Why do bigger tires cause worse mileage?

[bulldog-yota]

Why are wider tires put on to improve traction and handling? The only advantage the 70 series tires have over 65s is ground clearence and price, PERIOD. The height difference is less then 5/8". Unless you off-road frequently the 65s are better for your 4Runner.

Koz

Let me explain.

Foot print is a function of width and diameter of tire. Basically footprint is area a tire is on contact with the road surface. Area = Length x Width.

SO wider tires is wider and taller (more diameter) has more length. So increasing one or noth will increase footprint.

Bigger footprint increase rolling traction and rolling resistance, thus less MPG for that aspect of the equation.

Keep in mind that for on road sistuations you have to look at traction driving in a straight line, i.e. for braking and accelaration and also traction through corner, i.e. emergency handling and going through fast curves.

SO just looking at footprint (ignoring rotational weight and gearing) a 70 progile will do better in braking and accelration in terms of grip, however the increased rotaional weight and diameter works against the brakes and engine.

Looking at cornering, the reason a lower profile tire does better is because the sidewall deforms less than a taller tire. When the sidewall deforms you actually loose footprint and grip. Thus a lower profile tire gets better grip. However using a low profile tire with bigger wheels can actually increase your diameter, ask members that are running 18-20" wheels with overall bigger diameter tires how their cornering has improved.

SO you might be right in terms of 65 vs 70, but you reason is not correct. Footprint on the 265/70/17 is more than on the 265/65/17. However for practical on road driving it will do little but decrease your gas mileage.

 

[Koz]

Let me explain.

Foot print is a function of width and diameter of tire. Basically footprint is area a tire is on contact with the road surface. Area = Length x Width.

SO wider tires is wider and taller (more diameter) has more length. So increasing one or noth will increase footprint.

Bigger footprint increase rolling traction and rolling resistance, thus less MPG for that aspect of the equation.

Keep in mind that for on road sistuations you have to look at traction driving in a straight line, i.e. for braking and accelaration and also traction through corner, i.e. emergency handling and going through fast curves.

SO just looking at footprint (ignoring rotational weight and gearing) a 70 progile will do better in braking and accelration in terms of grip, however the increased rotaional weight and diameter works against the brakes and engine.

Looking at cornering, the reason a lower profile tire does better is because the sidewall deforms less than a taller tire. When the sidewall deforms you actually loose footprint and grip. Thus a lower profile tire gets better grip. However using a low profile tire with bigger wheels can actually increase your diameter, ask members that are running 18-20" wheels with overall bigger diameter tires how their cornering has improved.

SO you might be right in terms of 65 vs 70, but you reason is not correct. Footprint on the 265/70/17 is more than on the 265/65/17. However for practical on road driving it will do little but decrease your gas mileage.

First I wouldn't be surprised if the 65 and the 70 are very close in footprint size considering the 65 has a wider tread and the 70 has a slightly larger diameter. If I had to bet I would say the 65 has a bigger footprint. The one thing I'm sure of is that the lose of gas mileage is not from the difference in footprint size or the frontal area (wind resistance) of the tire.

The loss of mileage is due to the change in final gear ratio and added rotating mass.

When it comes to braking/acceleration and suspension effectiveness, Rotating mass (and its location) and un-sprung weight are very important and can drastically affect a vehicle's traction and stability when cornering.

Changing the air pressure can also have a big affect on the performance/handling. I run 40 psi all the way around.

Changing the tire size (even slightly) can negatively affect a lot more then just gas mileage.

Again, unless you go off-roading frequently stay with the stock size (diameter) tire. ANYTIME you change the size of the tire or rim you WILL be affecting some other aspect of your vehicles ride design.

How many people put larger rims and wider tires (lower profile) and end up with a shit ride?

Koz

 

[00328]

How many people put larger rims and wider tires (lower profile) and end up with a shit ride?
Koz

My guess is a lot more than would ever admit to it.

 

[bulldog-yota]

First I wouldn't be surprised if the 65 and the 70 are very close in footprint size considering the 65 has a wider tread and the 70 has a slightly larger diameter. If I had to bet I would say the 65 has a bigger footprint. The one thing I'm sure of is that the lose of gas mileage is not from the difference in footprint size or the frontal area (wind resistance) of the tire.

