New road bike or change tyres on CX for (faster) century ride?

Just noted you did it in 6.40 first time. To go around in 5 hours is a massive jump in performance, obviously 20mph average. You will need to be doing 22mph for long periods. And don't forget there will be 30000 other riders on the route. You will remember how busy the Surrey lanes were and had to go with the flow mostly.

I run two wheel sets on my cross bike, one for off-road/cross and the other for on-road use. For on-road use I am finding that 32mm gator skins are not bad.   I have mixed feelings on the gators skins. On my road bike I used to have 23mm ones and on cold wet days I never felt confident on front wheel grip. I swithed to 25mm GP's and they are better. But on the cross bike the 32mm Gators roll well and the grip is good.

Regarding new bike or stick with what you have. If you have the money and want one, go for it. Is the weight that important?, I think it depends on your weight. My cross bike weighs about 10Kg, my road bike just under 8Kg. I weight 84Kg so me and bike about 94Kg on cross, 92Kg on road bike.  About 2 to 3% difference in all up weight. Remeber a 500ml bottle adds 0.5Kg. add two for a long distance event 1Kg. It is easier (in principal) to take 2 kg off me than the bike.  Iam not convinced I am much slower up the hills on Cross bike.  But road bikes can also have a more aero position. 

a good bike shop should let you roll up on your existing bike and take a new one for a spin.  It all becomes very personal, and what is one persons responsive darting bike is another twitchy nightmare. 

I've got 25mm schwalbe ones on my summer bike, done about 2000 miles, one puncture so far and wear is minimal, just put Conti gp4000ii's on my winter bike and had my first puncture last week, but the flint was sizeable, som no fault of the tyre, grip is great, particularly in the wet.

having ridden both schwalbe ones and gp4000s, the durability of gp's wins every time (and they're faster). schwalbe's are fine but my experience was one lock up (an emergency stop to avoid a car cutting a corner) and the tyre scuffed and "50 pence pieced" necessitating a replacement. I have emergency stopped plenty of times on gp's and never had this problem.

While the correct answer is always "Get another bike" the more pragmatic option is to get some 28mm Schwalbe Ones (tubeless or folding, up to you) and enjoy your bike at full speed. I've got them on my Jake the Snake and its like a magic carpet.

I bet I'm heavier than  you (116 kg) and run those tyres at 85/90 over potholes with no issues.

I'm riding my CX with GP4000s 28mm at the moment, used them for RideLondon last year on a different bike. With the wider rim of a CX wheel, I found they actually expanded to 32mm as measured in place on the bike.  Lots of grip, lots of comfort, great.

I just looked up your bike and there's one other thing you might want to try.  If you've got the same straight seatpost I see in the picture, consider a setback seatpost with a bit of flex.

I ride a Ridley X-Bow Cyclocross 5000 miles a year , its about 10 kilos. I run 700x28 Conti four Seasons, its plenty fast. I have done several back to back 100 milers on it. Can always keep up with road bikes, club runs, chaingangs , etc.

If your brother is faster than you, you will regret trying to match his pace and will blow up at some point. If he thinks it's a race tell him to try and tag on the back of a fast group and you will see him at the Mall. Fast rider should slow down to slower rider.

Re tyres if you weigh 85 kg+ use 28mm. Less than 75 kg use 23/25mm. I weigh 95kg and found 25mm tyres needed maximum pressure to give good performance. I have just fitted tubeless 28mm Schwalbe pro ones and already are better

All new bike purchases should be wholeheartedly supported- that is a rule.

Being sensible though, for one ride new tyres are the way to go- 28mm schwalbe ones would be my suggestion.

If you are looking to do more cycling for leisure rather than communting then a new lighter bike will be more fun.  Could you borrow someone elses light road bike (your brother's for example) and see the difference for yourself?

PS- tell your brother that one of the unwritten rules about the rules is that they are not to be taken seriously.....

GP4000's every day over gatorskins. They're faster, more grippy and have just as good puncture protection. Get yourself over to wiggle where they 59 quid for a pair.
The only question is width. I commute on 25mm but my summer bike is 28mm. I would be inclined to say go for 28mm given you are used to wider tyres and the narrower you go, the more skittish . For reference, I completed the ride London last year on 28mm's in under 4 1/2 hours, faster than my previous year on 25mm.

I'd like to say get a new bike... but it's all a matter of degrees..

If I were you, I'd swap out the tyres for some £60 25mm ones, can't go wrong with Conti GP2 tyres (I use schwalbe pro ones but same thing really).

However you will lose comfort with smaller tyres and it'll be noticable over 100miles.

My issue you have is that your bike is a little on the how to say this nicely... 'big boned' side of the scale. I don't know the weight but I'd hazard that large sized £1000 specialised with metal fork and triple chainset is probably touching 12 kilos. A typical road bike of the same budget will be more like 8kgs. So unless your bro in law is less fit than you, you're at a significant disadvantage, regardless of new tyres.

Take a look at some of the buying guides on road.cc for new bikes, budget - Triban 3 /  bit more - Giant Defy / bit more - Carbon Giant Defy.

Note you'll pick a decent hardly used Giant Defy up for about a third of the RRP on fleabay.

Oddly enough my bro in law rides a large specialized tri cross also and can make it round a lumpy 40 miles, but he just blows out of his arse on the hills. I personaly admire his tenacity and endurance.

I also use my CX for everything at the moment and have been riding with either 32 or 28's Durano which are very forgiving. Using 28's at the moment I still like the 32's though.

I have Gatorskins on my road bike (25's) and have never had any issues with them, I find them very good.

Your cyclocross bike already puts you in a relatively aerodynamic position so there isn't much point in splashing out on a new bike.  

New tyres will make big a difference but don't go for the god-awful gatorskins!  Far too crashy and they don't have any grip.  

Get something more supple for 2 reasons: 

1) Lower rolling resistance will make you faster.  Narrow isn't necissarily faster, no need to go smaller than a 28.  

2 ) Comfort - you will ride faster for longer if you are comfortable and not fatigued by being rattled around by narrow tyres.   

Continental GP 4000 are a real benchmark and I think they go up to 28mm, Schwalbe One is also supposed to be very fast at 28mm width.  For a real plush comfort choice then go for the open tubulars that are in vouge at the moment, something like the challenge Strada Binanca in 30 mm would be good - I had a set of these and (with nice low pressure around 50 psi for me) they were so fast and so so comfortable! 

I ride my CX bike for everything, and go between full CX tyres (Continental Cross King 35mm and Challenge Grifo Pro 32mm) and Road tyres (Bontrager R1 23mm) and the bike is significantly better handling on road tyres without changing anything else.

I rode a centry last year on mine using 23mm Vittorias and it was absolutely fine (if a bit uncomfortable as they were at 110PSI to avoid pinch flats). If I were you I'd go down to a good set of folding 25mm (which is what the pros use nowadays generally) and you'll be fairly comfy and fast enough for your century.

Good luck!

AEB