All the decades old debate is just noise. The difference is simply marginal

Availability is another story. Quite simply, 650c is a tiny market and choice/availability is less than 700c. For that point alone, I’d go with 700c unless it significantly affects fit.

I think the bike industry has made the decision for you by (nearly) all new bikes being 700c.

(Just like in mountain biking- I never asked for, and was never consulted about, the change from 26" wheels to 27.5" or 29". It was simply imposed by the industry (to ensure all that high quality equipment many riders had was obsolete and not transferable to anything new). Just like 1x, boost axle spacings, press fit, 12 speed, etc etc etc.
(Try getting a new suspension fork - axle spacing changed, wheel size changed, steerer diameter changed. So for a new fork, it's a new headset, new wheel, and a new frame 🤬)

This article?

https://www.slowtwitch.com/Tech/650c_and_the_Future_of_Wheel_Sizes_7123.html


I don't believe there's anything inherently "slow" about 650c wheels, especially since the shorter riders are likely also not as heavy and therefore penalized less for any Crr difference. The difference in MTB between 26"/650/29" is also poorly understood and largely nonsense. I think it's a shame that women and shorter riders in general are forced onto the 700C standard, but from a quick google search it seems like continental makes gp 5000 tires and there are some aero wheels and discs out there so it's not truly limiting.


Canyon makes their Speedmax 650b compatible: https://www.canyon.com/...hlon-bikes/speedmax/


I would base my decision on riding your closest fitting options and seeing if it really makes an ergonomic or handling difference for you, and largely ignore what bike shops recommend (unless you really trust them for some reason, many will try to shoe horn people onto whatever they have in stock, usually too large).

How tall are you?

You might have been looking at 650b GP5000 tires. Sadly there are very few 650c tires or wheels being manufactured anymore. I have a couple of smaller friends riding 650c tri bikes and the options keep getting more limited. None of the fastest race tires are available and AFAIK to use latex means folding over 700c tubes. One friend wanted a rear disc race wheel and I could only find Renn (if he’ll make one) and Premier in 650c.

Manufacturers stopped making them because of low demand. Some then made up stories to make it look like something other than a profitability based decision. In truth - not many brands made a good 650 (the whole point of smaller wheels is to have a smaller bike) so most of them weren't much of a loss.

As it stands now the Cervelo P-Series 48 and QR PR 48 are shorter reach than the original Cervelo 48 650c and 24 and 19mm taller respectively. So it's often possible to replicate the position a rider had on a 650 bike.
The problem arises if you had the 2nd generation Cervelo size 45, which is the smallest tri bike ever made.

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Which is pretty pointless - the XS is 2mm taller stack than the QR PR4 48 and only 6mm shorter reach. So it's just inconvenient to have 650B.

Smaller wheels need to be accompanied by a meaningfully smaller frame to justify the hassle of being different.

FYI, Conti only has made the GP5000 in a 650B size, not a 650C size; and even that seems to not be currently listed as one of their offerings. Availability of fast tires has become a problem for my wife, we have a whole drawer full of the now discontinued GP4000 650C tires.

While the Cervelo 45 is indeed a small bike, the Felt DA 47 is slightly longer but nearly 20mm lower.

The only reason I've moved from a 650C bike is because of tire selection. If more than one company (Schwalbe) was making good race tires for a 650C I'd be back on them.

I am right there with you on my Cervelo 45 650c. Since I have already lost an inch at age 60 and shrinking (currently 5'1"), this is interesting to me as well.

I got some of the last Hed tri-spoke wheels made 5 years ago and I am holding on to them. As for tires, I just bought new Gatorskins that I train on. My tubular race tires get so little use that they may go to the grave with me.

If I had to buy new race tires, I am sure that I could find them. But like most everything else at my size, it takes a little more effort than for others...

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DFL > DNF > DNS

I just bit the bullet and bought 700 for the first time in my life. Still have a 650 Felt DA w/ tubular training and race wheels. I’m going to have to buy new race wheels to get the 700 Felt IA as fast as the 650. I’m 5’-9”.

