First point to make... these are merely my opinions based on experience, feel, personal preference, geographical riding conditions and the good lord Strava.
There are no scientific facts to back up any statement or evidence to prove my beliefs are reasonably sound.
Item two is frame clearance. This is something we are all limited by and can sometimes seriously restrict your choice.
Third on the agenda is honesty.
Seriously consider how or where you intend to ride your bike. There’s no point dragging around 50mm of tractor rubber if you mainly ride along a gritted canal path ... although that is arguably suitable for some of the UK’s roads!
Finally... YOU HAVE TO GO TUBELESS
650b is best... there you go, I said it.
With the majority of the current crop of gravel/all road frames I’ve owned or ridden, the limiting factor of 700c tyre widths would nudge me in the 27.5 inch direction.
This is very much a UK based opinion... we don’t have miles and miles of manicured gravel roads. Most of the riding I do tends to take in light mountain bike territory and surfaces that rattle your teeth out... with stretches of pavement in between.
The extra comfort and control afforded by a smaller wheel/larger tyre combination is a winner.
Over a varied terrain I would also wager a bet that it’s faster.
Anybody old enough to remember the joy of a nimble handling 26” wheeled mountain bike will also attest that smaller wheels are more agile and fun.
If you want to smash out the miles and you’re lucky enough to live somewhere that you have access to fire roads and well groomed trails/paths then a fast rolling 700c tyre is arguably the better tool.
I’m not really going to get into tyre model specifics as this is the internet and people will aggressively disagree with anything... and I don’t want an argument.
To contradict that, I absolutely love the WTB Byway in 650b 47 flavour.
Why? It’s fast on the road and compacted surfaces. It’s chubby and comfortable. There’s an element of grip on the edge of the tread for loose corners. It’s tan walls and bloated good looks make any bike look the nuts. Fact.
That statement may sound ridiculous? Surely grip is a pretty important factor and only having a little of it is not enough and you’d be an imbecile to choose to ride with as little as possible?
Yes and No.
Crashing isn’t fun. I’m going to jinx myself by saying that I rarely crash.
But pedalling a bike is a joyful escape...Two wheel drifts are fun. Riding too fast down a fast and loose descent, not knowing if you’ll make it round the corner is thrilling.
Going where the terrain tempts you and your appetite for adventure allows. This whole gravel biking thing is an element of going back to your childhood and just riding bikes... anywhere.
Gravel bikes are a joyful compromise... a do it all bike. One bike for everything. Hahahaha ...yeah right!!
It’s a constant see-saw of over and under biking.
Under biking is brilliant as it makes a once familiar mountain bike section interesting , a technical climb more challenging and a warp speed descent seem that much faster.
650b makes the compromises less of a factor, slightly less frequent and more tolerable.
The variation of 650b gravel tyres is also massive.
My opinion is that the larger surface available allows greater tread variety over a similar dimension 700c (approx 35mm).
You have supple, fast rolling slick tyres such as the Rene Herse Switchback Hill or WTB Horizon for road/light gravel applications. At the other end of the spectrum you have lots of more aggressive, gravel specific tyres or 27.5” xc mtb treads.
You don’t get that level of choice in 700c before you head into 29er territory and you’ve long run out of frame space anyway… That's not to mention the bottom bracket height issue.
When all is said and done, it’s just riding bikes!
Tyres though, are probably the single most important decision you can make in how your bike rides.
They are also one of the more cost effective ways of upgrading or varying your ride.
Fat, thin, aggressive, supple, fast or tough... go with whatever suits
BUT GO TUBELESS... two wheel drifts, mud covered faces and wheelies are fun. Punctures are not.
Fit the fattest rubber your flavour of gravel bike will take.
If you’re more interested in speed... get a road bike
If you like technical, more challenging riding... get a mountain bike
I have both sizes of wheel and a shed load of tyres.
Choice and variety is brilliant.
I told you this would be pointless... I’m not sorry.
Convert an older 29er? That’s an idea…