Got some bigger tires and mileage is now horrible

Typical. On my 01 v6 I went from 265 70 17 street tires, to same size all terrain and lost about 1.5 mpg. Getting roughly 13.5 mpg now.

 

Interesting. They must have been C load, as kos are heavy to begin with. Look up the weight difference between the tires. I found a 16lb difference between each wheel resulted in a 12%-15% mileage drop and much slower acceleration even in the same size.

 

Two rims, same size and each weighing 20 lbs can have a difference in mpg's by where the weight is placed on the rim. The further out from the axis of rotation, the more it affects mpg.

That being said, a 15" rim and a 17" rim both weighing 20 lbs, the barrel of the 17 is farther from the axis of rotation and is going to get slightly worse mpg's. But let's be real, most 17" rims don't weigh the same amount as a 15".

I did a shit ton of research on this matter. lol...

Edit:
I should add that a 4 cylinder is affected easier than the 6 cylinder models which would be barely noticeable.

 

Doesn't sound to far off. On my v6 I got about 17 stock city. Now with 285/17s load E I get almost 14.

 

The size difference is a bit more compared to original.

Previous vs new:


Stock vs new:



The weight of the entire wheel is going to make a difference too. Rotational mass is a lot "heavier" than static weight on the truck, so it's going to take a lot more power to move than something loaded in your bed.
All the theories are going to be different from each person, but your real world experience is all the proof you need that it is negatively effecting your MPG. Rest assured though anyone who changes to a larger wheel and tire always loses MPG, you are not alone.

 

I have 31's, and have terrible mileage. That's part of the price you pay to drive a mean truck. Hope yours looks mean.

 

Weight and tire pressure. Larger tires all else the same should actually yield better mileage due to engines being more efficient at higher loads.

 

You didn't mention if your truck has a manual transmission or auto, but if it has the auto turn on the ECT button. Also wouldn't hurt to check your actual speed and distance with GPS on a smart phone.

 

Get some 31” pizza cutters and you’ll likely see some mpg regained.

 

Yeah, the 235/85r16 size may be going on my second set of rims soon. Seems like a lot of people here really like the skinnies.

 

Ya I like the skinny’s, once I get through my current set on my 96 I’m gonna go with those. Stupid tires are so expensive I’m getting every ounce of rubber out of these i should invest in a second set of rims to have 2 sets but haven’t yet.

 

I found a set of steelies for $50 and couldn't pass it up. lol

 

I’ve been lurking Craigslist and let go hoping that someone is ditching some, but it’s all damn terdgen wheels

 

I have a 99’ 3RZ Manuel with E Rated KO2’s 265-75R16’s with heavy steel wheels. I easily get 21mpg tank after tank. And that’s using GPS to calculate. I drive mostly highway and keep it around 70mph tops. I have a 1/4 tank right now and the trip odometer shows 267 miles.

Somethings wrong.

 

1996 with a 3RZ and came with the 28" stock tires... you probably have 3.58 axle gears, truck is working too hard now with the larger diameter. 4.10 axle ratio would be like stock again

 

rotating mass, dude.

 

Weird. I cannot seem to sell my 16" steelies with 1/4" plus of tread on the mounted and balanced 245/75 BFG KO's, including centre caps, TPMS sensors, and lug nuts, and I'm asking $225 CAD or about $175 USD

 

235 is too wide/ tall. 215/85R16 is closer to stock, and the real skinny. It's the skinniest 16" you can get. I love mine. The E rated Bridgestone Destination XT's are as light as the P-rated stock tires.

 

Go on E Bay for tires. You can't beat their prices and supply. I have some really nice mudders for 450 bucks 30 10.5 15

 

Yeah the 215/85 would definitely be better for a 4 cylinder. I have the 4.0, so opting for the 235/85 which is not quite an inch taller than stock.