That's where we get into deciding what matters for the individual
The DA 47 with the short/low stem has bar XY 458/468 (roughly, their fit data was never great)
My Sister in laws P3 45 wit 60x-30 stem has bar XY 422/474

In general I'd be advising 650 in order to reduce the reach - which the 45 is far more effective at. And not too limited on the stack front.

I've always felt that the smaller wheels are faster, for many different reasons. But as someone else stated, the measuring devices that we have just are not accurate enough to measure that difference. I suppose some math geeks could sus it out to some degree, but so many variables that you would just have to pick one condition and then write the equation..

I got to race at a time when we all had 700c, then we all went to 650c, and then after about 8 years or so, the bike companies made everyone migrate back to the 700's again. All the top pros did perfectly well on the smaller wheels, many never going faster once they were forced to ride the bigger wheels again. Most of you probably have no idea how prevalent the 650c wheel was in the pro ranks, I believe one year that 28 of the top 30 in Kona were on them, while some women were even riding 24 inch wheels(setting a new bike and overall course record on them too!!)

5'1"

On a reasonably smooth surface I'd expect the rolling resistance difference to be negligible except when it's effected by availability of fast tyres in 650.
Aerodynamic drag can be expected to be lower since it's both a smaller wheel/tyre and also is lower to the ground where wind gradient will make it less susceptible to headwinds. If I had a bike with 650 wheels and had appropriate wheels and tyres, I wouldn't be thinking about changing for efficiency reasons.

My 13yo Daughter rides a 650C bike. Replacement tires are difficult to find as there are not many options available.

I have also been looking for some lighter rims because the factory rims on her Argon 18 are crazy heavy. Again not many options available outside of a custom build which is not practical for a 13yo. I was really hoping for something to come on the used market but anytime we find something it is really really old or beat.

Conti currently list 25 and 28mm 650B tires on their website. Doesn't solve your 650C problem though.

Fundamentally, yes, 650c is going to be a little slower than 700c unless the 650 bike is re-geared.
53x11 @ 100 rpm with 650c yields 34.8 mph
53x11 @ 100 rpm with 700c yields 37.1 mph

To equal the smaller wheel, you'd need a 57 tooth front ring, or a 52x10

However, potential lower cDa and lower frontal area make it almost equal. I'm not doing that math.

I don't think lower gearing equates to "slower"; you just choose a different gear and it only makes a difference when you reach your top gear. As for your highest gear, you do need to adjust gearing to achieve that; my wife uses a 55t chainring.

I think it's a lot more accurate to equate slower with rolling resistance and aerodynamics. Smaller wheels have slightly higher rolling resistance but the current lack of 650C fast tires is the bigger problem. Smaller wheels can have lower wind resistance but a lot depends on the design of the bike.

Gearing is not what makes a bike faster or slower, unless it's selected inappropriately. In your sample calculation for 100rpm on 53x11 with the tyre circumferences you used, the power required would be dramatically different due almost entirely to the rolling resistance and aerodynamic drag associated with the very the different speeds.

Discussing gear ratio as a limitation without context of rider ability, terrain, distance and conditions, is ludicrous..... or perhaps this was a jokey answer and I just missed the tone?

It took me all of two clicks to theoretically replace the Continental Sprinter 650c racing tubulars on my Hed tri-spokes. I also imagine that if I was willing to wait 4-6 months (again) that Hed would pull out the old molds and make me new wheels. The small amount of lost crr from tires is nothing compared to the positive effect of me getting my creaky, tired, injured, old, fat ass in shape ;-)

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DFL > DNF > DNS

A data point appropos of nothing: I have two road bikes, both frames built by me, round-tubed steel. One 700c (Mavic Open 4CD rims, Kenda 23mm slicks), the other 650c (Alex semi-aero rims, Pasela 25mm file-treads). The 650c bike is slightly lighter, and uses an old Kestrel aero fork and SRAM mix of Rival 11 with Red cranks, the 700c bike uses a Wound-Up fork and Ultegra 10-speed components. Same position, same bars, same bar tape.

On flat ground, the 650c bike is a good .5mph faster.

There are other far more important factors than the wheel size. And I'm a fan of 650c.

BTW -- the Pasela's are available in 28mm, check Rodriguez cycles.

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