UKC

Have entered my first 150k road race in a few weeks, which I will be doing on my gravel. Currently have 40mm tyres fitted, so looking for some 28mm road tyres. Does anyone have any advice / suggestions, as this is a new area to me?

As with so many thing cycling-related, the answer at the moment seems to be 'whatever you can get'. I would normally recommend Conti GP5000, but I don't think you'll find any (in the UK). You might find 30mm or 32mm version and they won't be significantly slower. Of course if it's really a race (as opposed to a sportive) and you're at the pointy end then 'significantly' may differ.

https://www.probikekit.co.uk/bicycle-tyres/continental-grand-prix-5000-clin...

If they really are in stock 

I'm no expert, but I think the width of your rims on your gravel bike may limit how narrow you can go anyway - having said that I think it was my 32 mm Marathon Plus tires that I put on my gravel bike that came with 38 as stock, although it might have been my Marathon Plus Tours which are 35 mm. I definitely used the 32 mm ones on my old CX bike for several years, but I'm not sure if they were a narrower rim. 

A lot of modern real world and lab testing seems to show that narrower tyres don't make such a massive difference anyway, and they will be hard on your bum if you're used to nice big fat cushty gravel tires! I bet a road-focused slick tyre at 32 or 35 will be just as fast as 25 or 28 and more comfy on a long ride. 

All I can add is that my endurance road bike came with 32mm tubeless slicks (Giant Gavia Fondo 1) and they feel plenty fast.

Skinny tyres are not necessarily faster. They actually have a longer contact patch than wide tyres, and aero arguments have kind of gone out of the door. I have never raced in my life, but I'd have thought a smoother less fatiguing ride would be preferable to a super fast skinny tyre? And Toby is right, there is an optimal ratio for rim to tyre. I think its on Sheldon Brown...

It's a sportive in France, but they take it super seriously so I don't want to look much out of place. I could complete it with my current tyres (I just got back from riding from Paris to the Alps with no issues) so really all I want is to increase my average speed closer to 30kph, as I generally cruise at 24. Apart from getting fitter, tyres is the only real change I can make (without changing chainrings which is hassle for little use). I would just buy a cheap road bike, but I don't wear enough lycra, and I have too much respect for other road users / rules of the road to be a roadie...

Changing tyres is not going to make that much difference IMHO. Some time back I did a test to compare a gravel bike with 40mm knobblies (brand new Maxxis Ravagers @ 30/35psi) with a road bike on 25mm GP4k @ 60/70psi on a rolling tarmac loop (alternating laps, 4 on each). The difference was ~5%. At the pointy end of a race that's massive, but in a 150km ride you might find the fatigue from narrower tyre on less than perfect surfaces may negate the slight RR advantage - particularly if you're not used to riding a 'buzzy' road bike.

I once tried putting 25mm slicks on my gravel bike (instead of 40mm knobblies). They worked fine, but they looked too weird to tolerate. Bit like seeing big Range Rovers with low profile tyres. Just wrong.

I resort to 38mm slick GravelKings occasionally if I want to pretend lighter slicker tyres make a difference. At least they sound nice.

I currently have Vittoria Terreno Dry - they are not at all knobbly so nice for road riding, but also good for forest tracks. Maybe I just stick with what I know, as the event is on rural French country roads so likely to be a bit bumpy. 

+1 for Contis. The GP5ks are great, but I just removed my knobblies and replaced them with Conti GP 4 seasons (new job, no more gravel bits on my new work commute).

The 4 seasons are a bit more robust than the 5000s, at least in my experience.

I went for 32 mm, but 28mm would also have been fine.

CB