I ride an SF ~1/3 off-road, ~1/3 gravel roads, and ~1/3 pavement. I realize the SF isn’t the best suited for off-road use but I have 50/50 DS tires on it and that’s how I ride it regardless.

Stock final drive on an SF is 14-39T. I am currently running 13-39T and I am very pleased with the overall results that reduction has given me in all types of riding. However, I was in some particularly gnarly & steep off-road terrain the other day and I found myself continually hunting for another lower gear. It got me thinking about trying a 13-42T combo.

I calculate that it would cause me to spin an extra 700-800 rpm in top gear at 100kmh (60 mph), and I hope it would give me better engine braking on steep 1st gear descents, and a little extra rear-wheel grunt on steep ascents and in mud.

A quick search for others running a 13-42T final drive on an SF came up inconclusive, but I’d still like to hear recommendations from anyone who’s tried it.

 

I think you'll find you have too much engine braking on steep downs.
13-42 on a S.

Depending on the limit of the highway you're on it may get wrung out.

 

Ya, sigh, I am probably outside the limits, both off-road & highway, of what the SF was designed for. The SF really is out of its element on both highways and gnarly off-road terrain. It’s much better suited to dirt roads/trails and urban pavement where speeds are over 10kmh (6mph), but under 80kmh (50mph).

I am quite sure that an extra 3 teeth on the rear sprocket isn’t going to unleash some kind of secret off-road beast lurking within the SF. I am at a crossroads of considering a 351S, DR400, or something bigger like a DR650 or XR650. I’d really like to keep the weight & size down so something like a KLR is a non-starter.

 

My s model with stock gears will run 75mph(actual) all day long, granted it's slighly buzzy but if I had sf gearing even that would be improved.

 

The final drive of the S and SF gear ratios are the same, the SF uses a 39 rear cog to compensate for the smaller rear wheel.

 

I know these things dude and I was talking about my S model.

 

Just makin sure, some people think that by going to sf wheels and sf gearing they get better final drive ratio .

 

chain and sprockets are toast after a busted chain link...i'm switching to a 13t/40t and 15t/40t combination...i think that will end up being the perfect combination for me...

 

That 15t on the front & 40t on the back sounds like it might be the perfect combo. I'm just a little annoyed that i went for a Supersprox stealth 39t, because it will last forever from what i've read about the quality of them!!

Keep us posted with your review on the 15/40 combo.

 

Few things cause a missed heart beat than discovering a broken link plate!
I hadn't even realized that there is a 40T available for the KLX, but I can see that pairing nicely with a 13T. I look forward to hearing how it works out.