37s on My F350 – Did I Ruin My Fuel Economy? (Real World Test)
- Dan
- Apr 11
- 4 min read
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Fuel prices right now… they’re not exactly getting cheaper. And if you’re anything like me, standing at the pump watching the numbers climb, you start questioning your life choices a little bit.
Especially when those choices involve throwing 37-inch tires on a diesel truck. So that got me thinking, did I mess this up? Or are 37s not as bad for fuel economy as everyone says?
Real World Fuel Economy Matters More Than Perfect Data
Before getting into the numbers, let’s get one thing straight: this isn’t lab testing.
This is real-world fuel economy. Real driving. Real conditions. Winter diesel, short trips, highway runs, hauling the family around… all the stuff that actually matters when you own a truck like this.
I’ve been tracking the fuel economy on this truck for a while now, and while the data isn’t perfect, it’s honest. And honestly, that’s what most of us care about anyway.
What Changed – 33s to 37s on My F350
If you’ve been following along with the build, you’ll know we recently stepped up to 37s on the F350.
That decision was always part of the plan. This truck is being built to haul our family of six, get us into the backcountry, and handle whatever adventures we throw at it.
What I didn’t fully account for at the time was everything happening around that decision; fuel prices climbing, driving habits changing, and how much real-world conditions actually impact fuel economy.
If you’re new here, this all ties into our full F350 overland hunting build, where we’re slowly turning this truck into a capable family adventure rig.
February Fuel Economy Comparison (33s vs 37s)
Looking back at February gives us a pretty clean comparison.
In 2025, running 33s, the truck averaged 16.68L/100km over just over 1000km of driving.
Fast forward to 2026 with 37s, and over roughly 1200km, the average came out to 15.74L/100km.
That’s not a typo.
The truck actually did slightly better with 37s.
Now, before getting carried away, the difference is less than a liter per 100km. But still — it’s not the direction most people expect when stepping up in tire size.
March Fuel Economy – Where Driving Style Shows Up
March tells a slightly different story, and this is where context matters.
In 2025, we did a lot more highway driving — longer trips, more consistent speeds — and the average landed at 13.48L/100km.
This past March, life looked a bit different. New baby, fewer long trips, more in-town driving, and shorter runs overall. That average came out to 14.41L/100km.
So yes, fuel economy got worse. But not because of the tires alone. The biggest factor here was how the truck was being used. Even at that, we are still talking only a litre difference.
The Real Difference – Highway vs In-Town Driving
This is where things really start to make sense. Around town, short drives, stop-and-go — 37s absolutely use more fuel. There’s no getting around that. More rolling mass, more effort to get moving. But once you’re up to speed on the highway, things change.
The truck settles in, RPM drops, and the difference between 33s and 37s starts to level out. In some cases, it’s nearly identical. That’s something I didn’t fully expect going into this.
Why 37s Didn’t Kill Fuel Economy (Like I Fully Expected)
A big part of this comes down to how the truck is set up. Once the truck gets moving, the RPM drops thanks to the larger tire diameter. Pair that with the 10-speed transmission, and it does a pretty good job of finding an efficient gear.
But the real factor here is gearing.
The Importance of Gearing When Running Bigger Tires
This truck came from the factory with the 3.55 gearing as part of the tow package.
At the time, that was one of the main selling point and it might have been one of the most important decisions for this build.
With the jump to 37s, that gearing effectively balances things out. It keeps the truck usable, keeps RPMs reasonable, and prevents fuel economy from taking the hit most people expect.
If you’re planning on going bigger with tires, this is something worth thinking about early. Fixing gearing after the fact is a lot more expensive than planning ahead when originally shopping for a new vehicle or something you should factor in if your building up a used vehicle.
So… Did 37s Ruin My Fuel Economy?
Short answer?
No… not really.
Averaged out over the two months, the numbers are almost identical with a 15.08l/100km and a 15.075l/100km (rounding makes it 15.08l/100km ). In-town driving takes a hit, sure. But highway driving balances things out more than expected.
So if you’re worried that stepping up to 37s is going to completely destroy your fuel economy, based on this real-world data, that’s not necessarily the case.
This Is Just the Beginning
The reason I wanted to get this data out now is because this truck isn’t staying the way it is.
Not even close. We’re about to add weight and a lot of it!
Camper setup, gear, full family loadout, everything that turns this into a true overland rig built to sustain us in the backcountry.
And once that happens, fuel economy is going to change again.
What’s Coming Next – Fully Built Truck Fuel Economy
The next test is going to be the real one. Empty truck vs fully built overland setup. Same tracking, same real-world conditions, but now with added weight and a true adventure loadout.
That’s the data I’m really interested in and I’ll be sharing it once we get there.
Final Thoughts – Bigger Tires Aren’t Always the Problem
Bigger tires get blamed for a lot when it comes to fuel economy. But after running the numbers, it’s pretty clear they’re only part of the equation. Driving habits, gearing, terrain, and how you actually use your truck play a much bigger role than most people think.
Related Builds and Gear
If you want to dive deeper into this build and how everything ties together:
(Coming Soon) Fully Built Overland Fuel Economy Test
Support the Build
If you want to support what we’re building here:
“Roads Were Meant for Roaming” merch is live on the site.
Because even with fuel prices climbing… we’re still getting out there.
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