About 2 weeks ago I purchased a set of BFG 285/65R20s for my new 2008 FX4 to replace the POS Pirelli 275/55R20s. A friend/co-worker asked me how the new tires had affected my gas mileage. I just thought I would share my email response to him with you:

"The other day you asked me how my new larger tires had affected my gas
mileage on my F150 and since I always try to give accurate answers, here's
the facts - they didn't!! I know the first thing your going to say is "BS
- I've done it before and larger tires do cause gas mileage to decrease".
Hold on, let me explain.........

When you install a larger diameter (taller) tire** without recalibrating
your speedometer, the odometer does not record the correct miles traveled -
it records a shorter distance. When calculating gas mileage, you are now
calculating the accurate number of gals. used but for a SHORTER distance
which results on lower MPG. I proved this to myself by Mapquesting the
distance to/from work and using that distance for my fuel calculations -
the gas mileage was the same as before I installed the larger tires!!

Now let's look at another factor. A larger tire (just like a tire with low
air pressure) has a greater rolling resistance. This is the exact reason
why Funny Cars & Dragsters use "skinnies" up front. However, even though
you have a greater rolling resistance with a larger tire, the engine is
turning at a slower RPM at a given MPH. The problem is, mostly people
don't get their speedometer/odometer recalibrated. So instead of driving
70 MPH - they are now traveling 75 MPH at the same RPM which causes fuel
mileage to decrease.

So what's the moral to this long-winded email?
Larger diameter tires have a greater rolling resistance than a smaller
tire = less fuel mileage
Engine RPM is decreased @ a given MPH = more fuel mileage
Without recalibrating the speedometer (and odometer) the MPG calculation
is skewed = less fuel mileage

Conclusion: The installation of larger tires** results in a greater rolling
resistance which is offset by a decreased engine RPM provided the actual
driving conditions (MPH) have not changed. The key is to recalibrate the
speedometer/odometer.

**Disclaimer: eventually I'm sure that a huge tire would detrimentally
affect MPG. However, that scenario was "out of scope" for this
evaluation."

Please feel free to offer your opinions

 

Hmmm.

Your logic is a bit misguided.

Larger tires DO, repeat DO have an adverse affect on MPG.

First off, why would you even touch on the subject of a non-recal'ed speedo when installing tires of a different diameter? This is pretty much a given.

The fact is, a taller & wider tire (your 285's for example), are indeed taller than stock and MUCH HEAVIER. It's this additional rolling mass that contributes the MOST to the lower MPG's, and here's the important part.......*** city MPG's *** will take a nose dive. That is very simple to understand. Getting a 6000lb truck moving from a dead stop takes a lot of energy. Adding a few hundred extra pounds (via larger tires) makes this task even more so.

I went from the OEM 275/55/20 to a 305/55/20. My MPG's did indeed drop.

My speedo is corrected via the Edge. I have verified the Edge's conversion via my wifes car driving besides me too.

My City MPG's went from ~14-15MPG, to ~11-12. This is with my VERY conservative foot too.

My highway MPG's did NOT suffer as dramatically. Highway MPG's went from ~ 18-19MPG, to ~17-18.

It's common sense to know that MPG's will decrease due to a substantially heavier tire. It's also common sense to know that said MPG's will drop more in City driving than in Highway driving. Takes more to overcome that heavier mass. Once that mass starts rolling, it's inertia helps a bit, but only a bit.

The camshaft profile in a vehicle is designed to be more efficient in a certain RPM range than others. Putting that cam in a different RPM range by altering the rolling diameter of the tire changes this. Installing a different gear set would help bring things back into balance.

 

i call BS, there is no way your mileage is the exact same. i'm knocking down 13 city (it's not true city driving either, small town, no stoplights, only stop signs_) and 17+ highway, with smart driving. and gearing is why my highway is so good. but i totally agree with rsm8. now if you have say 4.10 gears stock and put bigger tires on and got better highway mpg that's a give, less rpm's on the highway, but in your scenario, no.

