New tires/rims MPG dropped from 16-18 to 10

What did I do wrong? I had an old pair of KO2 265 70 16 on steel rims and I bought some takeoff 2023 Tacoma 17 in TRD Pro rims and Wrangler Adventure AT 265 65 17. My tire guy said dropping from 70 to 65 would compensate for the change from 16 to 17 rims. My MPG went from 16-18 down to 10. I checked the Speedo on a 100 mile stretch and it seemed accurate. Any wisdom from those who know more than I?

 

Well, tire diameter is identical, so your tire guy was right.
That's a pretty drastic change in mpg.
The only thing I can think of is the total weight of the new wheels went up, but it's hard to believe it would make that much difference.
If you still have your old wheels and tires, put them back on and see if your mpg comes back.
If it does, then clearly it's the new wheels and tires causing the difference. More likely the tires.

 

Interesting
Agree that’s big decline

IMO
Drastic, sudden changes as your experiencing come from multiple factors
(Like an airplane crash)

Rim weight….Y
What about tire weight?
Big change in outside temperature recently?

 

Agree with @ControlCar that there's likely multiple culprits. Even something like winter blend fuel might be responsible for a portion of the decline. Other than the wheels/tires being changed, was any other work done on the truck recently?

Also regarding the new wheels, are they maybe rubbing on the calipers due to incorrect backspacing?

 

Something else happened at the same time. It’s not the wheels/tires.

Was there any other work done around the same time? Are your brakes dragging?

 

Thanks guys. I had a bunch of other work done around the same time, but the low mileage seemed to be right after the wheel change. I had front suspension work, valve gasket, plugs and wires, and the throttle body cleaned. Those last two are potential culprits, but not sure what to do about them, how to diagnose. Also, these wheels have a different offset. The wheels stick out further. But that makes for greater clearance, not less, from what I can see.

 

Pull a plug or two and check the gap.

Jack up each rear tire and see if they spin easily (check for brake shoe dragging)

Check for vacuum leaks around the intake. They had to remove the whole intake and everything connected to the throttle body to clean it. This sounds like it would be the most likely culprit to me.

 

OP, try to see if your caliper has any signs of rubbing the inner hub face of the wheels. I bought my truck 4 years ago with some no name aftermarket wheels on it. It had the really dangerous style adapter plate spacers fitted as well. I decided to remove the adapter plates and quickly learned why they were on there to begin with:





I then promptly ordered and fitted Bora hub-centric spacers to rectify the situation safely. I've been wanting to get new rims (to eliminate the need for spacers) since and have passed on the exact rims you have several times because I've read from multiple threads that they have improper backspacing (and would rub the calipers).