The loss of mileage is due to the change in final gear ratio and added rotating mass.

When it comes braking/acceleration and suspension effectiveness, Rotating mass (and its location) and un-sprung weight are very important and can drastically affect a vehicle's traction and stability when cornering.

Changing the air pressure can also have a big affect on the performance/handling. I run 40 psi all the way around.

Changing the tire size (even slightly) can negatively affect a lot more then just gas mileage.

Again, unless you go off-roading frequently stay with the stock size (diameter) tire. ANYTIME you change the size of the tire or rim you WILL be affecting some other aspect of your vehicles ride design.

How many people put larger rims and wider tires (lower profile) and end up with a shit ride?

Koz

Read what I wrote at the beginning of the thread, there are a number of factors involved:
- footprint which is a factor in rolling resistance (for tires with the same thread pattern adn make up) it is the biggest part of rolling resistance difference
- Rotational weight
- Gearing
- And obviously the odo will underread

You discount rolling resistance way to easily. Ask people who have gone from highway to AT ot MT tires in exactly the same size and the majority will tell you that they lost gas mileage.

Wonder why handling is better at higher psi? The sidewall deform less leaving you with a bigger footprint. Sidewall deformation is one of the biggest problems in handling with passenger vehicles, even some of the "low end" race vehicles. Obviously the suspension needs to be tuned for the specific setup that will be used. The funny part is my handling on road improved going from Dunlop 265/65/17 to 265/70/17 Revos.

Agreed changing tires and wheels will affect the dynamics of the vehicle. Pending on what you are looking for their are different options. If on raod handling is your main concern you should not have started with a 4500lb SUV anyway But the handling on the 4R can be improved with the right tires/wheels and suspension. All these things are interconnected and designed with a compromise from the factory. So some people will want better on road handling, some will want better offroad capability, and each will make changes accordingly.

Going to 18" wheel and 265/60/18 tires will not suddenly make the ride bad, besides the 06s will probably come this way on some models. That is not what I want as I'm going in the opposite direction, but for some it is better. Some member with these tires and wheels can comment themselves how much their ride has been affected.

 

[KMX3060]

Wonder why handling is better at higher psi? The sidewall deform less leaving you with a bigger footprint. Sidewall deformation is one of the biggest problems in handling with passenger vehicles, even some of the "low end" race vehicles.

This is patently incorrect.

If you don't believe me, go ahead and air 'em up to the max. You will feel like you are driving on marbles or ice when you are cornering. The sidewall of radials is made to flex. It keeps the tread in contact with the road.

And also, air resistance is the biggest factor in larger tires getting less gas mileage.

 

[KMX3060]

In the United States, vehicle manufacturers are required to maintain an average fuel economy for the "fleet" of new vehicles they sell each year. Currently, the government Corporate Average Fuel Economy (CAFE) mandate is 27.5 miles per gallon (mpg) for cars and 20.7 mpg for light trucks (includes minivans, vans, and most pickup trucks and Sport Utility Vehicles). However because it's an "average" fuel economy, in order to sell large cars or trucks (that use more fuel), the vehicle manufacturer must also sell small cars and trucks (that are fuel efficient). The vehicle manufacturer can be fined if their annual vehicle

 

[bulldog-yota]

This is patently incorrect.

If you don't believe me, go ahead and air 'em up to the max. You will feel like you are driving on marbles or ice when you are cornering. The sidewall of radials is made to flex. It keeps the tread in contact with the road.

And also, air resistance is the biggest factor in larger tires getting less gas mileage.

Well from practical experience on the 4Runner. I had very little difference, after lifting my 4R 2.5", in gas mileage compared to when I changed my tires. Wind resistance from lifting the 4R 2.5" increases far more than from slightly bigger tires. Changing my tires had a far bigger penalty on my gas mileage. This is from experience, and I'm sure when other members chime in they will have the same experience.

Also go race your 4R and you will see the tires buckling under cornering. You will actually notice the scuff marks well up on the sidewall. This means your actual thread on the inside has long since stopped making contact with the road. Decreasing the sidewall length minimizes the possibility for the tire to buckle under hard cornering. Increasing tire pressure effictive makes it more resistant to this, obviously up to a certain point. Take your 4R and run it on exactly the same type of tire with 265/70/16 and 265/60/18 and then see which one handles better. I have raced enough cars and experimented enough. Also notice the difference in the scuff marks on the sidewall.

