I am new to the forum, but i was wondering if anyone has had this to happen. I was getting close to 20 mpg on my truck, before i changed the tires, the truck came with 235x17 on it, they looked like crap, so when they wore out i put 265 on. My speedometer was 2 mph fast with the 235"s its dead on now with the 265's. But my fuel milage now is about 16.5 mpg.. my question is do you think its the tires, or maybe a coincidence and is something else?

 

Welcome to the forum!

What air pressure are you running and what pressure are the tires rated for? Low tire pressure is the only thing that comes to mind...

 

running 40psi, and I didnt tell what my truck is, 2010 f150 5.4 ext cab 4x4 sorry!

 

Have you accoutned for the fact that your odometer is now reading differently like your spedo is?

 

2 mph fast at what speed?

Speedo's and Odo's are off a percentage. Not just 2 mph across the board.

As stated the 2mph will account for some of the diff.

 

yes, i have done all the calculations..it should not have made that much difference! 3.5 mpg?

 

I check it with my GPS, I have check from 55mph to 70 mph, with the new tires its dead on with GPS speed, now. It used to be 2mph fast with the 235 tires at those speeds...I dont know!!, makes no sense to me!

 

2 mph at 55 will account for about .75 mpg.

Only other thing I can think of would be difference in rolling resistance of the 2 sizes and extra resistance getting a larger tire moving.

What is the max tire pressure of the new tires. Try running the pressure up close to the max and check to see if that makes a difference.

 

I was told that running at or near max on my Toyos would trigger my TPMS... The max is 80 PSI. I am running about 42. Anyone try going higher? Any TPMS faults?

 

Bigger tires are heavier, and will hurt your city fuel economy. The size difference acccounts for at least part of this "loss"

what are the full metric sizes of each tire?

 

I went from the stock michelin's to Nitto's and lost over 1 mpg. And I kept the stock size.

Bigger tires are heavier and have more rolling resistance.

 

sounds to me like you are comparing a 235 standard load to a 265 load range E ....

a 10 ply tire is going to offer more rolling resistance, so will the wider profile.

also, if you are comparing a less aggressive tread pattern to one with more bite, that will make a difference as well.

add that to the fact that your MPG was artificially high with the speedometer being off with the smaller tires ... all of those factors added up could account for the total difference.

as for pressure, take your (RGAWR)/(Max load on tire) x 80psi .... that's what i would run for pressure.

for a more in-depth discussion on tire pressures: https://www.ford-trucks.com/forums/1...tle-stiff.html

 

Tires can do this. Taller tire, wider tire, and more aggressive tread patterns will hammer your mileage.

If the speedo was 2 mph high between 55 - 70 mph then you were traveling 3.6% - 2.9% less distance than you thought. So instead of 20 mpg you were getting 19.4 - 19.3 mpg. Now you're getting 16.5 mpg. Not an impossible event if the tires you chose are more aggressive.

 

Heavier, larger, and more aggressive tires will reduce gas mileage.

2-3 mpg is not at all unheard of. I have an '02 E150 van which came with 235/75x15" Michelin LTX M/S's stock, 29" diameter. I upsized to Firestone Destination AT's in a 30x9.50 size, actual diameter 29.5". I could feel the engine (a 4.6, so it does not have a lot of power to spare) working a LOT harder on mild freeway uphills with the cruise on--roads that I was familiar with.

My freeway mileage went down from 17 to 15 mpg with just a half inch of tire size increase and a blockier tread pattern. It was summertime, and I could feel the rubber on the Firestones being really hot after a stint on the freeway. City mpg went down from 15ish to 13ish.

The Firestones had a 30 day money back guarantee, and I took them back, was out of the cost of mounting/balancing, but the loss of mpg over the life of the tires would have been maybe a grand or more. I had already put on a new set of Michelins in the stock size that are still on the van.

The Firestones looked cooler and I wanted a more aggressive tread for Michigan winters, but it was simply not worth it. I have had four full size vans, and have put 255/70's on a couple of the older ones in place of 235/75's....same diameter but wider. MPG loss was a measurable 1 mpg with this change on both of the vans I changed over.

Your upsize is far greater than what I did on my van, so yeah, not a surprise. Any chance your tires have a satisfaction guarantee? It's up to you whether the "looks like crap" stock tire size is worth $2 grand in fuel over the lifetime of the new tires. (Diff for 50k miles, 16.5 vs 20 mpg, $4 per gallon.)

George

 

Tire size calculator


punch in your original tires and your new tires and see the difference in size and it will tell you the odometer difference.