What caused change in my avg MPG?

I’ve been getting 21-22 mpg hwy. I went in a trip this weekend, the same trip to visit my parents that I’ve been getting 22-22 on, and I’m hovering at 18.3-18.7.
I got new tires Thursday (Goodyear wrangler workhorse AT 265/65R17) to replace the Toyos that came with the truck 50k miles ago (in the same size).
My transmission also spent a LOT of the drive hunting and my engine spent a lot of the drive straining to maintain the posted 75 mph on virtually flat land.
What gives? Does my truck suck now?

 

bad gas is my first guess

 

Wind direction? Did you use 87 or 88 octane gas?

 

The tires are more than likely heavier, and even the slightest bit of headwind will have a dramatic impact on MPGs. Since you were road trippin, you could have kept a higher highway speed, which also dramatically reduced MPGs.

I just went Friday to shoot a Saturday match. The drive is 140 miles, 100 being straight highway. Going out I had a slight tailwind, going home I had a slight headwind, going the same speed, I got just over 1.5 MPG less.

 

Same as I’ve been using. 87 I think

 

Truck is fine, the tires suck.......suck gas that is. I give kudos to the good years on my truck but I took an almost 4mpg hit when I put them on.

 

wind, speed , gas are all variables. But if it does it again trade it in for the 4th gen

 

Maybe the new tires were underinflated

 

Hm

Headwind is a given here. No getting away from it. Didn’t consider the tires might be heavier despite being the same size.

 

Drive the trip six more times and calculate from there

 

Maybe the old tires were over inflated??
Hahaha, just messin with ya! Good point though, tire pressures will also have a noticeable impact too.

 

Oof

 

This! One data point is an anomaly.

 

If you check the websites of the respective tires (old and new) they should have the weights of each.

I also agree that it could be due to under-inflation. Although I would guess it’s probably just the build of the tire itself.

 

Both actually. 3 tires stayed at 31psi, but the 4th got bumped to 38 last time I had a flat fixed, and I’ve been waiting ever since to see if it would go down

 

Ha ha, I actually over inflated mine to see if it helped and it does, not 4 mpg better but maybe two?

 

Got this yesterday doing 75-77 mph...
Stock everything tires are at 35psi.

 

Even though the new tires are the same size, they could be as much as 1" taller than your old tires. Tires wear down as they wear and by the time you have 40,000 miles on most of them, they are in reality one size smaller than when new. You were driving about 5% faster, and farther than you were on the old tires.

When your speedometer said 75, you were closer to 79. And in my experience right around 70 mph is like hitting a wall as far as fuel mileage goes. Anything over 70 kills fuel mileage; getting close to 80 really kills it. You also drove farther. If your trip meter said 200 miles, you drove closer to 210.

Chances are good you were getting false, optimistic readings with the old tires since they were undersized in relation to your trucks computer. In my experience moving up to 265/70/17's give a more accurate speed and odometer reading. The numbers you're getting now are probably accurate and the old numbers were inaccurate.

 

New tires have greater rolling resistance. It'll get better over time but fresh tires are usually always mpg killers.

 

I get about 16-17 MPG on 90% hwy driving.
I don't accelerate hard. How are you guys getting such high MPG?