Bigger tires look great and add capability, but they cost you at the pump. How much? We collected real-world data from diesel owners to give you actual numbers, not marketing claims. Here's what to expect when you upsize.
The Physics: Why Bigger Tires Hurt MPG
Three factors combine to reduce fuel economy with larger tires:
- 1.Increased rotational mass: Heavier tires require more energy to accelerate and decelerate
- 2.Aerodynamic drag: Larger tires, especially with aggressive treads, create more wind resistance
- 3.Effective gear ratio change: Larger diameter means your engine works harder at any given speed
Real-World Data: What Owners Actually Experience
We surveyed 127 diesel truck owners who tracked their fuel economy before and after tire changes. Here's what the data shows:
MPG LOSS BY TIRE SIZE INCREASE
Based on owner-reported data, stock gearing
Breakdown by Tire Type
Not all 35" tires are equal. Tread pattern significantly impacts fuel economy:
Highway/All-Season
-2.0 to -2.5 MPG
Least impact, best highway efficiency
All-Terrain (A/T)
-2.5 to -3.5 MPG
Moderate impact, good balance
Mud-Terrain (M/T)
-3.5 to -5.0 MPG
Highest impact, loudest, most drag
"Same truck, same driving, same routes. Went from Toyo Open Country AT3s to Nitto Trail Grapplers in the same size (35x12.50). Lost almost 2 MPG just from the more aggressive tread pattern."
, RAM 2500 Cummins owner
The Annual Cost: Real Dollars
Let's translate MPG loss into actual money, assuming 15,000 miles per year and $3.80/gallon diesel:
| Scenario | MPG | Annual Fuel | Cost | Extra/Year |
|---|---|---|---|---|
| Stock tires | 18 MPG | 833 gal | $3,167 | Baseline |
| 33" A/T tires | 16.5 MPG | 909 gal | $3,455 | +$288 |
| 35" A/T tires | 15 MPG | 1,000 gal | $3,800 | +$633 |
| 35" M/T tires | 14 MPG | 1,071 gal | $4,071 | +$904 |
| 37" M/T tires | 12.5 MPG | 1,200 gal | $4,560 | +$1,393 |
Over five years of ownership, 37" mud-terrains could cost you nearly $7,000 more in fuel compared to stock tires. That's money that could go toward other modifications.
How to Minimize the MPG Hit
1. Regear Your Axles
This is the most effective solution. Regearing restores your effective gear ratio, allowing the engine to operate in its optimal power band. Owner data shows regearing can recover 1-2 MPG of the loss from larger tires.
Common Regear Recommendations
Cost: $1,500-$2,500 for both axles, professionally installed
2. Choose Lighter Tires
Rotational mass matters more than you think. A 35" tire can weigh anywhere from 55 lbs to 85 lbs depending on construction. Lighter tires (without sacrificing durability) can save 0.5-1 MPG.
Look for: hybrid or crossover all-terrain tires that use less rubber in the tread pattern without compromising off-road capability. Nitto Ridge Grappler, Toyo Open Country AT3, and Falken Wildpeak AT3W are lighter options in their categories.
3. Narrow vs Wide: Consider Narrower Options
Wider tires look aggressive but create more aerodynamic drag and rolling resistance. A 35x12.50 will typically get worse mileage than a 315/70R17 (roughly 34.4" x 12.4"). The visual difference is minimal, but the efficiency difference is measurable.
4. Keep Tires Properly Inflated
This is free and effective. Every 5 PSI of underinflation costs approximately 1% in fuel economy. Larger tires need higher pressures, follow the load range recommendations for your specific tire and weight.
Pressure Guidelines for Lifted Trucks
- • Daily driving (unloaded): 45-55 PSI for most LT tires
- • Towing/hauling: 65-80 PSI depending on load
- • Off-road: 25-35 PSI (air back up before highway driving)
5. Driving Habits Matter Most
The biggest variable in diesel fuel economy isn't the tires, it's the driver. Aggressive acceleration, high highway speeds, and heavy towing will dwarf the impact of tire choice.
- •Every 5 MPH over 55 MPH costs approximately 7% in fuel efficiency
- •Aggressive acceleration uses 30-40% more fuel than gradual acceleration
- •Idling a diesel for 10 minutes uses approximately the same fuel as driving 5 miles
The Sweet Spot: Maximum Look, Minimum Loss
Based on owner data, here's the configuration that gives you the best balance of presence and efficiency:
THE MPG-CONSCIOUS BUILD
- •Tire size: 295/70R18 or 305/65R18 (approximately 33-34")
- •Type: Hybrid or light-duty all-terrain
- •Lift: 2-3" leveling or mild lift
- •Expected loss: 1-2 MPG from stock
This setup still looks significantly more aggressive than stock while minimizing the ongoing fuel cost penalty.
Accept the Trade-Off or Don't
Here's the honest truth: if you want 37" mud-terrains, you're going to lose fuel economy. There's no magic solution that eliminates the physics. Regearing helps, tire choice helps, driving habits help, but you won't get stock fuel economy back.
The question is whether the capability and appearance are worth the ongoing cost to you. For some owners, $1,000+ per year in extra fuel is a reasonable price for the truck they want. For others, a more moderate tire choice makes more sense.
THE BOTTOM LINE
Know the real costs before you upsize. Plan for regearing if going to 35" or larger. Choose the lightest tire in your size category that meets your needs. And accept that bigger tires mean bigger fuel bills, forever. Build accordingly.