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If you own a diesel truck, there is a good chance you have either already installed larger tires or thought about doing it.
Bigger tires can completely change the look of a truck. They can improve ground clearance, help with off-road traction, and make a lifted truck look like it was actually meant to be lifted.
But they come with a tradeoff.
Larger tires can hurt diesel fuel economy.
How much depends on the tire, the wheel, the gearing, the truck, and how you drive it.
And with diesel prices climbing, that tradeoff becomes a lot more noticeable.
The good news is that if you understand why bigger tires affect MPG, there are some things you can do to minimize the loss.
There is not one single reason larger tires hurt fuel economy.
Usually, it is a combination of several things happening at the same time.
The biggest factors are:
Each one matters.
Put them together, and the truck may need noticeably more energy to travel the same distance.
This is one of the simplest parts of the equation.
A larger tire is usually heavier than the factory tire.
And often the aftermarket wheel is heavier too.
That matters because rotating weight affects the truck differently than static weight.
The engine has to accelerate the tire and wheel every time you pull away from a stop.
Then the brakes have to slow it back down.
The heavier the rotating assembly, the more energy it takes to get moving.
That can reduce fuel economy, especially in stop-and-go driving.
This is one reason a heavy mud-terrain tire can have a bigger MPG impact than a lighter all-terrain tire of the same diameter.
Larger tires also tend to create more rolling resistance.
That can come from:
Mud-terrain tires are a good example.
They are designed for traction, not maximum fuel economy.
The tread pattern that works great in mud may take more energy to roll down the highway than a highway-terrain tire.
That does not mean you should not run them.
It just means there is a tradeoff.
This is one of the biggest things diesel truck owners sometimes overlook.
When you install taller tires, you effectively make the axle gearing taller too.
For example, if the truck came with a certain axle ratio and factory tires, the engine and transmission were calibrated around that combination.
Install a significantly larger tire and now the engine turns fewer RPM at the same road speed.
That may sound like a good thing for fuel economy.
Sometimes it can be.
But if the gearing becomes too tall, the engine may fall out of its efficient operating range.
Now the truck needs more throttle to accelerate.
The transmission may unlock the torque converter more often.
It may downshift more frequently.
And when towing, it may spend more time hunting between gears.
All of that can hurt fuel economy.
This is worth pointing out because it sounds counterintuitive.
A lot of people assume:
Lower RPM = better fuel economy.
Not always.
A diesel engine operates most efficiently within a certain load and RPM range.
If the engine is lugging because the tire size has effectively made the gearing too tall, it may require more throttle and more fuel to maintain speed.
The goal is not simply to get RPM as low as possible.
The goal is to keep the engine operating efficiently.
That is why tire size and gearing need to work together.
Modern diesel trucks rely heavily on transmission strategy.
The transmission is constantly deciding:
When tire size changes, the truck may no longer be operating exactly the way the factory calibration expected.
This can result in:
And all of those can reduce fuel economy.
This is one reason a tire-size recalibrator or programmer can be important after installing larger tires.
Depending on the truck and application, products like the Edge EZX Module or the Bully Dog GT Platinum Diesel Tuner may offer calibration and tuning functions that help the truck account for a different tire size.
Thoroughbred Diesel also carries tuning and calibration products from DiabloSport and Superchips.
You can also browse our full selection of diesel tuners and programmers.
This is another important one.
If you install larger tires and do not recalibrate the truck, the odometer may no longer be measuring distance correctly.
That means your MPG calculation may also be wrong.
Let’s say the truck thinks you drove 300 miles.
But because of the larger tire diameter, you actually traveled farther than that.
If you divide the wrong mileage by the amount of fuel used, your calculated MPG will look worse than it really is.
The dash display can also be affected.
So before assuming you lost a huge amount of fuel economy, make sure the truck is properly calibrated for the new tire size.
It is important to be clear about what recalibration does.
A tire-size recalibrator does not magically increase fuel economy.
It corrects the truck so the computer knows what size tire you are actually running.
That may help with:
The biggest benefit is that the truck is operating from correct information again.
That is the first step before making any bigger changes.
If the tire size change is large enough, recalibration may not be enough.
At some point, the physical gearing of the truck becomes the bigger issue.
That is where re-gearing the axles comes in.
Changing the ring and pinion ratio can help restore some of the effective gearing lost when larger tires were installed.
