It’s the classic truck trade-off: you want the 4-inch lift and off road tires for off-road performance, but you don’t want the on-road fuel economy of a lead-weighted brick. My 2018 RAM 1500 (2WD) has been a work in progress for years. Bilstein 5100s, a 4” Rough Country lift, and an Eaton Truetrac rear diff. It’s built for dirt or inclement weather that it rarely sees—it usually lives on the highway at 72 mph.

For the longest time, I was stuck at a crossroads. Moving from the stock 33-inch tires to a 34 or 35 usually means sacrificing precious MPG. But I wanted to find the inflection point—the maximum size with the least efficiency loss.

After a lot of math and a little experimentation, I got it dialed in. Here is the breakdown of the physics and the build.

The Problem: The Toughness Tax

Most people lift a truck and immediately throw on 35x12.50 MTs. It looks great, but you’re adding massive rotational mass and wind resistance. On a half-ton RAM, that’s a one-way ticket to the 14 MPG club.

The engineering math is unforgiving: for every 100 pounds of added weight, you lose about 1% in fuel economy. When you’re trying to get 6,000 lbs of steel moving from a dead stop, that weight is a parasitic tax on your kinetic energy.

My goal was specific:

  1. 34-inch diameter to fill the wheel wells.
  2. Maintain 20 MPG at 72 mph.
  3. OEM+ Aesthetic: No poke, no rubbing, and keep the factory chrome 20s.

The BOM (Bill of Materials)

Milestone 1: Shedding the Pendulum

Before the tires even went on, I had to rethink mass. I was carrying 400 lbs of sand in the bed for winter traction and a toolkit that was a literal kitchen sink.

Weight is the enemy of stop-and-go MPG because of static inertia—it takes significantly more torque to get those first few rotations. I ditched the 400 lbs of sand but kept the utility: a barebones daily toolkit, recovery straps and boards, and a trailer hitch. It’s about 80% utility for 20% of the previous mass. I treat the truck like the high-speed cruiser it more often than not is, and when I need the traction or larger toolkit I can always add them back.

Milestone 2: Finding the Safe 34

I landed on the 275/65R20. It’s a 34.1” tire, but crucially, it keeps the stock width. This prevents the tires from rubbing the Upper Control Arms (UCAs) on stock wheels and keeps the aero profile as slim as possible.

I went with the Nitto Terra Grappler G3. Why? Because it’s a modern on-road AT. It’s lighter than the Falkens (61 lbs anchors) and rolls smoother than the BFG KO3s. At 55 lbs, it’s the efficiency winner of the heavy-duty bracket.

The G3 uses a silica-rich compound that effectively keys in to the road surface. Unlike harder, chip-resistant rubber that favors off-road toughness, this compound reduces energy loss from tread squirm, which is vital for maintaining a baseline MPG at 70+ mph.

Milestone 3: Programming the Truth

If you don’t calibrate your speedometer, your ECU is getting bad data. Moving from 33s to 34s means your truck thinks it’s going 3.3% slower than it actually is.

I used the JScan app and a bypass cable to tell the BCM the tires were exactly 34.00 inches. Instantly, the shift points smoothed out. The 8-speed Torqueflite finally knew when to hunt for 8th gear and stay in MDS (Multi-Displacement System). If the engine load stays too high because of uncalibrated shift points or excessive tire weight, the MDS won’t activate—effectively locking you out of 4-cylinder mode and your 20 MPG target.

Note on the bypass: I deliberately did a behind-the-dash temporary bypass rather than a permanent one. I didn’t want to make the truck more susceptible to theft by making the bypassed security gateway readily available 24/7. I plug it in for maintenance and pull it for daily driving.

The Result: 20.0 MPG Verified

Metric Result
Highway (72 mph) 20.0 MPG
City 15.5 MPG
Steering Feel Planted (at 42 PSI Front / 40 PSI Rear)
So What? I have the capabilities I wanted without paying the California 35s gas tax.

FYI: If you’re running a Truetrac like I am, remember that you can’t run a mismatched spare on the rear axle. If you get a flat, it’s the Double Swap protocol: spare goes on the front, good front tire goes to the rear. It’s an afternoon of work, but it saves the diff. To handle the swap, I paired the Unijack with a dedicated jack stand.