Effects on Chain Geometry: Decide on whether to have a smaller front sprocket or to have a bigger rear sprocket
When changing the final ratio, you can go 3 ways:
1) change all, front-, rear-sprocket and chain (always best !)
It is a lot cheaper to buy a smaller front sprocket and keep your current chain and rear sprocket then to buy a bigger rear sprocket and possibly have to buy a longer chain. That is why people are always wonder how much the front must be smaller to have the same effect as making the rear 2 or 3 teeth bigger.
Of course there is more to it then just money. A smaller front sprocket will put
more strain on the chain (and thus wear it sooner) and might even cause the
chain to scrape the swing arm and damage it !
Besides having impact on chain wear, changing the sprockets also has an impact on the chain length needed or the sprocket center distance. By entering both options (front-smaller and rear-bigger) you can see the effect they have on your chain geometry and depending on your current options for moving your rear wheel, decide on one of both.
Let's assume you had a stock setup of 15-36, then changed it to currently 14-36 but now chain and sprockets are worn out and you want to replace all. Since you like the current final ratio but would like to return to a stock front sprocket, you need to choose need determine the size of the rear sprocket. So the question is what size rear sprocket will give you the same final ratio you have now when using a 15 teeth front?
To find out, you will have to experiment by trying different values. Fill in all the know sizes for Stock and Current. Then in Custom setup fill in the new 15 front sprocket. Now you have to choose bigger rear, lets start with 38:
As a result you can see that difference between Custom and Current is -1.5 % (less RPM at same speed). So this is not exactly the same as you have currently. Maybe a even bigger rear sprocket will get the same result as the Current setup. Now enter a 39 rear sprocket and check the differences:
Now the difference between Custom and Current is + 1.1 % (more RPM at same speed). This is already closer to the wanted current final ratio but still it differs. Now it's up to you which version you prefer, the one with 38 teeth (1.5% more RMP at same speed) or the one with 39 teeth (1.1% less RPM at same speed). So there seems to be no exact match for the 14-36 ratio but both 15-38 and 15-39 come close.
You can also use the "Final Drive Ratio Graph" which shows the custom final drive ratio in a graph and 48 other ratios based on it. Here you can easily see which other sprocket combination comes close to the one you are looking for.