If you take some time to do the math, cyclingpowerlab.com and a gear calculator website can figure all of this out for you as far as what type of gearing would need.
For example, if I know that my overall target wattage for a 40k is 300w, total weight (bike, rider, gear) is 85 kg, preferred cadence is 97, and that my cda is .25, then I can run multiple scenarios of power required for both the 10% uphill and the 3% downhill and you can then use a gearing calculator to see if such power/cadence will match the resulting speed.. Which I assume are the gearing factors you're most concerned about. How easy of a gear do you need for the steepest section, and how big of a gear do you need for the extended downhill where you will be pedaling.
So lets take an example from cycling power lab.
At all the assumptions above with a 10% grade (adjusted cda to .35 since I assume you will be on the horns...not that it matters much)
300w = 11.81 km/h
310w = 12.19 km/h
320w = 12.56 km/h
330w = 12.93 km/h
340w = 13.30 km/h
Now take those speeds and use a gear calculator (my favorite is mike sherman's site) with various gear combinations and cadences. Also note that a 750m section at 12.5 km/h will take you 3.6 minutes...not just over 1 minute for us mere mortals. I believe Kevin is on a different level than all of us... ;)
So if you assume 97 rpms, even a gear of 36/32 only gets you 13.8 km/h. So if you want to maintain that cadence, you'll need more than 340w for 3.6 minutes. But lets say you're willing to drop down to 80 rpms for that period of time. Then it becomes clear that even a 36/28 gearing allows you to maintain 13 km/h, so you'd need to do maybe 332w for 3.3 minutes or so...or roughly 110%. Not that bad considering you should have an immediate recovery going down the 10% grade. Would be tough if the 10% grade was at the 10 km mark and you still had to climb a 2-3% grade for another 9 km right after the steep section with no recovery.
Ok...so now for the downhill...assuming -3%. Here the spread of wattages don't make that much of a difference because at high speeds, additional watts don't matter as much compared to going uphill. But the calculation basically says that anywhere from 275-310 watts is 57.4 to 58.7 km/h. So at 97 rpms, what type of gear do you need?
The gear calculator says that with a 52/11, you can hit 58.4 km/h. With that same gear at 102 rpms, you'll hit 61.5 km/h. If you solve for 54/11, then it's 60.7 and 63.7 km/h respectively.
So just by sheer math...with all of the assumptions I made, I would think that 52/36 with 11/28 should be fine. But you need to plug in your own numbers and figure it out yourself. The bottom line is that there are useful tools that if your information is correct, you don't really have to guess.
For example, if I know that my overall target wattage for a 40k is 300w, total weight (bike, rider, gear) is 85 kg, preferred cadence is 97, and that my cda is .25, then I can run multiple scenarios of power required for both the 10% uphill and the 3% downhill and you can then use a gearing calculator to see if such power/cadence will match the resulting speed.. Which I assume are the gearing factors you're most concerned about. How easy of a gear do you need for the steepest section, and how big of a gear do you need for the extended downhill where you will be pedaling.
So lets take an example from cycling power lab.
At all the assumptions above with a 10% grade (adjusted cda to .35 since I assume you will be on the horns...not that it matters much)
300w = 11.81 km/h
310w = 12.19 km/h
320w = 12.56 km/h
330w = 12.93 km/h
340w = 13.30 km/h
Now take those speeds and use a gear calculator (my favorite is mike sherman's site) with various gear combinations and cadences. Also note that a 750m section at 12.5 km/h will take you 3.6 minutes...not just over 1 minute for us mere mortals. I believe Kevin is on a different level than all of us... ;)
So if you assume 97 rpms, even a gear of 36/32 only gets you 13.8 km/h. So if you want to maintain that cadence, you'll need more than 340w for 3.6 minutes. But lets say you're willing to drop down to 80 rpms for that period of time. Then it becomes clear that even a 36/28 gearing allows you to maintain 13 km/h, so you'd need to do maybe 332w for 3.3 minutes or so...or roughly 110%. Not that bad considering you should have an immediate recovery going down the 10% grade. Would be tough if the 10% grade was at the 10 km mark and you still had to climb a 2-3% grade for another 9 km right after the steep section with no recovery.
Ok...so now for the downhill...assuming -3%. Here the spread of wattages don't make that much of a difference because at high speeds, additional watts don't matter as much compared to going uphill. But the calculation basically says that anywhere from 275-310 watts is 57.4 to 58.7 km/h. So at 97 rpms, what type of gear do you need?
The gear calculator says that with a 52/11, you can hit 58.4 km/h. With that same gear at 102 rpms, you'll hit 61.5 km/h. If you solve for 54/11, then it's 60.7 and 63.7 km/h respectively.
So just by sheer math...with all of the assumptions I made, I would think that 52/36 with 11/28 should be fine. But you need to plug in your own numbers and figure it out yourself. The bottom line is that there are useful tools that if your information is correct, you don't really have to guess.