The goal is to bring the engine and transmission back into a more useful operating range.
Re-gearing may make sense if you notice:
If larger tires have changed the way your truck drives, re-gearing may be worth considering.
Thoroughbred Diesel carries drivetrain and gearing products from Motive Gear and Yukon Gear & Axle, both of which offer axle and gear components for a wide range of truck applications.
You can also browse our ring and pinion gear packages.
Re-gearing is not a universal fuel economy fix.
The correct ratio depends on tire size, transmission, axle ratio, engine, towing needs, and how the truck is used.
But if the gearing and tire size are badly mismatched, correcting that relationship can make the entire truck work better.
Even with larger tires, basic tire pressure still matters.
An underinflated tire creates more rolling resistance.
That means more energy is required to keep the truck moving.
Aftermarket tires may not use the exact same pressure as the factory tire.
The proper pressure depends on the tire, load rating, truck weight, and how the vehicle is used.
Do not simply assume maximum sidewall pressure is always the answer.
You want the tire properly inflated for the application.
And if one tire is constantly losing pressure, fix the leak instead of continuing to run it low.
Do not forget about the wheel.
Large aftermarket wheels can add a significant amount of weight.
A heavy wheel combined with a heavy tire can create a lot more rotating mass than the factory setup.
If fuel economy matters, pay attention to the weight of the entire wheel-and-tire combination.
Two 35-inch tire setups can have very different weights.
That difference matters.
Large tires often come with a lift kit.
Now you are dealing with another factor.
Lifting the truck increases frontal area and changes how air moves around the vehicle.
At highway speed, aerodynamic drag becomes a major part of fuel consumption.
The taller the truck sits, the more air it has to push through.
Add wider tires, aggressive tread, heavier wheels, and more ride height, and the MPG loss can add up quickly.
Again, there is nothing wrong with building a lifted truck.
Just understand what you are trading for the look and capability.
Not all large tires affect fuel economy the same way.
A highway-terrain tire generally rolls easier than a mud-terrain tire.
An all-terrain falls somewhere in between depending on the design.
Things to consider include:
If fuel economy matters but you still need a larger tire, choosing a less aggressive tread pattern may help.
Larger tires can have an even bigger impact when towing.
Now the engine is already under more load.
If the effective gearing is too tall, the transmission may have to downshift more often to maintain speed.
That can raise RPM, increase fuel use, and make the truck feel like it is constantly working.
This is where re-gearing can make a big difference in how the truck drives.
The goal is not just fuel economy.
It is keeping the drivetrain operating in a range where it is not constantly fighting the load.
The point here is not to tell everyone to go back to stock tires.
Plenty of diesel trucks run larger tires and perform very well.
The key is setting the truck up around them.
That may mean:
If you make a major tire-size change and do nothing else, you may not like the way the truck drives.
If you build the truck around the tire size, the results can be much better.
There is no universal number.
One truck might lose a small amount.
Another might lose several MPG.
The result depends on:
That is why it is difficult to say something like:
“35s cost exactly 2 MPG.”
There are too many variables.
What matters more is understanding what changed and correcting the parts of the setup that no longer work well together.
When diesel is cheaper, losing a little fuel economy to larger tires may not bother you much.
When diesel prices rise, that same MPG loss becomes a lot more noticeable.
If you put a lot of miles on your truck, even a small difference can add up over the course of a year.
That does not mean you need to remove the tires you like.
It just means now is a good time to make sure the truck is properly calibrated and geared for them.
Yes, bigger tires can hurt diesel fuel economy.
They add weight, increase rolling resistance, change effective gearing, alter transmission behavior, and can make your MPG reading inaccurate if the truck is not recalibrated.
But bigger tires do not automatically mean the truck has to drive poorly.
A properly set-up truck can handle larger tires much better than one that was simply lifted, fitted with big tires, and left otherwise untouched.
Start with tire-size calibration.
Then look at gearing.
Make sure tire pressure is correct.
Consider the weight and tread design of the tire.
And pay attention to how the transmission behaves.
If you have installed larger tires and your diesel truck feels sluggish, shifts constantly, tows poorly, or has lost fuel economy, Call for Free Advice at Thoroughbred Diesel.
Tell us your truck, current tire size, factory tire size, axle ratio, and how you use it.
We can help you figure out whether calibration, tuning, gearing, or another part of the setup makes sense.