Sorry for the slow reply. I am the OP of the thread.  My gf and I have not put bigger tires on. I stand by what I said about actual measurements. She still has the 42 WAM tires on now and I am 100% certain it would fit 50 WAM tires from what I can see.   I do want to say that, as someone coming from a MTB background, I don't understand why more people do not just consider going with a smaller rear tire.  This has always been VERY common in MTBing, going back decades. There is no need, especially on gravel, for a tire as big in the back as the front, so people could just size down a bit. I actually often didn't even use the same tire (not just size) when I competed in XC MTBing. I realize some people don't like the look of two different-size tires, but I am not one of those people who are bothered by that.

This is setup for my gf. It fits her and her proportions, riding style. It is a 90 stem. 90 stem is not long for a gravel bike. I know people are going longer if they want a race bike, but that is not how she is using it.

Those photos are with tires measuring 38.  I actually put on 38s, which measure 42 now. I have it set up for the Champagne gravel (rails-to-trails) gravel we have around where I live. I would never need more than 45mm for anything within 3-4 hours of me. When I did use 50mms on my other gravel bike (Lauf Seigla), I still used a 45mm on the back.

I don't mind the spacing. I spend almost all my time in 18-33 on the back anyway, as my area has lots of rolling hills. I also come from MTBing originally, so even after 25 years of road riding, I am still fine with the bigger jumps when at the big gears in the back. The simplicity of any button or lever on the left being one shift and the right the other is such a joy. 

I don't think the SRAM braking is better than Shimano. I think it is different, and some people prefer one over the other due to lever feel and design. SRAM D1 was horrible, of course, but the E1 is what people like now, and I think that upgrade was huge, but not really over Shimano. If people are super concerned with braking, a bigger improvement would be 180/160 rotors instead of 160/160 or 160/140! 

I have nothing against SRAM, and I have it on two other bikes (one road bike with Red/Force and one Gravel bike with Rival/Red XPLR mix).   Shimano is cheaper right now, by a lot. I bought the groupset pieces off AliExpress from the Anrancee store. The equivalent Red E1 XPLR would have cost MUCH more.

7.2kgs with 40mm tires (size M)

Dura-Ace/GRX/XTR build details below!

Tire clearance detailed below!



https://drive.google.com/file/d/1WTlJCgMTGyZIGCSC4D-jL-Pu9TamX5sA/view?usp=drive_link

Tire Clearance on Evolve Cima GR: (Photos: https://www.facebook.com/groups/975602387895509/posts/1339069638215447)

With a 40.69mm measured tire, I have 11.92mm of space on EACH side on the back wheel. With 4-6 mm of space around the tire, this allows you to go with any gravel tires I am aware of that are 50-622. Going larger on the back will be determined by whether you have a 2x and the tread type you are putting on.

Is it going to fit the big Thunder Burts or MTB tires on the back? No.

Will it fit the Dubnital on the back? That will depend on your rims.

50s easily fit on the front... with the same tire on the front, I had 13.00mm on each side. IF you felt the need to go big with tires, you could run larger on the front than the back, of course. IMHO, when truly on gravel or single-track, you should be doing that anyway!

BUILD DETAILS:

Evolve Cima GR (Medium / Sailfish Pearl)
Direct Mount Hanger   Evolve
Cockpit Handlebar/Stem   Farsports F1 Ultimate 37x90
Computer Mount   Farsports
Bar Tape & Plugs   Fabric Knurl Gravel tape
Saddle   WTB Volt Carbon
Wheels w/tape   Elite Wheels G42 (28mm int/36mm ext hooked)
Tires   GP 5000 S TR 38 (measures 40.2mm)
CL Rings   Wolf Tooth MK0 Center Rings
Tube/Sealant   Fillmore
Rotors   Dura-Ace CL 160mm / Galfer CL 160mm
Bottom Bracket   Alpha Ceramic Speed BSA 68
Pedals   Wolf Tooth Del Pedals
Levers   Shimano Dura-Ace 9270
Brake Calipers w/hoses   Shimano Dura-Ace 9270
Front Derailleur   —— none
Rear Derailleur   GRX RX 827 w/ battery
Cassette   XTR 10-51t
Chainrings / Crankset   Wolf Tooth 1x Aero 46t & 160mm Dura Ace
Chain   M-9100 Shimano
Oversize Pulleys   Kogel Hybrid Ceramic
Ti Bolts   S-Parts

I am waiting for the May 10th release of the Evolve Cima GR. Then I will make a decision between these bikes, their geometry, and colors. This will replace my gravel bike and my endurance bikes. I MIGHT one day get down to two bikes (until N+1 creeps back in). I have the evolve Cima and it replaced my 2.5 year old Sworks. It is better than the SL8 and the parts that came with it are phenomenal, and the claimed weight tracked (which almost never happens).