My wife bought a Sirrus X5.0.
Has a 40T crank. The rear cassette is 11-51T

I wonder if switching that 40T to a 42T would make a big difference in terms of speed. We don't live in an area with lots of hills.

Thoughts?

The difference is 5%. If she kept the same cadence and was going 15 mph with the 40T, she'd go 15.75 mph with the 42T.

Why not a 44t or even 46t? If there aren't lots of hills, how often does she (or anyone) really need a ratio that is that low?

Is your wife looking for a higher top speed or better acceleration? If she is spinning out on the 40:11 gear combination, then by all means, install a larger chain ring, perhaps even a 44T.

A 11-51 cassette seems to be grossly unnecessary if you "don't live in an area with lots of hills," and the huge gaps between cogs in this cassette cannot be good for maintaining a steady cadence or for acceleration.

Is she complaining about the gears? An extra couple of teeth on the chainring is neither here nor there with such a large range cassette on the back. She probably wouldn't even notice the difference as she would simply compensate on the cassette. She would only notice if she's already spinning like crazy to keep up with you in the 40-11 gear. In which case you would want a considerably larger chainring anyway.

Is a new bike. Sirrus X5.0
Has a 40T single ring on the front and a Shimano SLX M7100, SGS, 12-speed rear Derailleur. The cassette on the rear is 11-51t.

What would be the easiest way to improve speed?

Change the cassette? If yes, which ratios?

And would a large cage rear derailleur accommodate that cassette?

Top speed would occur in the 40/11 combination. Changing to a 44 tooth chain ring and a 1 inch longer chain would produce 10% more speed potential. So would a change to a cassette starting with a 10T sprocket, if available.

I would definitely go to a 46t, Increasing the ring by 2 teeth just is not a big enough difference to bother with, usually. However, if she only needs a small increase in top end, that might be enough. Personally, I would still go to at least a 44t.

Have you looked at what the gear calculators on line show you? This is my go to one for just it's simplicity, but it gives more info about gears you may not have in the results.
https://www.bikecalc.com/speed_at_cadence

With a 1x front changing the front ring will impact the entire range of gearing. So if the current low ratio gears are needed for any of the riding done on that bike, then they will be gone if you increase the size of the front rings.

If the bike is only ridden on pavement or packed gravel, it's hard to imagine the low gears are currently needed though.

Spinning out a 40x11 on flat roads is not as easy as people think.

With the stock 38mm tires, 80rpm in the 40x11 = 23.81mph. 100rpm = 29.76rpm
If you upsize to a 42T, those numbers will go up to 24.91mph and 31.23mph.

I run a 44T on my 1x CX bike for summer gravel/dirt becuase I was occasionally spinning out the stock 40T (with an 11-32 cassette) when hammering in group rides, big tailwinds, descents, etc.

I can't imagine spending enough time at those speeds on a Sirrus to warrant swapping to a larger ring, but that bike also has a massive 11-51 12sp cassette, so even with a 44T there would still be plenty of low-end gearing for any small hills.

I just want to point out that on this bike with 38mm tires, 15mph in the 40x11 would only be 48rpm cadence.
I don't think taller gears are the answer if 15mph is the top speed we're talking about.

Not sure where the "15 mph" number came from.

I would ask the OP, does your wife use the top couple of cogs? I might go for both a bigger chain ring and a smaller cassette. 40x51 is ridiculous unless you are climbing mountains. Apparently there are 10x45 cassettes available, which would (very, very) slightly increase top end, while providing closer gearing and still plenty of climbing potential .... which you say she doesn't need anyway.

With a 46-tooth ring and a 10-45 cassette your wife would have decent road gearing and still have the same climbing capacity as a 50-34x11-32 combo which is pretty normal for a road bike used for climbing.

In any case .... 44 or 46 teeth on the ring sounds like a good place to start.

