MPG vs. Tire Size (Information to Ponder)!

[helmsa]

Prompted by a discussion that started in another thread, I decided to post some information for all to ponder when selecting tires. There seem to be a lot of questions going around the modified area about MPG after upgrading tires.

HERE IS MY DISCLAIMER: THIS THREAD CONTAINS NO MPG INFORMATION OR DATA AND IS BASED SOLELY ON MATHEMATICS AND MY THEORIES, NOT NECESSARILY REAL WORLD APPLICATION! I AM NOT ADVOCATING FOR OR AGAINST UPGRADING TIRES, ONLY PROVIDING INFORMATION TO CREATE A GREATER UNDERSTANDING OF POSSIBLE EFFECTS.

Here are some different setups (information I can speak to):

• Stock Tires: 235/70R16 (28.95").
• Upgraded Tires 265/75R16 (31.65"). This tire is 9.3% larger in diameter then the stock tire size.

What does that mean? Well that until corrected by whatever means, your actual speed will be 9.3% faster then what your speedometer shows and actual distance traveled will be 9.3% further then what your odometer reads.

For example:
70 mph (speedometer) = 76.51 mph (actual)
100 miles (odometer) = 109.3 miles (actual)

There is a cure for this problem though...recalibration.

Larger tire size has two effects on MPG.

The first effect is that the larger diameter requires more energy to get the tire rotating given the exact same weight. The second effect is the increased weight that is being placed at the wheel. Combine these two effects and you get a double negative (that doesn't turn positive).

Actual data that I have collected looking at tire weights is as follows (for the most similar load rated tire I could find):

BFG A/T (KO)

• 235/70R16 - 38 lbs
• 265/75R16 - 51 lbs
• Increased weight of 34.21%

Pirelli Scorpion ATR

• 235/70R16 - 37 lbs
• 265/75R16 - 52 lbs
• Increased weight of 40.54%

BFG M/T (KM2) - New M/T

• 235/70R16 - 37 lbs
• 265/75R16 - 49 lbs
• Increased weight of 32.43%

There is no direct cure for this increased weight/diameter, however, one could replace the gears (making the work done by the engine easier). Through some researching, if a person with the stock setup of 235's and 3.55 gears upgraded to 265's, the most comparable gears in terms of engine RPM's would be 3.90's.

 

[IZwack]

One thing that also plays a big factor is tire width -- because as the tire gets narrower, the PSI increases which means there is the tire patch in contact with the ground gets smaller (which has a possibility of reducing rolling resistance)

 

[helmsa]

Good point. I didn't want to get too much into that subject (rolling resistance, friction and tread patterns) because there is not a real way to put numerical values on them the same way that I did everything else.

But just to sum that up:

Narrower tires have less rolling resistance then a wider tire. This is due to the decrease in surface friction caused by tread contact.

Tread pattern effects rolling resistance/friction. Highway tread will have create less surface friction because the tread pattern is smoother (therefore tire rolls smoother) then that of a M/T. Every time the lug of a M/T tire hits the surface, friction is created and therefore an increase in rolling resistance.

 

[chris's2005]

How the heck do you know all of this, jeeze your one smart guy

So i have 265/75/16 and LS 3.73 so i should re-gear towards 4.10's?

Thanks for ya help

Chris

 

[Thelt]

Would larger tires not result in lower rpms at highway speed? Would that not translate in to better highway mileage when the detriment of using more gas to get the bigger tires rolling is minimized?

Seems like it should be worse MPG city and better Highway with larger tires.

 

[helmsa]

How the heck do you know all of this, jeeze your one smart guy

So i have 265/75/16 and LS 3.73 so i should re-gear towards 4.10's?

Thanks for ya help

Chris

Chris, you are correct (assuming you had 235/70R16 tires to start with). To remain near the same RPM as stock, 4.10 gears would be needed.

Would larger tires not result in lower rpms at highway speed? Would that not translate in to better highway mileage when the detriment of using more gas to get the bigger tires rolling is minimized?

Seems like it should be worse MPG city and better Highway with larger tires.

Thelt, to answer your question, yes larger tires will result to lower RPM's at highway speeds, but that doesn't directly translate to better MPG. The engine still has to work to keep the tires rolling.

At this time, I don't have a complete explanation of this yet, although personal experience (2001 Dodge Ram1500 5.2L) has showed a negative impact on MPG in both city and highway applications.

