If you pay $10/mo extra, you can cut about a year off the term of the loan. As stated before, remember to specify the excess to principle. Paying it off early won't hurt your credit score, it might just not be improved as much as it would have. If you pay if off early by making consistent payments of a higher amount, you will likely improve your credit just as much if not more.
An option if you can swing it, would be to pay it down part way fast, to a point where you can sell it for more than you owe. Just to get out from under the loan. Than buy a cheaper car. Not only would you save on interest, you would be able to cut your insurance cost as well. Over the long haul you would save big.
Once you get out from under the loan, make car payments to yourself. Put the money away and when it would come time to replace you will have the cash ready.
Very dependable cars can be had for $5000 or less. My 01 Altima was $5000 when I bought in 2011. It has 121K miles. today it has 240, and all I've needed to put into it has been gas, oil, tires, a battery, brakes and rear struts. Run Synthetic oil and only change the oil every 15,000 miles. It is starting to show body rust under the rear doors. Other than that, it should make 300,000 miles. That is unless my daughters who will be driving in another year decide to remodel it.
The world needs ditch diggers too Danny!!!