Big time mpg drop...?

so lately my trucks mpg has dropped pretty significantly, and i seem to be getting horrible hwy mileage specifically. About 17 or 18. city hasnt really been affected much around 19, 20. but i used to be getting around 24hwy. i do about 75% hwy driving so i'm starting to get a little worried here. i noticed it dropped dramatically after i filled up with octane94 fuel from a sunoco station and since then ive been using 89. but ever since that tank of 94 it hasnt been the same at all. i recently got a larger tire size as well, 265/75's from the stock 245/75's. i know this would have some effect on mileage but not the huge decrease im experiencing. any suggestions? perhaps a bad batch of fuel contaminated the system? ive filled up 4 or 5 times since using the 94 so i'm out of ideas...

 

i recently got a horrendous fill from a tempo around here.... so pathetic..... went 60 km on a quarter tank with a load, but no excessive speed etc... filled up at a petro can.... and got over 110 km on a quarter tank... maybe bad gas?, will stay away from tempo for now...

 

The simplest answer is usually right...its probably the larger tires. Added rotational weight can make a huge difference in acceleration and gas mileage, especially on torque deficient engines like our 4 bangers.

 

How much bigger tires?
You can simplify the analogy to a lever arm working against your axle shaft. Larger diameter tire, longer lever arm the axle has to move against the rolling resistance, inertia of vehicle weight, as well as wind resistance.
The rolling resistance will be a bit different with different tire construction/materials. Could be better, or worse than previous tires.
The engine has to constantly work harder with larger diameter tires.

 

How many miles are on your truck? It could be time to clean the intake out.

 

An easy experiment would be to disconnect the battery for ~ 5 min. to reset the ECM. Really reaching here, but it's possible the Sunoco fuel (in Canada, Sunoco uses up to 15% ethanol) caused the ECM to adapt to the Sunoco, and hasn't re-adapted.
Seems logical a sudden change in economy like that has to have a root cause.
Tires, gas ... any other changes near then?

 

+1- Between the larger/heavier tires and the changes they make to the measurements you are using to calculate gas mileage...You might find alot of those lost mpgs.

 

I get crappy gas milage with my 32's as well. You should keep in mind that if your ODO says you have gone 100 miles, with 32's you have actually gone more than 100 miles. Well actually that's not true either probablly, since the ODO is off from factory.

I figure if it bothers me too much, I have a few choices. 1. Switch to 265/70/16's. 2. Regear. 3. Put even bigger tires on and regear. 4. Or just pay thru the nose for gas. So far I'm just buying lots of gas and whining about it, till I figure out what I'm going to do.

 

it started happening before the larger tires
i'm almost positive the sunoco station i fill up at is 10% ethanol
my truck only has 10000km (~6200mi) on it, all on-road so far so i really shouldnt have to clean anything out on the intake side yet, but ill look
ill give resetting the ecu a try as well but i think i already did that when i installed my hids and had the battery disconnected

thanks for the suggestions

 

i agree with the two above i noticed the same thing with my truck when i changed the tire size. i went from the stock size to about 30's. but i know this is kinda a weird way of thinking about it but out trucks (4bangers) are like little kids, when a little kid puts on their dads shoes it takes more power for them to get going, to stay at the same speed, and also to slow down. haha

 

Whats your definition of "pretty". Its reads about 2.5 mph faster at 55 mph. Thats not as accurate as most other vehicles I've driven.

 

True statement. This was discussed ad nauseum in another thread. The gist of it is that although the speedo and odo receive their input from the same source (the VSS - vehicle speed sensor on the output shaft of the transmission), the odo is a direct digital-to-digital representation (and is accurate) while the speedo converts that digital signal to an analog representation (needle movement) - and it's not totally accurate.

 

My 2008 5-lugger read 60 at a real 55, and the odometer reads 101.5 miles for every 100 interstate mile markers. Have checked the odometer on 3 different stretches of interstate and they all read the same.

 

Have you checked to make sure a brake isn't hung up. Happened to me on my "other" truck, and lost about 4-4.5 mpg until I figured it out. Checking the temperature of the rim, carefully, after a drive is a pretty good indicator.

 

I noticed this on my scangauge after i filled up. i usually do alot of highway driving and get about 18-19 mpg, but now i am getting like 11 or 12. i don't know what the deal is. maybe im not restting my scangauge right after a fillup?

 

You beat me to it...exactly the prob