 

[sjr033]

The first effect is that the larger diameter requires more energy to get the tire rotating given the exact same weight. The second effect is the increased weight that is being placed at the wheel. Combine these two effects and you get a double negative (that doesn't turn positive).

The double negative, that is not a positive, can be fixed by combining a third negative...A taller tire and the lift needed to fit a taller tire, raises the height of a truck and in return causes increased wind resistance. I read something like for every inch you go up, is 1-2% decrease in mileage.

 

[JCUZ]

Here's some real world experience, just to add confusion to the fire. When running 245/65/17 Cooper H/Ts we get around 320km to a tank of LPG. Now that we are running 265/75/16 Hercules A/Ts, we are getting around 320km from a tank of LPG.

 

[Thelt]

I have not had the new tires on long enough to see what effect it will have. I will report back once I do. I was getting about 16.8 mpg in mostly city driving according to the computer. I was running 245/70/16 HTs and no lift. Now I have 2" of lift and 265/75/16 ATs.

 

[helmsa]

Here's some real world experience, just to add confusion to the fire. When running 245/65/17 Cooper H/Ts we get around 320km to a tank of LPG. Now that we are running 265/75/16 Hercules A/Ts, we are getting around 320km from a tank of LPG.

Not everything can be explained. This one just leads me to ask questions.

1. Did you do the "Brown Wire Mod" and if so, when?
2. What gears do you have?
3. What effect do you think that the 4.6L has vs. the 4.0L?
4. What effect do you think running LPG has vs. unleaded gasoline?
5. What wheels did you have when running 17's?

My experience would lead me to believe that several factors that I have questioned are playing some effect. Wheels tend to increase weight, by going to 16" wheels vs. 17" weight was equaled out (just a thought). The increased power from the 4.6L translates into a lessened effect caused by the increased size. LPG may (not sure on this one) have higher BTU's then gasoline translating into a lessened effect from the increased size. BWM done around the tire change and/or stock 3.73 gears could have played into the entire equation as well.

 

[IZwack]

Thelt, to answer your question, yes larger tires will result to lower RPM's at highway speeds, but that doesn't directly translate to better MPG. The engine still has to work to keep the tires rolling.

To add to this, the lower RPMs will mean you are probably not cruising at the optimal RPM range which means your engine is struggling. One could argue that you can go faster to get the RPMs back to its sweet spot but of course going faster means an increase in aerodynamic drag (which is a function of the square of the vehicle's speed R ∝ v^2 -- which is not a straight line but a rising quadratic curve especially at highway speeds).

 

[reissiga]

Prompted by a discussion that started in another thread, I decided to post some information for all to ponder when selecting tires. There seem to be a lot of questions going around the modified area about MPG after upgrading tires.

HERE IS MY DISCLAIMER: THIS THREAD CONTAINS NO MPG INFORMATION OR DATA AND IS BASED SOLELY ON MATHEMATICS AND MY THEORIES, NOT NECESSARILY REAL WORLD APPLICATION! I AM NOT ADVOCATING FOR OR AGAINST UPGRADING TIRES, ONLY PROVIDING INFORMATION TO CREATE A GREATER UNDERSTANDING OF POSSIBLE EFFECTS.

Here are some different setups (information I can speak to):

• Stock Tires: 235/70R16 (28.95").
• Upgraded Tires 265/75R16 (31.65"). This tire is 9.3% larger in diameter then the stock tire size.

What does that mean? Well that until corrected by whatever means, your actual speed will be 9.3% faster then what your speedometer shows and actual distance traveled will be 9.3% further then what your odometer reads.

For example:
70 mph (speedometer) = 76.51 mph (actual)
100 miles (odometer) = 109.3 miles (actual)

There is a cure for this problem though...recalibration.

Larger tire size has two effects on MPG.

The first effect is that the larger diameter requires more energy to get the tire rotating given the exact same weight. The second effect is the increased weight that is being placed at the wheel. Combine these two effects and you get a double negative (that doesn't turn positive).

Actual data that I have collected looking at tire weights is as follows (for the most similar load rated tire I could find):

BFG A/T (KO)

• 235/70R16 - 38 lbs
• 265/75R16 - 51 lbs
• Increased weight of 34.21%

Pirelli Scorpion ATR

• 235/70R16 - 37 lbs
• 265/75R16 - 52 lbs
• Increased weight of 40.54%

BFG M/T (KM2) - New M/T

• 235/70R16 - 37 lbs
• 265/75R16 - 49 lbs
• Increased weight of 32.43%

There is no direct cure for this increased weight/diameter, however, one could replace the gears (making the work done by the engine easier). Through some researching, if a person with the stock setup of 235's and 3.55 gears upgraded to 265's, the most comparable gears in terms of engine RPM's would be 3.90's.

