I have a newish gravel bike, and a 13yo road bike (recent fitting). I ride my gravel bike like a road bike and wanted to get a similar feel to my road bike.

Gravel bike is okay most days, but now and then I get sore sit bones and feel too close to the bars. Same saddle both bikes. I’ve done rides as long as the road bike long rides, and the discomfort is infrequent, but when I’m feeling it...it sucks.

I can try and pick up some parts and solve it myself. Unfortunately I can’t get back to the shop where I had the fitting on the road bike. I fear that if I get a fitting on the gravel bike the guy will tell me all is well or make only the slight adjustments (same as when I got fit when I bought it 8 months ago).

Seat height is the same (inside heal of shoe to top of saddle...same area both bikes).

Setback on gravel bike is 10mm further back. I can’t go more forward as I run out of rail. Post has 20mm setback. Thinking I need a 0mm post.

If I match setback I’ll then be that much closer to the bars, which is already 10mm shorter and 10mm less drop). Guessing I’ll need 10mm more stem. I’m okay a bit more upright, but could drop spacers off needed.

Thoughts? I’m not sure which route I’ll go...

You sound like you are on the right track. Just double check all measurements to make sure you know what you need to do.

Just had a though, make sure the crank length is the same on both bikes or the results will be out.

Thanks. Gravel bike has 2.5mm shorter crank arms. When I measured height that’s why I took it from inside shoe with pedal at 6 o’clock, thinking that would even things, but yeah, that difference may still affect things...

OK, well with shorter cranks on the gravel bike I would have the saddle setback a little further than the bike with the longer cranks. Maybe not 10mm further back but a little further back non the less.

Setback probably should be measured from a vertical off the BB. If your bikes have different seat tube angles then you won't get the saddle in the same relation to the BB by measuring from the seat post.

You could also just set your reach to your hand position on the bars, but if everything else isn't the same, then still your saddle distance behind the BB might make things feel drastically different for you.

I measure setback by placing my bike in the corner of a room, measuring from the wall where the rear tire is touching to the BB, and then to the saddle (same saddle each bike), I'm less concerned with the actual setback # for a bike, then the difference between them (with one bike feeling spot-on).

OK, sorry that's not going to work without a lot of extra maths taking tire sizes and chainstay lengths and more into account.

The way that you should measure saddle setback is from the centre of the Bottom Bracket (BB) and you measure it to the tip of the saddle.
Place a rule/measure from the tip of the saddle and forwards towards the head tube. Drop a plumb line down to intersect with the centre of the BB axle and measure the distance on the scale. As a comparison this works best if you have the same saddle on both bikes.

Anyway, the true zero, zero point for all fit measurements on a bicycle (and probably some others as well) is the centre of the BB.

I’m going for a fitting on the gravel bike (2nd bike).

I thought I had everything dialed in, went for a couple rides and ended up with a really sore Achilles’ tendon (I’ve had in the past). Despite the numbers I’m getting between bikes, the road bike feels like I’m more over the pedals whereas the gravel bike feels like I’m behind them (and I can’t go any more forward on the rails).

Well listen to the fitter. Be willing to try their suggestion for at least a dozen or more rides. New positions sometimes require you to get used to them before you actually feel better.

However for calf and achilles tendon issues I'd have to ask if you have your foot too far back on the pedal and simply let it flex too much as you pedal. Moving the foot forward on the pedal may feel funny for several rides, but for me it has been an easy thing to get used to.