Fat is the new thin. Oversizing is no longer limited to frame tubes and handlebars. The days of 'bootlace' road tyres are numbered; 25mm is where road tyres are at. This is a trend that has been gathering pace for years. Most of the big tyre companies have had a fat, lightweight road clincher or two in their range for some time.
The main benefit claimed for them is greater comfort over a 23mm (700x23c) tyre of the same construction. True enough, but perhaps of more interest to many road cyclists is their improved rolling resistance, even when run at lower pressures. They can be run as low as 80psi (7bar) with excellent results, although the sensation initially feels strange.
Despite this, the 25c format is too fat for racing. The width of the tyre increases frontal area and air drag, ruling it out for time trials on smooth roads. The increased weight is a significant handicap in any acceleration, so they aren't much use in a bunch. For training, touring and randonnees, however, the fatter tyre is simply a superior option, provided it will fit in your frame. Here are three of the best takes on the idea.
Michelin Krylion Carbon 25c £19.99
Michelin may have started the lightweight 700x25c ball rolling with the Axial Pro Service Course, which was based on Frederic Guesdon's 1997 Paris Roubaix winning tyre. The French manufacturer's latest effort is a 25mm version of the new Krylion Carbon model. It gets the same long-life, high grip tread compound and puncture resistant carcass as the original 23c tyre, the two combining to bump the weight to a substantial 290g.
In exchange you get one of the most durable lightweight tyres around, but with levels of comfort and rolling resistance normally associated with much lighter rubber. The twin tread compound has a high percentage of carbon filler in the centre for durability. Michelin claims 30percent extra mileage over an all-silica tread, or a potential 5000km from the back tyre. No wear is apparent after 500km.
It is very resistant to cuts, and with the four-ply anti puncture layer in the carcass makes life very hard for sharp flints. The one (slow) puncture so far was caused by a 5mm long shard of glass on a Belgian cycle path. There's no escaping the sluggish response of this weighty tyre, but once up to speed it runs very nicely. A great touring and training tyre, if a little heavy for cut and thrust riding.
www.michelin.com
Michelin Tyre 01782 401754
Vittoria Open Corsa Evo-CX 25c £27.95
The legendary Italian firm of Vittoria has vast experience with oversized tubulars, and with 13 victories in Paris-Roubaix knows how to build a tough tyre. The proven Open Corsa CX is no exception, and the current Evo model is the best yet. This now comes in the tested 25mm format, with the same Corespun K290TPI Polycotton carcass and SiO2 Kevlar-enriched tread and a respectable quoted weight of 245g.
This is a truly luxurious tyre, with a floating ride quality at 7bar that must be experienced to be believed. Although thin and supple, the Evo is tough enough for the foulest conditions. Flint-laden Surrey lanes have inflicted several cuts but no flats so far, suggesting a very resistant carcass. The rubberised sidewalls don't shrug off muck as well as the others. The tread is thin, and expected life may be as low as 2000km. Definitely a special occasion tyre, it is perhaps the ideal clincher for the Ronde van Vlaanderen or the Paris Roubaix randonnee.
www.vittoria.com
Michael Elson 01664 444513
Continental GP 4000 25c £29.99
The lightest tyre on test at a quoted 215g, this has the same tread compound as the 23c GP4000. Lack of wet weather grip in early examples has been fixed, and the current tread is a good
mix of wear resistance, grip and speed. Moulded wear indicators let the owner know when replacement is needed. A Vectran breaker belt under the tread is claimed to improve puncture resistance. Beware the rotation direction indicator moulded in the sidewall, although fitting the tyre the wrong way round probably makes no difference.
The GP4000 is rightly popular, but the 25c version does not excel in any one area. At the same pressure, it feels slower and less luxurious than the Vittoria. On the other hand, resistance to winter grime is better. Wear rate is faster than the Michelin, but the Conti rolls faster. One flat in 500 winter miles is acceptable, and the compound is acceptably cut resistant. Little wear is apparent after 500km.
www.conti-tyres.co.uk
Cambrian Tyres 01970626777
Verdict
All three tyres can be recommended in 25c format. The Michelin Krylion is hard-wearing but sluggish, the Conti GP4000 a good all-rounder and the Vittoria fast, comfortable and surprisingly tough but surely not as long-lived as the others. 
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Gordon Daniels
