Originally Posted by scottfsmith

Noonievut suggested on a recent thread here that there should be a formula to pick how wide a tire to run on gravel. I thought it would be fun to take a stab at it.. here it is.

1. Look at the Cycling Tips 1-5 scale of gravel difficulty and pick the number which is the max you will frequently be riding on (i.e. you will be under-biked if you had smaller tires on that).
2. If you are rarely on gravel (less than 10% of the time) subtract one from your number from step 1 -- you don't want to compromise your road riding too much and should be willing to be a bit under-biked sometimes on the gravel. Otherwise keep number the same. Lets call this your RGG, your raw gravel grade.
3. Now, take RGG * 3 + 30, call that RGW, your raw gravel tire width. This is the initial approximation of what size tire to run and we will now go through some potential tweaks to your RGW.
4. If you want a nice compromise of speed and smoothness, make no change. If you value smoothness over speed, add 3. If you really value smoothness over speed, add 6. If you really value speed over smoothness, subtract 3.
5. For every 20 lbs over 160lbs you are, add 1, and similarly for every 20 lbs under subtract 1.
6. Apply all the tweaks to your RGW and that is your gravel width, GW, the size of tire to run.

Applying this to me, I am about at a 2.5 in terms of the max gravel I ride by the CyclingTips rating system. I have special gravel rims which are mainly on gravel so no minus one for me. So my RGW is 30 + 2.5 * 3 = 37.5mm. I don't have any of the other corrections so my GW is 37.5mm. This is pretty close to what I run, my Terra Speed 35c are 37.2mm mounted.

Of course this is all just a stab in the dark with numbers pulled out of my head. But I did a couple calculations imagining various scenarios and it didn't seem too far off. Feel free to punch up your numbers and see. It is also mainly for fun and maybe for beginners to get a start, don't take it too seriously.