There are 5 harness slots. All measurements are with me pushing on the padding.
9 inches (8 with infant padding)
The bottom slots can only be used rear facing. The next 3 slots up can be used rear facing or forward facing. And the top slot can only be used forward facing.
The seat shell measures 24 inches. My Marathon measures 25 inches. I put my daughter in both seats. In the Marathon she had at least 4 inches above her head. In the My Ride she had barely 4 inches above her head.
The widest part of the seat is at the cup holders, and it measures 20.5 inches. The widest at the head is 18.5- 19 inches. For comparison, the widest part of the True Fit is 19 inches.
The seat width at the knees is 9.5 inches. The width at the butt is 10 inches. The width of the base is 11 inches. The harness adjuster is 7 inches from the floor.
The harness adjuster is similar to the Graco Nautilus. The harness material is the same as the Nautiuls and the Safe Seat 1 and Snug Ride 32. The chest clip and crotch buckle are also the same as on the GN, SS1, SR32.
The seat is lined with EPS foam from the top of the head, down to below the bottom harness slots. It lines both the back and the sides of the seat. The seat cover is "Spiral" which is 2 shades of brown. It is nice and soft. There is no mesh on this seat (like on the GNs). I am having a hard time keeping 1 side of the cover on now, but it's not a big deal.
BabyS is 4 months old and 16 pounds. She is on the 2nd slots in the Safe Seat 1, with a few inches to grow above her head. She's still on the bottom slots of the My Ride, and the top of her head is even with the top slots.
Sofi is 2 years 7 months and 25 pounds. She is on the 2nd slots of the Marathon and True Fit. She's also on the 2nd slots of the My Ride. I didn't try her in the seat installed in my car (but I did on the test bench at BRU) but she has a little more legroom in the My Ride than in the Marathon, but less than in the True Fit.
J is 6 years old and 50 pounds. He is on the top slots of the Nautilus. He is over the top slots of the My Ride by about 1 inch. A few weeks ago he was dead even, or a hair above the top slots on the Marathon.
To switch from forward facing to rear facing, you need to flip 2 feet on the bottom of the base. You can see me flipping the foot on the left. You need to do the same to both feet. You simply flip the feet under/in the base for rear facing (which is what I'm doing in the picture). Or flip the feet out and on the sides for forward facing.
The Harness has 3 length adjustments. The picture I show it on the middle adjustment. Both Sofi and BabyS could use the harness there. As the child gets bigger and harness gets tighter, you can adjust it in the back to give you more length. Similar to the Safe Seat 1/ Snug Ride 32.