Have you ever looked at the weight of a steel wheel vs. an alloy wheel? If you originally had factory steel wheel and then went with a larger tire on an alloy wheel, theoretically, you could compensate for some of the increased tire weight (assuming that the alloy wheel is lighter).

Of course, there are a lot of vaiables that can have an effect on mileage...I'm sure that there are a lot of ways that you could make up for the mileage loss.

 

[Thelt]

To add to this, the lower RPMs will mean you are probably not cruising at the optimal RPM range which means your engine is struggling. One could argue that you can go faster to get the RPMs back to its sweet spot but of course going faster means an increase in aerodynamic drag (which is a function of the square of the vehicle's speed R ∝ v^2 -- which is not a straight line but a rising quadratic curve especially at highway speeds).

How can there be a sweet spot when the factory gearing is different depending on whether you have the tow package or not. If you have the 3.73 you are turning a higher RPM at 60mph than if you have the 3.55. Which one is in the sweet spot?

 

[helmsa]

How can there be a sweet spot when the factory gearing is different depending on whether you have the tow package or not. If you have the 3.73 you are turning a higher RPM at 60mph than if you have the 3.55. Which one is in the sweet spot?

The factory programing in the computer is "optimized" (general term with regards to factory tunes) run at a certain RPM. Maybe it is optimized for the range of 2k to 2.5k RPM and by placing larger tires on both setups, 3.55 drops below the threshold and the 3.73 stays within.

The factory is also going to have different programing with the two different gearings.

 

[IZwack]

How can there be a sweet spot when the factory gearing is different depending on whether you have the tow package or not. If you have the 3.73 you are turning a higher RPM at 60mph than if you have the 3.55. Which one is in the sweet spot?

Engines always have a sweet spot -- that's why you see people running around with huge tachometers with shift lights like they are the greatest thing in the world . After the sweet spot, the hp/torque being produced by the engine has surpassed its "inflection point" on the curve and starts to diminish per unit volume of fuel being consumed by the engine ("diminishing returns" for the business folks) -- which means its time to shift (and hopefully the transmission's gear ratio puts the next gear you're shifting to at the bottom of the sweet spot). Of course not all engines are the same and a racing engine can be shifted much higher (RPM wise) than our engines because their volumetric efficiency / sweet spot is also much higher than our engines.

 

[CALDWELL]

great post HELMSA !!!!!!! just what we needed from the other thread...

info may not be EXACT like you said... but Damn Good Attempt !!!!!!

GET U SOME !!!!!


100 post YAY lol

 

[helmsa]

How the heck do you know all of this, jeeze your one smart guy

So i have 265/75/16 and LS 3.73 so i should re-gear towards 4.10's?

Thanks for ya help

Chris

Hey Chris, sorry that I didn't answer the first part of this sooner. I've always been one to understand anything mathematically or scientifically and I've always taken an interest in vehicles. I just research what I want to know. I know that you have talked in other posts about your age, and want this to be an encouragement to you as I am not much older then you. You can do anything you set your mind to.

great post HELMSA !!!!!!! just what we needed from the other thread...

info may not be EXACT like you said... but Damn Good Attempt !!!!!!

GET U SOME !!!!!


100 post YAY lol

I just tried to lay things out as issues I have looked at in changing tires. What is the effect on my speedometer/odometer because I am keep track of my mileage very closely, so I can monitor the effects of changes like CAI or exhaust. How much weight am I really adding (some tires I have looked at, 265/75R16 can only be found in Load Range E). How can I ease the transition and effects to my vehicle.

 

[Thelt]

I want to run a whole tank out before I judge how much my mileage has changed with the bigger tires but so far it looks like a drop of about 1mpg. If you allow for the fact that my speedometer is off about 7% then the drop is pretty small. I ran my rig with the lift installed but the same tires for a while and my mileage dropped from 16.8 to 16.3 or about half a mpg. I assume that is due to poorer aerodynamics from the lift. I can live with losing a couple mpg because I like the lift and tires a lot.

 

[1_v8_merc]

Who cares? Gas prices have been cut more than In half the past month!

 

[IZwack]

Who cares? Gas prices have been cut more than In half the past month!

Some of us do .. I'd rather put money towards something that lasts more than a week or two instead of burning it in the engine day in and day out. Plus I'm poor .