Yes sidewalls flex, but I'm refering to the sidewall buckling under lateral force. Which means a part of the sidewall actually touches the road surface instead of the actual thread. This means the inside of the thread has long since stopped making contact with the road surface. Your VSC will be going bonkers at this point in time.

 

[Koz]

Read what I wrote at the beginning of the thread, there are a number of factors involved:
- footprint which is a factor in rolling resistance (for tires with the same thread pattern adn make up) it is the biggest part of rolling resistance difference
- Rotational weight
- Gearing
- And obviously the odo will underread

You discount rolling resistance way to easily. Ask people who have gone from highway to AT ot MT tires in exactly the same size and the majority will tell you that they lost gas mileage.

Wonder why handling is better at higher psi? The sidewall deform less leaving you with a bigger footprint. Sidewall deformation is one of the biggest problems in handling with passenger vehicles, even some of the "low end" race vehicles. Obviously the suspension needs to be tuned for the specific setup that will be used. The funny part is my handling on road improved going from Dunlop 265/65/17 to 265/70/17 Revos.

Agreed changing tires and wheels will affect the dynamics of the vehicle. Pending on what you are looking for their are different options. If on raod handling is your main concern you should not have started with a 4500lb SUV anyway But the handling on the 4R can be improved with the right tires/wheels and suspension. All these things are interconnected and designed with a compromise from the factory. So some people will want better on road handling, some will want better offroad capability, and each will make changes accordingly.

Going to 18" wheel and 265/60/18 tires will not suddenly make the ride bad, besides the 06s will probably come this way on some models. That is not what I want as I'm going in the opposite direction, but for some it is better. Some member with these tires and wheels can comment themselves how much their ride has been affected.

Nowhere in my post did I discount rolling resistance, as a matter of fact I NEVER even mentioned rolling resistance. What I did discount was the difference in footprint size between 65s and 70s. I'd like to see a formula or some proof that there is a difference, not personal opinion.

As far as footprint; it is a very small percentage of the rolling resistance, which makes up 15 to 25% of the total resistance to movement. The footprint, weight of the tire and wheel (rotating mass), tread depth, construction, and the material itself (rubber compound) all add up to the rolling resistance. The air pressure in the tire itself can have more affect on gas mileage then any one of these components in rolling resistance. Again, the footprint difference is NOT causing the decreased gas mileage.

I don

 

[bulldog-yota]

Nowhere in my post did I discount rolling resistance, as a matter of fact I NEVER even mentioned rolling resistance. What I did discount was the difference in footprint size between 65s and 70s. I'd like to see a formula or some proof that there is a difference, not personal opinion.

As far as footprint; it is a very small percentage of the rolling resistance, which makes up 15 to 25% of the total resistance to movement. The footprint, weight of the tire and wheel (rotating mass), tread depth, construction, and the material itself (rubber compound) all add up to the rolling resistance. The air pressure in the tire itself can have more affect on gas mileage then any one of these components in rolling resistance. Again, the footprint difference is NOT causing the decreased gas mileage.

I don

 

[bulldog-yota]

As regards to handling I'm referring to at the limit handling. And yes a lot of variables come into play. And the only way to determine the best air pressure for handling is to drive it at the extreme and experiment with the air presure that performs the best. When racing with stock cars using street tires my experience has found more often than not going 50-60 KPA above recommended pressure produces the best results. It also produced far less sidewall scuffing than lower pressures. Higher pressure tended to make the front unsettled. Pending on rear or fwd pressure would vary. FWD needed more pressure upfront.

The chalk trick works nice for DD, but not for optimum handling. It is to increase tire life and generally ride comfort. Upping the air pressure give better milage and even better handling for a lot of tires and vehicles (not all). Do the chalk trick and then add 3-4psi and see what your handling and MPG does.

I guess it depends on your definition of handling. Cars on the road live with a set of compromises engineered into them to balance what would sell the best. A number of sacrifices in terms of handling are made to increase ride comfort, gas mileage etc.

