The truth is a few miles per gallon are not going to make much of a difference on your gas bill. Gas mileage is way overrated. I should know. I drive 50 miles to work, one way, four times a week. I used to have a Toyota Yaris because I thought it was saving me so much money. Well, one day I sat down and actually did the math. Here is how it pans out.
400 miles a week going to and from work. That equals roughly 20,000 miles a year.
At 30 mpg, you will spend somewhere around $2,200 in gas throughout the year.
At 20 mpg, you will spend somewhere around $3,300 in gas. So you save $1,100 during the year. Or $22 bucks a week. Now that seems like a lot, I know. But that is the difference in 10 mpg. Even going from 331's to 373's, you will not drop anywhere near 10 mpg. Maybe three or four.
Now, those numbers are found if you are driving approximately 20,000 miles a year just to work and back. Most people don't drive that much. And there was a 10 mpg drop in mileage, which you wont have. The less you drive, the smaller the price gap.
While saving approximately $90 a month on gas sounds great, you can easily cut that much out of your budget on other things and drive the car you want.
I went from my Yaris and getting 37 mpg to my Mustang getting 24 mpg (now 28 mpg with the cat back). It was a little cheaper to drive the Yaris. I guess you could say I would have an extra $100 a month in my pocket if I was still driving one. But I was driving a YARIS! I am much happier driving my Mustang. I can actually pass people on the highway now. And no one makes fun of my car anymore. The real savings was that the Yaris was cheaper to buy and cheaper to insure. That is where the savings are. But you already have a Mustang. The rear end change will not change your insurance.
So if you want more pep, I would get the 373's. I don't think it will make that much of a difference, either in mpg or performance. But the 373's will be faster than the 355's, and the change might be noticeable if you go from the 331's to the 373's. I bought the 355's to save gas, too. But now I doubt it saved me much at all. Next time I am getting the bigger rear end.
You got the car because it was the best performance bang for the buck on the market. It gets respectable mpg. You won't kill the MPG unless you drive fast and race around town.
If you want the lower rear end, don't pass on it because you will save $10 a month on gas. I say eat at home once more a week and get the hot set up. The only real savings you will have is not changing out the rear end. But you have to have a car you like driving.
2011 black C/S. Stickers are gone, 5.0 badge in place. Flowmaster cat back Pro Series/Hushpower exhaust that comes with the worst drone in history! FRPP tuner. Drop in K&N filter. Auto Tans
I sit in traffic a lot. HID/security package. 3.55 rear end.