As with many words that sound similar when spoken, accel and excel are often confused in writing. Being vigilant about the intended meaning and following a few simple guidelines will ensure that you don't mix up these two words.
Define excel as: to do better, surpass or be superior. Someone who excels at something is more than just good at it; they are outstanding, well beyond the norm or the average. Synonyms for excel include skillful, proficient, outdo and outshine.
Use excel as a verb. Here are some examples of proper usage: "She excels in math" and "Bill excelled beyond his competitors."
Define accel as speed up or accelerate. Accel is actually an abbreviation of accelerando, a word used in music to indicate a gradual increase in pace. More often, however, we use accel as an abbreviation of the word accelerate, usually referring to driving a motorized vehicle or a physical activity such as running.
Use accel as a verb. Examples include "He accelled when he got to the straight away" and "Press the gas pedal to make the car accel."
Remember the usages for these two words with this hint: Excel is part of excellent and is similar in meaning, while accel is part of accelerate and is similar in meaning.