IMO, there is no benefit. 40/11 on 700x38 is a 100 inch gear and already at the maximum gearing allowed for junior racing. Plenty of gear on a hybrid. Changing the chainring just shifts all the gears a little bit. YMMV.

Otto

What about her speed on the bike are you wanting to improve? If you are a stronger rider and are wanting her to equal your speed, then she is either going to have to get stronger legs or an e-bike.

As long as she doesn't have the same power to weight ratio as you she will always be slower for most things.

What sort of speed are you talking about?

In the 11 tooth sprocket in the 40 tooth chain ring (with a 700C wheel with a 38mm tire), the following shows the speed for different pedal RPM (cadence).

40 11.9 mph
50 14.9 mph
60 17.9 mph
70 20.8 mph
80 23.8 mph
90 26.8 mph

With some practice, people can easily do 80 RPM.

The current set-up is likely good enough for the flats (it might be on the low side when going down hill).

She is not going to ride any faster just because you change the gearing. I’m guessing she doesn’t have monster power and so won’t be speed limited by a 40-11 gear if there are no major downhills.

I would let her ride the bike and see what gears she uses. If she never goes anywhere near the 51T on the back then you might want to get a closer spaced cassette. But I would try it first.

If the OP's wife wants to go faster .... then we can assume that at her comfortable max revs in top gear, she is not going as fast as she wants. He does not mention his own pace.

My question would be, is she spinning as fast as she comfortably can, but she Could spin faster, but nor with pleasure? if that is the case she probably has power to turn a higher ratio. If she is spinning as fast as her power allows .... then changing cogs or rings wouldn't matter.

I think most of assume (possibly incorrectly, but I did say "assume") is that 40x11 is simply too low a gear combo to get the speed his wife wants, comfortably and consistently, on the terrain they normally ride (mostly flat.) The fact that she might only be spinning 80 rpm isn't the issue .... if that is where she is comfortable.

I have a 48-38-28 triple with a 14x34 (it think 34) 7-cog cassette on my old Cannondale tourer. I often find myself wishing for the extra couple teeth I am missing compared to my other bikes, most of which are 50-34 with 1- or 11-28 or -32. Sure I could definitely just spin faster in 48x14 .... but it is not a pleasant experience for me. I have a certain natural cadence where I feel most comfortable.

So I would assume that the OP's wife, if she Could spin the 40x11 faster, could also be happy with a 44 or 46 ring.

The goal here is a happy rider. She isn't racing for cash or anything. She just wants to enjoy her ride, and finds her bike to be undergeared .... as far as I can tell.

As far as I can tell she hasn’t even ridden the bike yet.

We really don't have enough information.

It's fairly common for new riders to think that a bigger gear means they'll go faster. It's fairly common for new riders to use a low cadence.

Do you think it's likely she's going 23.8 mph?

The issue might be that the OP is seeking a equipment solution for something that really isn't a problem (or one that would be better addressing with some experience). She might not be a "happy rider" if the expectations aren't realistic.

I do love the thought of someone out on their new hybrid and just cruising along on a flat road in top gear at 25mph and being like "why is this bike so slow?".

I think this is the trap the OP has probably fallen into here. She needs to ride the bike and get used to the gears before even thinking about changing the chainring or cassette. But for sure going from a 40T to a 42T chainring is going to be a complete waste of time.

yeah .... i reread the thread with this assumption and it fits. Could go either way ..... Hilarious. "My bike is too slow when I don't ride it .... how can i go faster?"

Anyway ..... it's only BF, after all .......

Perhaps someone only thinks this because the illegal e-bikes are flying by at 28 mph.

Thank you all for the comments. We changed the order for a "non x" Sirrus that comes with a more road-oriented 48/32

Lol OP your wife sounds like a beast if she can spin a 40/11, that's a super heavy ratio on the flats for a new rider...

also not to be a jerk, but she probably won't really notice the difference between any of the setups mentioned. The top end is too high for her to use anyways unless she's a monster (and even then she should just spin faster)