 

[Zaraspook]

Formulas and educated guesses are nice but let's hear some real world experience from guys who have changed from the stock tires to 70 series Revos. Has your gas mileage changed and how much?

 

[bulldog-yota]

Formulas and educated guesses are nice but let's hear some real world experience from guys who have changed from the stock tires to 70 series Revos. Has your gas mileage changed and how much?

Going from 265/65/17 Dunlop AT20s to 265/70/17 Revos I lost around 1.5-2 MPG.

I also lifted my 4R which lost me around 1 MPG or more pending on how much highway I drive and how fast. Around town lifting made very little difference in MPG.

SO now I have supplied my experiences, formulas and educated guesses. Way too much time on this thread.

 

[HuntRunner]

Going from 265/65/17 Dunlop AT20s to 265/70/17 Revos I lost around 1.5-2 MPG.

Going to a 70 you lost 1.5-2 mpg....How much mpg did the folks lose that stuck with the stock size Revos? This might help us to determine how much fuel loss is contributed to size vs. resistance due to the tread pattern?

 

[orangeblast22]

would have been a good thread to finish.

I am thinking of going from my 265/65/17 dunlops to some new 265/70/17 REVOS and I am also planning on doing a 3"front and 2" rear lift with those cornfed spacers.

Main thing I'm worried about is overall performance given that this my runner would be a daily driver and only plan to take if offroad occasionally.

would like to hear more people opinions and such.

 

[Koz]

Main thing I'm worried about is overall performance given that this my runner would be a daily driver and only plan to take if offroad occasionally.

would like to hear more people opinions and such.

Here's my personal opinion; First, how much is OCCASIONALY and what type of off-roading are you going to do? If your occasionally is a couple times a year and you plan on easy to moderate trails the stock 4Runner is fine for that. You could remove the running boards and you would be surprised at how capable a stock 4Runner is. Your vehicle was engineered and designed to provide the best and most stable ride, on/off road, with the OEM specs. Once you change them, weather it be lifting, larger/heavier tires, etc, you will have to make adjustments to compensate. When you are done you have compromised the vehicle in some way. You may have gained a little more ability off/on-road but you WILL sacrifice somewhere else. Also you have laid out money to do so. What you have to look at is the gain worth the money and what was sacrificed? I have gone off-roading with a 4Runner that had the lift and larger tires and because I was a little more careful I got through without any scares.

With this said, the best upgrade in overall ride, handling, and all types of traction is to get a set of REVOs!

Koz

 

[orangeblast22]

then REVOS it is!

the only thing i'm battaling is going to 265/70 from the stock size and doing a lift...it woudl seem like most people on here have done at least a 2-3" lift...

I've been eyeing those Cornfeds from Burtman Industries...the price is right and everyone says it make the 4runner look the way it should have looked from the factory...not to mention I DO now finally own a 4WD vehicle so it would be sensable to say the least. Not to mention also..that most people notice no difference in handling and so on and so fourth...most say if anything there is a very VERY slight increase in body roll...and maybe it just their imaginations...so they say.

then theres the bigger tire and the whole acceleration and MPG decrease that scares me.

if Toyota would just come out with a 4.0 supercharger then I'd get the lift and tires no problem

 

[HuntRunner]

Since this thread, I have changed to a 265/70/17 and added a lift. I lost about 2 mpg, and most of this came from the change in tires, as I did the tires first. This is what I expected and I do not regret it. The new look of the 4Runner out-weighs the loss in mpg!
No noticable power loss either.
I put over 46K miles on the stock dunlops with 3/32 tread left on them.

 

[broncosi]

while were on the topic of gas milage somewhat, does a V8 really use that much more gas than a V6? just curious, no complicated answers needed just yes or no

 

[OUTRNNR]

while were on the topic of gas milage somewhat, does a V8 really use that much more gas than a V6? just curious, no complicated answers needed just yes or no

Short Answer: not really.

2WD (V6; V8) 18/22; 17/20
4WD (V6; V8) 17/21; 16/19

The only significant improvement is highway driving, so it depends on where you do the bulk of your driving, city or highway. If its a mix, average of 1.5-2 mpg more in the V6, hardly mind-blowing.