I had mine changed to 28mph but with a 36T chainring it wasn’t really useful - I don’t pedal along at over 100rpm. I installed a Praxis 44T chainring and now I can maintain 28 at a cadence of about 80 and can actually pedal into the low 30’s if desired. Much better. Its now very similar to a Vado 5.0 running a 48T chainring with the same 11-42 cassette and 650’s. I need to make an adjustment in chain length, but initial results are good. If I really miss my .86 ratio I can change to an 11-50 cassette (SRAM PG1230) and get it back, but I never used 1st anyway and don’t think I’ll ever need gears that low. Also the new chainrings from Praxis have a plastic guard which is nice. They aren’t listed on their website but they are available as a maintenance item - just have to request it. As noted elsewhere the guide has to be removed (mine was broken anyway) but if a guide was needed a new mount could easily be fabricated.
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One thing to note is that the class 3 upgrade and bigger chainring are only marginally related. I don’t really notice the assist at higher speeds and for the most part don’t even try to ride that fast. But I didn’t like running out of pedaling ability so early on downhill runs. With a 36T chainring the bike was often pretty much just a coaster. Similarly you don’t need assist when going down hill - just higher gears (I often turn the assist off). So if you are really hung up on the class1/3 thing, my recommendation would be to leave it at class 1 and go to a bigger chainring to make the bike more useful. Personally I don’t see a problem with the differing classes - if you just ride responsibility it wont be an issue.
Regarding losing the 1st gear ratio, as I said above it was a non-issue for me because I never used it, or second gear for that matter. I get more satisfaction riding in a higher gear with lower assist (eco) and working harder. After just two months I can now stand up pedaling up hills which - something I couldn’t do when I first got the bike. If you have any concerns about gear rations and want to plan your mods a little more purposely than trial and error, use this:
https://www.bikecalc.com/speed_at_cadence. I find “speed at cadence” and “ratios” to be most useful. This will also quickly show you that a minor change in chainring size (38 or 40) will only have a minor effect on road speed.
I guess if you really want to use your Tero as a mountain bike, and ride steep challenging single tracks, you might miss the .86 ratio of 1st. But I quickly found that this bike isn’t really suited for that kind of thing (nor am I), and while they advertise the bike as having “mountain bike DNA” they really created either an overweight, less than agile, tank of a mountain bike, or a very capable but somewhat under-geared SUV type touring bike. The Tero is listed under “Active” and not “Mountain” on the website for a reason. I like the heavy duty aspects of the bike, but I use it more like a Vado than a Levo, hence some of my mods. I suppose its a little more capable on trails than a Vado but it is nowhere near the Levo/Kenevo level and if thats the kid of riding you’re planning on, the Tero will be a disappointment. As a heavy-duty touring bike, I think its wonderful.