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Discussion Starter #1
I recently had to change the alternator on my Corolla and the belt was original and in bad need of changing as well, so I got a new one. Everything worked out well but now my gas mileage is reduced .7 mpg. It was getting up to 38.2 and now it's sitting at around 37.4-6. Now for the 3,000 miles I've had and been driving it it's been mostly highway miles at 50-55 mph, hence the higher end mpg. But when I first changed the alternator and belt I did the 10 minute idle relearn while in drive and have been taking 1 hr long interstate trips a few times, along with that usual highway route.

I'm no longer driving the interstate and back to solely driving that highway route so I'm wondering does the car just need more time to get back up to previous average mpg, is the mpg lower because the newer serpentine belt affected it, OR have I tightened the belt too much? I tried adjusting it to about 1/4-1/3 in slack in the longest stretch of belt, as recommended from my research, but there's no way I see of accurately measuring that.

From trying to research this I've found lots of claims of reduced mpg after changing TIMING belts, but nothing about serpentine.

I'd be glad to hear any tips on the subject.
 

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Welcome to TN. I am going to move your thread to the 10th gen Corolla forum.

It will take time for the computer to relearn driver inputs.

IME, mpg improvents can be made by checking your tire pressures, air filter, and extra weight in both the passenger cabin and trunk cargo areas.

How are you calculating your mpg?
 

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Give it some time. Check your tire pressures. I watch my mileage like a hawk, but the margin of error in fills is at least .7MPG, see if the lower average is the same long term.
 

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Discussion Starter #4
Thanks for the responses! I tried deleting this thread as I posted here by mistake but didn't see an option. I already posted a duplicate in the 10th gen forums. You can delete this one.
 
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