gas milage dropped from 20 mpg to 16.5- what is causing this

Late this summer I finished up a lot of the standard "maintenance" I wanted to accomplish on my truck thinking it would not only ensure the truck will serve me well for years to come but also that it may improve the gas mileage. Unfortunately the opposite has occurred and gas mileage has dropped significantly. Keep in mind this drop also coincided with the installation of a A.R.E. Z Series topper, so not sure if that has had a major impact on the drop.

Here is what was all done shortly before I noticed the drop in mpg:

New plugs (Denso and they were gapped)
New spark plug wires and wires,
fresh front and rear differential fluid (synthetic),
upper and lower ball joints,
new toyota coolant,
timing belt and water pump,
new lower radiator hose.
New Tie Rods
Fresh Transmission Fluid (Synthetic)
MAF Cleaned
Throttle-Body Cleaned

Of course we are in the dead of winter and winter gas usually hits MPG negatively but in my experience I have not seen it impact any of my previous autos with a 20% drop, especially when you consider all of the items above that I would think would help offset some of that poor gas.

Any thoughts or insight into what I can do or check to get this thing back to where it was 3000 miles ago?

 

Winter gas can be a huge killer of MPGs in my experience. And yes, 20% seems like a lot but I've seen it happen in winter before.

The extra weight of the topper (200lbs?) has an impact too.

Also check your tire pressures. Many folks forget to do this when winter hits. It can impact fuel economy.

 

02 sensors?
Cats?

 

I lost a couple miles with my softopper.

 

I forgot to mention.... idle times. So many people let the vehicle run for a while to "warm up" the motor or the cab before they drive. This will destroy your overall MPG calculation.

 

Kind of funny you say the mileage drop started with the cap install but don`t think it had any impact.

Then your mileage calculations are made on nice calm days with no wind.

Cross winds and head winds can reduce mileage

It is not only the extra weight (How much is in your cap now) but wind resistance

Add in idle time and winter blend gas .

I am quite content if my mileage does not drop below 10 mpg

 

Are you using 4x4 around town for snow?

 

Essentially I added the topper and did a majority of the maintenance listed in a very short period. I didn't check mpg after that.

I recently noticed it seemed like a tank of gas was going a lot quicker so I calculated my mpg on the last 3 trips back to my home town and back (all interstate) and got the following 268 miles on 15.3 gallons , 223 miles on 14.8 gallons, 252 miles on 15.1 gallons for a total of 743 miles on 45.2 gallons (16.44 avg)

This was on dry roads, no 4wd, just filled up reset the trip odometer and on the way.

When checking mileage I run just under 75 mph per my speedometer and I make sure it includes a same day round trip so that if I am benefiting from wind one way, I am paying the price on the other.

I just have not seen a 20% drop in the winter before. Maybe the topper cost me a couple and the winter fuel cost me a couple and all of my maintenance gained me a half mpg for a total net -3.5 mpg.

I know this is only 3 tanks and if I throw out the low one I am over 17, but before the topper and the maintenance I had some tanks at 21.5 and some at around 19 so I was just surprised to see it drop that much with all of the updates. I guess once it warms up I will find out as the topper comes off once ice fishing season is done.

 

268 miles / 15.3 = 17.1
223 miles / 14.8 = 15.06
252 miles / 15.1 = 16.688

When my truck was COMPLETELY stock with the stock shit highway tires I got 19.1 MPG average on semi highway drives to work during warm months. Granted I have the 4.0L V6.

 

I read somewhere where the amout of hp required to travel at 65 than 55 is pretty good, something like 2 times the power for 20 mph more. its an air resistance thing.

 

I've been seeing the same thing!
When I first bought the truck I had about 19 mpg but more recently I tested it and got 16

 

Just had a drastic drop myself. In December I changed the spark plugs and cleaned the MAF sensor but didn't have time to get into the throttle body. Yeah I know I was right there but in winter I have to make jobs quick as possible. It'll be a summer job.
I noticed a 3-4 mpg drop immediately. I haven't see a drop like this before. Was getting 21 now struggling to get 18. I use fuelly to track the MPGs.
So I'm wondering is there a retraining the computers need to do ? Is there a way to do that? Reset the computers and have it re-learn my driving. I've read that someplace I think but can't put my fingers on it. Yep I'm in the 1st gen area and have a 2nd gen but I don't care where answers come from.

Double Deuce, I bet your addition in weight is your biggest factor.

 

You can disconnect your battery for about half an hour, this will force the ECU to recycle and reset all monitors. It will also do this on its own after about 40 miles of driving (above a certain speed, as I recall).

I am always amazed at how much the winter ethanol/gas blends affect the gas mileage of these trucks. If you're really concerned about it, find somewhere that has ethanol free gas, I bet you will notice a huge improvement.

 

When I used to meticulously track my mileage on all my vehicles, there always was a significant dip in MPGs over the winter do to the regular gas blend and extended engine warm-up time. Using 4WD dropped it even lower.

 

Agreed but I've never seen it this bad. I've had this taco since it had 8 miles on the OD and after 125K+ it was a shock.

 

I seem to remember reading somewhere that driving at 70 vs 75 in these trucks had a noticable positive impact on gase milage. I'd guess it's a combo of that and the things you've listed already

 

I can tell you first hand, highway speed drinks the fuel on our trucks. They have the aerodynamics of a brick. I drive mostly highway to and from work, stay below 65 and have been averaging 21.5 - 22, Scangauge and hand calculated. Stock 2.7 4X4 Xcab on Michelin's. Winter gas obviously. Summer I will get 2+ miles per gallon better. Increase to 70 - 75 MPH and I will lose 3+ MPG without question.