Well this post ain't funny.
I have always tracked my mileage.
For nearly five years I got 17.5/18mpg summer 16.5/17 winter.
My low fuel light came on at about 350-365 miles.

These new larger tires must be killing my mileage and the throwing the speedo off.

The low light came on at 290 miles even still, I know I have about four gallons left.

Since it read 290 on the trip meter, I refused to take it seriously.
I was on my way to the gas station at 360 miles when the truck started to sputter. Whenever I gave it gas it hesitated. I had five miles and one traffic light to go. I put on the cruise and got off the gas pedal. I got the red light and when I tried to takeoff easily she shook, sputtered, coughed. The station was across the street and I rolled to the pump with the tach wavering between 500 and 700 rpms.

The trip meter read 365.2 and I pumped 25.347 gallons into the 25 gallon tank without any overflow.

that's 14.4 mpg and the only diff is slightly larger tires.

14.4 now I'm getting the same lousy mileage as others I used to secretly laugh at.

 

when I went from 235/70 to 265/70 I noticed over 5% on speedo, but a 10% drop in fuel mileage.

 

Hey Raoul, maybe I missed it someplace, but when you say

If you did not adjust the speedo, then the odometer is off as well. Since the tires are taller, the speedo and odo will both show less speed and miles then actually traveled.

That's what occured with me before I adjusted my speedo gear. I thought I was only getting 10 mpg!!

I took it out on the highway and started counting mile post markers and looking that the trip odo I found I was a full mile off after only five mile posts............

After the speedo adjustment, it's just about bang on now. I think I lose one tenth of a mile for every twenty traveled now.........

I think that when I reported this eons ago, YOU were the one that informed me it was due to my odo being off?????

 

I can accept part of my loss to the odometer but, not all of it.
I think the other portion of my mpg loss is I have effectively changed my gear ratio to the deteriment of highway mileage.

I had 255/70/16 with 3.55 gear. The tires were very worn(75K) So, the mileage was getting bogus on the plus side.

The 265/70/16 is 5% off on miles traveled and has changed my 3.55 gear to ? equivilent(3.38-3.08) I'm sure someone knows the formula.

This probably also accounts for the sluggishness I noticed after the change.

I know the truck isn't really getting 14.4 just as it wasn't getting the 18.5 I thought it was.

The bottom line, I noticed these things going from 255 to 265 with the same aspect ratio.

I can know understand the pleas to mike troyer for performance help after members here put 'BIG' rubber on their truck.

They must literally weep when they feel the difference.

 

Anyway this thread poses a question and my short answer is
Yes.

 

Raoul, I'll bet if you put a pea under 12 mattresses, you'd swear someone put a boulder under there! ha, ha.......

I went from a 235/70/16 to a 255/70/16 at one point and could not tell a difference at all. I then went to my current 265/75/16 and there was a difference to be sure......

I guess it just all boils down to is how sensitive you are to changes and what's acceptable or not as far as performance goes.....

I guess I should mention that I did get a nice 3" catback system that gave me some extra low end and that's probably why I don't feel the performance loss as others (or you) do!

Good luck man!

 

Regarding the mileage, I will second/third/? the drop in mileage. My best prior to the change was 19.5MPG. That was during the winter, all highway, 65mph cruise. My best since that time has been 16.5MPG with similiar conditions. My city mileage seems to be down about 1MPG. It is definately more noticable on the highway.

The big question is... I will problably have to replace my 265 this year. They will have about 70K on them. I will probably drive this truck another two years. What do I replace them with? I like the extra ground clearance, stiffer sidewalls, and extra plys with the tires I have now, but I would also like to gain some performance back. I am thinking I might give the Revos a try, and also might upgrade to a 3.73 or 4.10 axle. I just wish I could do that myself.

 

JSCOTT, I would go with the 275/60's instead of the 265/70's. There is only about a 3/4 inch difference in ride height.

I just went from 275/60 to 265/70. Big mistake, like the others here I have noticed a loss in power.

My new 265/70's are for sale. I will go to the 275's when these are sold.

On another note, I really like the Firehawk Indy 500 tires. Very nice traction on wet and dry pavement. Very quiet as well. TireRack has them for $71 (265/70) and $88 (275/60). It's about $12 each to ship.

 

MitchF150,

That's exactly what I was trying to explain to them.