I really can't make up my mind 😀

I've used both 700x40 and 650x47 over many, many miles and I did settle on the latter for my new gravel bike, even though the 700x40 felt slightly faster on shorter rides. I think the added comfort of the wider tyre pays dividends in less fatigue on longer rides.

My tourer is currently running 700x28 and I'm having similar thoughts about that. I could go wider on the 700 wheels or even wider on 650s but then I think the speed/comfort equation is a bit different when the amount of tarmac gets above 90%

I’m now on my fourth gravel bike. The first 2 were with both 650b (47mm tyres) and the latter 2 have been 700c  (40mm)
My most recent is at the road end of the spectrum (2x after all the others were 1x) but that was the direction that worked best for me.

If you need to fit mudguards, go for the 700s. If not then I'd probably go with 650s.

700s with a ~ 40mm tyre are pretty capable and comfortable on road and on proper "gravel", ie non-rocky bridleways and fire roads. That's what I've got fitted currently, but as it's my commuter bike, I've also got mudguards on it.

Anything more than smoothish gravel and it becomes a bit of a chore to ride. It's capable of doing it, but I'm wishing I was on something bigger.

I was looking at an Orro Terra S Steel but is limited to 40mm tyres. I will probably ride 80% shitty Hampshire lanes and 20% connecting paths, bridleways etc. Is 40mm. enough?  Is it a reasonable bike?

I’ve had the ally Terra and currently have the carbon Terra and love both. Currently running 700x37 which are fine for towpaths, bridleways and the half brick half pothole lanes we have round here. I can’t be bothered with tubeless and run at about 35 psi.

For the first time in ages I put the 700c 40mm on. It’s so long since they on that I’d that I have to move to skewer across on the rear. They do roll better than the 660b, although the tread is bigger on them so it’s not a fair test just on size.

The 40mm are still comfy on the road. Wider tyres are easier to run tubeless IMHO

So it’s still close..

If it was a mtb you’d go 29er (700c) over 27.5 (650b) as they “roll better over small obstacles and are quicker”. The fudge with gravel bikes is clearance so 650b gets chatted up as better, when really its the width that matters.

My gravel bike runs 700c x 45/48 as it has clearance so I use them, mates bike doesn't have that clearance so he goes 650b, we both go to the same places at the same speed.

I'm still in awe at how the bike industry managed to convince folks that 650B/27.5 was better for gravel and 29" was better for mountain biking, when they're essentially both riding off-road. It's almost like they're trying to encourage folks to buy different wheel sizes to what they already have 😄

I think that I'm coming to the realisation that, for me at least, it really isn't going to matter. It might simply be a case of going for the best deal.

For a loaded bike I'd want as big a tyre as I can get. As you will bash the rear wheel into trail obstacles quite hard due to the extra mass of the bike and cargo. The bigger tyre can help mitigate this.
However on a lightweight off-road bike (minimal or no luggage), you can get away with tyres around the 38/40mm size. Unless of course you're either heavy or ride heavy.

Is 40mm. enough?

As someone who's ridden 36mm 700c on and off road around the Highlands and Islands for 3 years, yes it is. And I'm 87kg and use panniers at times...

I do have to temper my speed on really rough sections. I do pick lines. I don't mind the short 'that's bumpy' bits because the 80% I'm on the road they zip along better.

And I use near slick touring tyres too... 😉

Edit: but then I have just bought a bike with 2.1" X 700c... 🤨😂

I’m still in awe at how the bike industry managed to convince folks that 650B/27.5 was better for gravel and 29″ was better for mountain biking, when they’re essentially both riding off-road.

It's certainly true the way 650 x 47 fitted into existing 700C CX/gravel bikes was part of the marketing, but there were a few people myself included pushing WTB for a 650B tubeless tyre at that time because the road geometry with fatter tyres thing was just really good to ride, but the tyres back then were expensive and flimsy or just smaller volume.

I think the difference with a gravel bike is you're optimising for road use as well as faster, less bumpy off-road terrain than typical MTBing. Gravel stuff tends to be higher frequency and lower amplitude bumps. A larger volume tyre does that well (though tyre volume and OD are linked or work together there) and fits in if you want a relatively short wheelbase even with guards fitted. The geometry point is subjective though. imo if it has drop bars it should be a reasonable road bike, or at least should if it's not to be quite niche like a drop-bar 29er. 650 to 700 is only ~20mm longer stays and a matched longer front-centre but on a road bike it'll change the feel a bit. Enough to matter? Depends... It's not unworkable.

It might simply be a case of going for the best deal.

+1. Get the bike with the best frame and forks and groupset that take both wheelsizes, if poss. You can always try the other option later.

I’m still in awe at how the bike industry managed to convince folks that 650B/27.5 was better for gravel and 29″ was better for mountain biking, when they’re essentially both riding off-road.

I don’t think the bike industry did that. There are loads of bikes coming along with Space for bike 700c tyres.

However I owned a bike that didn’t have space for big 700c tyres but had space for bike 650b tyres. So really the industry saved me money. As a bonus the bike handles nicely with the smaller rims and it’s good for luggage space

"The bike industry" didn't convince me of anything. Having a range of options to choose from is ideal as we can each make a decision based on our own preferences, riding style, routes, terrain etc. I think it's actually more bizzare that the off-road world settled on 26" for everybody and everything (CX at 700x33 excepted) for so long - as if a one-size solution was right for everyone.