Shimano GRX vs 105

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Shimano sells two mechanical 12-speed groups that a gravel buyer will genuinely be choosing between: GRX RX820, which it calls its gravel group, and 105 R7100, its road group. Both shift twelve sprockets, both use flat-mount hydraulic disc brakes, and a bike shop will hand you the same sentence either way: GRX is gravel-specific.

That sentence is true, and it is also useless, because it does not say what changes. So we read Shimano’s own published specifications for both groups and found exactly three differences that are printed rather than felt. One of them is arithmetic you can do in ten seconds, and it is the one that should decide your purchase.

On Shimano’s published tooth counts, every GRX RX820 configuration has a lowest gear under 1:1. Exactly one 105 configuration does — the 50-34 crank with an 11-36 cassette, at 0.94:1 — and it needs the widest cassette Shimano lists to get there. GRX pays for that bottom end with roughly four tenths of a ratio at the top.

Difference one: the crank is smaller, and that is the whole argument

Shimano publishes the RX820 group with an FC-RX820-2 crankset at 48-31T, paired with either a CS-R8101-12 HYPERGLIDE+ 11-34T cassette or a CS-HG710-12 11-36T. It publishes the 105 R7100 crankset at 50-34T or 52-36T, with 11-34T and 11-36T cassette options.

Lowest gear is the small chainring divided by the largest sprocket. It is the number this site judges every gravel drivetrain by, because it decides whether you ride up a loose climb or push. On the published counts:

GRX with 11-34 is 31 ÷ 34 = 0.91:1. GRX with 11-36 is 31 ÷ 36 = 0.86:1. The 105 50-34 crank with 11-34 is 34 ÷ 34 = 1.00:1 exactly, and with 11-36 it is 34 ÷ 36 = 0.94:1. The 52-36 crank goes the wrong way: 36 ÷ 34 = 1.06:1, or 36 ÷ 36 = 1.00:1 with the widest cassette available.

So the marketing claim survives contact with the arithmetic, which is not something we get to say often. GRX is genuinely geared lower, by a margin that matters on the kind of climb gravel produces. Gravel gearing explained works through why 1:1 is the line rather than an arbitrary round number.

The other half of the trade is real too. The GRX big ring is 48T against 105’s 50T or 52T, so at the same 11-tooth sprocket the top gears are 4.36:1, 4.55:1 and 4.73:1 respectively. If you spend a lot of time above 25 mph on tarmac, GRX will spin out before 105 does. If you spend a lot of time on loose 12% climbs, that is a trade you should take happily.

Difference two: the clutch, and Shimano’s naming gives it away

You do not need a review to establish this one. Shimano publishes the GRX rear derailleur as RD-RX820 SHIMANO SHADOW RD+and the 105 rear derailleur as RD-R7100 with SHADOW RD. The plus is the chain stabilizer — a friction clutch on the derailleur cage that resists the cage swinging forward when the chain is thrown around by rough ground.

What that buys you is not shifting quality. It is the chain staying on the chainring over washboard and rock, and it is the difference between a drivetrain that slaps the chainstay for two hours and one that does not. On tarmac it does approximately nothing, which is why 105 does not have it and is not worse for the omission.

This is the difference most likely to matter to you if your gravel is genuinely rough rather than a smooth fire road. It is also the one that cannot be retrofitted by buying a cassette: a clutch is a derailleur, and a derailleur is the expensive part.

Difference three: the front derailleur is moved outboard by 2.5mm

This is the least discussed and the most quietly clever of the three. Shimano publishes every GRX front derailleur — the current FD-RX820, and the RX810 and RX400 before it — with a 2.5mm outboard chain line design to allow wider tire clearance for mixed-terrain riding. That is Shimano’s own wording, not a reviewer’s.

Move the chain line out and the chainstays can be made wider at the tire without the chain fouling them, which is the actual mechanism behind “more tire clearance” on a gravel frame. It is a frame-design enabler rather than something you feel through the pedals, and it is why a GRX-equipped frame can carry a wider tire than an otherwise similar road frame. The tire width guide covers why that clearance is the number that defines the category in the first place.

The practical consequence for a buyer is narrower than it sounds: if you are building or buying a whole bike, this matters and the frame will already have been designed around it. If you are upgrading parts on a frame you own, your frame’s clearance is already fixed and a GRX front derailleur will not widen it.

What is identical, and it is more than the marketing suggests

Set the three differences aside and these groups have a great deal in common, which is worth saying because it is the part nobody selling you a groupset will mention.

Both are 12-speed mechanical. Both run flat-mount hydraulic disc brakes. The cassettes overlap directly — Shimano lists the CS-R8101-12 11-34T, an Ultegra-tier road cassette, inside the GRX RX820 line-up, and the 11-36T CS-HG710-12 is available to either group. And Shimano’s published brake pad table puts the same BP-L05A-RF pad in the GRX BR-RX820 and the 105 BR-R7170 calipers, which is the clearest possible illustration: at the point where the bike actually stops, these two groups are the same part. The pad roundup has the full caliper list.

Which reframes the decision usefully. You are not choosing between a gravel drivetrain and a road drivetrain. You are choosing a chainring size, a clutch, and a frame design assumption — and only the first of those three is cheap to change your mind about later.

What we cannot tell you

We have not ridden either group, and this page contains no ride impressions, no shift-feel adjectives and no durability claims. Every figure above is Shimano’s own published specification, read on 4 September 2026 and linked, or our division applied to Shimano’s published tooth counts and shown in full so you can redo it.

Three gaps are worth naming. We have deliberately not compared weights here: Shimano does publish them, but we did not verify them for these two groups in this pass, and a remembered gram figure would be a fabrication. Our 105 versus Ultegra pageis the one that does that work properly, from the same database. We have also not covered GRX’s 1x option, which is a different decision with its own trade-offs — 1x versus 2x covers it. And Shimano does not publish a maximum tire clearance figure for the RX820 group, because clearance is a frame property rather than a groupset one.