what does a bigger sprocket do

You basically lowered your gearing. Bigger rear sprocket gives more torque to the wheels but reduces top speed and raises RPM at any given speed. For tight trails that’s great, but it makes fire road cruising annoying. Standard trade-off.

Exactly what CK said. It’s all about the ratio. If you want the same feel but lower RPM on the road, you could go back to stock rear and drop a tooth on the front. That keeps the low-end but brings RPM down a bit.

I did the same thing on my Kodiak 450. Loved it in the mud but hated it on gravel roads. I ended up swapping back to stock and just using low range for the nasty stuff. It’s simpler than chasing perfect gearing.

“Lowered gearing” is correct but think of it as a trade-off: you increase mechanical advantage at the wheel. That’s why you climb better. But the engine works harder to turn the same wheel speed, so fuel economy drops and engine wear might increase if you run high RPM a lot.

You didn’t mess up – you just need to decide what kind of riding you do most. If 80% is trail and 20% is fire road, keep the big sprocket and accept the noise. If it’s the opposite, go back to stock.

I disagree with keeping it if you hate the RPM. You can split the difference with a +1 rear or -1 front. That gives a moderate gain in low end without making highway speeds unbearable. Check your chain length too – bigger rear might need a longer chain.

Chain length is key. I put a 14T front on my YFZ and needed two more links. Measure before you buy anything. Also, bigger sprockets increase chain wrap, which reduces wear on each tooth, so that’s a plus.

Bigger rear sprocket also puts more load on the chain and sprocket teeth because torque is multiplied. If you ride aggressively, expect faster wear. Use quality steel sprockets and keep the chain lubed.

What’s your front sprocket size? If it’s a 14T, going to a 13T front gives similar effect to a bigger rear but is cheaper. You might already have the best combo for trails.

I run a 15T front / 38T rear on my Rancher for all-around work. For climbing, I use low range. That keeps the RPM sane on roads. Consider if your ATV has a low range – use it instead of changing sprockets.

“Low range” is not an option on sport quads. For utility quads it’s great. OP has a 400EX, no low range. So gearing changes are the only way. I’d say big rear is correct for woods, but you’ll have to deal with the noise.

Make sure your chain tension is correct after sprocket swap. Too tight and you wreck bearings. Too loose and you skip teeth. I dropped two teeth on my Z400 and had to shorten the chain.

Sprockets are cheap compared to replacing a rear end if you mess up. Buy a quality set (Sunstar, JT, Renthal) that match your riding. Don’t cheap out – a stretched chain from poor gearing can snap and damage your case.

I’ve been riding with a +2 rear on my LTZ400 for 3 years. Yes, RPM is high but I never stall on technical stuff. I just wear earplugs on long straights. To each their own.

Partial disagreement – if you’re always at high RPM, your engine temp climbs faster. On hot days in slow trails that can cause overheating. Consider a cooler thermostat or fan switch if you keep the big sprocket.

I learned the hard way: bigger rear sprocket means your speedometer reads faster than actual. If you have a digital dash, it’ll be off by 5-10%. Calibrate or ignore it. GPS tells the truth.

Important: confirm your model year before buying. 400EX sprocket patterns changed after 2005? Not sure but double check counts and offset. A little research saves returns.

OP, your buddy wasn’t wrong – bigger rear sprocket does help low-end. But you should have asked here first. Now you know the trade-off. Try it for a few more rides before changing – you might learn to like it on the trails and just avoid the fire roads.

I put a 13/39 combo on my Grizzly for crawling. Same deal – wide ratio. But now I can pop wheelies easily. Fun factor went up. If you like that, keep it. If not, swap back. Sprockets are cheap compared to a clutch replacement from constant slipping.

One more thing – if you ever do trails with friends on stock machines, you’ll be faster from standstill but slower on top end. That can be annoying in open sections. Sounds like you already noticed that. My advice: go back to stock and use clutch technique for obstacles.