 

hahaha. then explain to me why i only got 15.7 mpg on the interstate this past week if tires do not effect mpg. thats like saying larger tires won't effect my HP. and you jumped up nearly no tire size at all so the effect will be minimal and probly not noticable. put on some 35's and come back and argue the same thing.

 

My reaction is you wasted a lot of words plus my time in reading it. I ALWAYS recalibrate. Only a numbskull wouldn't, or he doesn't care ifhis speedometer/odemeter is correct. When I went from 245/70/17 to 265/70/17, I lost about 1/2 mpg. When I went from 265/70/17 to 305/50/20, I lost another one mpg.

 

Larger tires take more power to get up to speed. They take less power than a smaller sized tire to keep them at a given speed. It is physics.

 

I welcome and appreciate your comments

 

I live in SLC, Utah. Elevation:4400'. I installed 285/70/17's (32") tires on my screw. I lost a solid 2+ MPG highway! The tires actually corrected my speedometer as they are 2 MPH low from the factory. I noticed at this elevation that my torque converter would not lock up in overdrive if there is the slightest wind or hill. I would imagine that the MT tread does not help either. Bigger tires DO affect gas mileage period!

 

but your not gonna admit your wrong. you barely increased your tire size, meaning minimal change in MPG. go slap some large tires on your truck and then post your results. if your correct, why do people re-gear? other than to take stress off the tranny to HELP WITH MPG and why? because larger tires effect mpg.

ok say i am going 70 with a stock truck, truck sits at around 2k rpm roughly, truck does not have to work hard, won't be shifting in and out of OD. now slap on some 35's (yes the speedometer is recalibrated) now your going 70mph at about 1850 rpm, you get to a slight hill truck shifts out of OD which in turns effects gas mileage because your driving at a higher rpm. now your going a 400 mile trip. who is gonna get the better gas mileage? the guy with stock tires being able to stay in OD the whole trip or the guy with 35's who is constantly shifting in and out of OD?

now in the city with both of the same trucks as above, your at a redlight, you accelerate, the truck with stock tires accelerates to 45 mph with relative ease, now the truck with 35's you need to give it more gas to get there due to the weight difference in the tires. now which one is using more gas to get to 45 mph? and if you use more gas that effects your gas mileage FYI

and heres another bit of info to prove you wrong.

me and my dad were driving to LA, me in my truck (35's) and him in his truck (stock) both supercrews same gears, why did he average 2.5-3 mpg better than me the whole trip if were drove the whole way together going the same speed limit?

 

You are obviously far more knowledgeable on this topic than I am. Perhaps I should have started this thread by stating that "it has been my experience" or "in my opinion" and for that I do apologize.

Now since it seems so important to you let me clearly state "I am wrong".

Happy now?

 

a little bit, yea. you didn't post and say i did not notice any decrease in mileage and these are my findings, you came and posted that YOU WILL NOT LOSE MPG AT ALL with larger tires and this is why. im not trying to be anything, but its just the way you posted it.

maybe in perfect conditions on the highway you could see a slight increase in mpg, but i would imagine that would need to be a flat highway going pretty slow with no winds. if i go anything over 65 my mpg plummets dramatically.

 

Some of you guys take comments way too seriously. Obviously the guy is posting an opinion if it is on an online truck forum. I bet you could find threads all over this site that directly contradict each other even though they didn't start with "In my opinion."

Just to clear it up though.... this reply was only my opinion.

 

I thought I clearly stated "my MPG" on "my F150". Next time I will be sure to start all posts with "It has been my experience" or "In my opinion" just to prevent issues just like this.

Hahaha - good call.

PS: I live in Florida - there are absolutely no hills, no downshifting, none of that here.

It's all good. I appreciate all of your comments.

 

I just put my stockers back on w/some 285/65 18 Silent Armors and my mileage went up, my acceleration increased, my braking is better, and the ride is better. Might be tough puttin' the 20's back on this spring.

 

yes, you did clearly state my mpg and my f150, but you went into that long explanation generalizing all trucks, not sure if thats how you meant it but thats how i took it when i read it.