I just had the engine replaced with a rebuilt from the dealer. I noticed I’m only getting 20 MPG from both city and highway driving. Thursday and yesterday I drove over 10 miles on the freeway, and still only got 20 MPG. I filled up the tank with 8 gallons and mileage was 180 miles on trip meter. I didn’t notice if I was getting this MPG before the engine was replaced.
driving 10 or 20 miles is not going to give you a true reading of your MPG.
fill your tank and drive until you are about a quarter of a tank and then figure out your MPG. also do not go by your MPG gauge in your dash if you have one. it might not be accurate.
Drive the vehicle for a while so the engine breaks in, then see if the mileage improves.
This may take a thousand miles or more.
Tester
Do you notice any other issues with how the car drives, as far as the engine goes? Are you getting heat in the passenger compartment after the same amount of driving as before (a stuck open thermostat can reduce mpgs)?
Is the check engine light on?
There could be various factors, such as:
- Alignment is bad
- Oxygen sensors making the engine run too rich
- Knock sensor / ignition timing advance cancelled issue
- Timing belt / chain incorrectly set
- Older engines gain bit of compression ratio and efficiency due to carbon build up as they age.
- The transmission isn’t going in to lock up / over drive.
That means this person does not even know if he is getting worse mpg . Where does this 30 to 51 number come from . If this is not a hybrid it would never have gotten 51 . If he looks up on the web he can find out what the EPA rating was for this mystery year Civic.
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Probably close to the same as before if you’ve never checked.
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Going 10 miles is barely enough to warm up the car unless you live where it’s warm now. Go for a half hour or more drive on the highway and tell us what the mileage is on the trip. I think you are using the MPG meter on the car. If so, make sure you read the trip gas mileage and not the mileage since the last reset. The best way to measure gas mileage is to fill up, set the trip odometer to zero, then record the gallons of gas you buy at the next fill up and the total miles driven for that tank. Divide miles driven by gallons purchased. Reading the trip MPG from the gauge will be within a couple MPG and is accurate enough for a quick check.
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Yes, that is an important detail that might not have occurred to the OP.
Additionally, if the new engine was running while it was stationary at the shop, it was getting zero mpg, and that is going to be part of the calculations done by the computer’s algorithm–until it is reset.
But, since the OP admits not knowing what the gas mileage was prior to the engine replacement, how does he/she know what type of mpg his/her driving patterns usually yield. Perhaps 20 mpg is typical for his/her driving patterns. 
The figures lead me to believe that his fuel economy was 22.5 which is good for cold weather. It’s not Civic great but tires, pressure and alignment are unknown factors. Break in counts too as mentioned.
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If using the car’s mpg calculation, it takes quite a few miles to reset the averages so the reading will not be correct for a while. Calculate it the old fashioned way. Miles travelled divided by gallons in.
180 miles driven and my tank is almost to 1/4.
Which means nothing . That is not how you determine your fuel usage .
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That’s what I did yesterday when I filled up my tank with 8 gallons and had 190 miles on the odometer, which gives me 20 mpg.
No, just the opposite. It takes the heater a long time to get warm.
190 divided by 8 equals 23.75 , me thinks your math pencil is broke.
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I know how to use the trip meter. Every time I fill up the tank, I reset the trip meter. I’m not relying on the car’s MPG. I’m figuring out in my head as I watch the fuel gauge go down.
Good Grief , 
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180/8 is 22.5 mpg on a cold, newly rebuilt engine. Give it some break in miles and if it continues to take too long to warm up, it likely needs a new thermostat.
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