You don't have to be afraid of using 25% nitro, its only a couple of mph and it depends on alot of factors. If you run shit fuel like jero glo etc.. i want say to much of the fuel that i mean is shit couse there are people here who use a lot of bad fuel, in many cases because their local hobby shop don't have anything else and it's easier to buy it there. I don't want to offend anyone. But there are 20 % fuel out there who will be stronger then some types 25 % nitro. I think you should think a lot more of the set up on your car like: The Clutch shoes, springs, flywheel, venturi, Manifolds, pipes, Clutcbells. It's a heck of things you can do to make your car go slower and faster without thinking about the nitro% and you dont need to reshim to go back to 20%, but stay away from bad fuel and the engine you got in your car..there is no problem to make the car go faster or slower with 25 % nitro. Relaxe use your 20 % nitro in the engine and maybe you have to tune the high speed needle leaner.