Why Adding Bigger Tires To Your Diesel Truck Doesn’t Warrant A Ring And Pinion Change
As if diesels weren’t different enough from their gasoline counterparts already, there’s another stark contrast, and it’s downwind of the engine. It’s called the ring and pinion gear, something every ICE-equipped vehicle has. But in a diesel-powered vehicle’s case the ring and pinion doesn’t require altering when larger wheels and tires are installed. Why? Predominantly because of a diesel engine’s abundance of low-rpm torque, but also because the increased drag the larger rolling stock provides helps keep the compression ignition powerplant in its happy place: under load.
Of course, there are limits to how big you can go on tire size before re-gearing (i.e. gearing lower) becomes necessary—but thanks to the aforementioned effects of running larger rubber, diesels can get away with quite a bit more than their gasoline counterparts. This time, we’ll explain why diesels don’t mind the extra workload, but also where you should draw the line if you want to retain stock-like drivability or sufficient towing manners.
Diesel Engines 101
The biggest reason for not having to re-gear a diesel vehicle in conjunction with installing larger wheels and tires lies in how the engine works. Low-rpm operators, diesel engines begin building immense torque right off idle and most produce peak twist before 2,000 rpm. This is perfect for getting loads up and moving, which in diesel truck applications includes the heft of the 8,000-pound vehicle itself. But not only that, their torque curves are broad and appear table-top-like when viewed in graph form. By the time peak horsepower enters the picture (around 3,000 rpm in most truck applications), solid torque is still being turned out, which essentially means that the truck’s pulling power doesn’t drop off throughout the course of the engine’s operating range.
Diesels Love Load
Another contributing factor to a diesel not requiring a re-gear in order to run bigger wheels and tires is its affinity for being under load. The additional rotating mass that comes from running bigger rubber, not to mention the decreased aerodynamics that come along with it, force the engine to work harder in order to both gain and maintain speed. When a diesel is placed under load, its turbocharger(s) produces more boost pressure. And in the modern age, where a variable geometry turbo (VGT) is employed on virtually every diesel engine, they respond instantly to load. For all intents and purposes, the engine is more eager to run with the extra load from larger rolling stock in the mix. Even better yet, there is more boost on tap at all times, which only aids responsiveness.
Diesels Love Heat
Yes, forcing an engine to work harder will make it run warmer. Luckily that’s exactly what diesels were designed to do. The additional drag (and weight) from larger wheels and tires merely allows a diesel to make use of its oversize radiator and added fluid capacities. Just as a loaded diesel is a happy diesel, the same goes for heat. A warmer-running diesel means warmer exhaust temps and a warmer turbo—with the benefit of improved transient response. The load created by retaining the factory ring and pinion with bigger tires in the mix is the key behind all of these benefits.
Diesel’s Bread-And-Butter Combo: Leveling Kit And 35-inch Tires
Have you ever wondered why 35-inch diameter tires and 18 to 22-inch wheels are so popular on late-model diesel pickups? Other than the fact that lock-to-lock steering clearance is unaffected with the addition of a simple leveling kit (another common add-on), they allow a much larger and more aggressive tire to be run without any real trade-offs. Even fuel economy is hardly compromised by upsizing to 35s. The F-250 shown here, fitted with 35x12.50R20LT Nitto Terra Grappler G2 all-terrains mounted on 20x10-inch Fuel Off-Road wheels, arguably has better drivability now than it did when it was bone-stock—thanks to the added load of larger wheels and tires. It still returns 18 to 19-mpg empty.
To be sure, a factory-geared diesel does have its limits. And while it will take longer to reach the limits of a truck with a 4.10 ring and pinion than it will with one sporting 3.42’s, eventually you’ll find a tire that’s big enough to noticeably hinder your diesel’s low-rpm performance. As a general rule of thumb, once you’re in the 38-inch range or venture beyond it on a late-model diesel, lower gearing (which is numerically higher) will likely need to be considered. Bigger than 40-inches and you’re almost surely going to need to re-gear.
Beyond axle gearing itself, the condition of the ring and pinion shouldn’t be overlooked. In the diesel performance game, it’s common to stack all kinds of horsepower and throw obscene amounts of torque at the factory axles—most of which aren’t even treated to new gear oil before the partying starts. If you’re modifying a 20-year-old truck, without question you should be giving the rear-end (and front differential if 4x4) a once-over before your horsepower journey begins. Trust us, installing bigger tires and doubling a truck’s torque output—all on a high-mile and highly aged ring and pinion—can end catastrophically.
Don’t Forget About The Speedometer!
Even with your stock axle ratio retained, it’s important to remember that running a taller tire will throw off the factory speedometer reading. The bigger the tire, the more inaccurate it will be. Specifically, your speedometer will read slower than the vehicle is actually moving. This is because the taller tires cover more distance per revolution than the factory sized tires did. To get your speedometer to read accurately again, you’ll have to recalibrate it. This can be done via a reflash at your local dealership, through the use of an aftermarket programmer or performed using software such as EFI Live.
Big Tires & Towing
If you’re planning to do the whole nine yards with your diesel—suspension lift, big tires and large, offset wheels—and continue to tow heavy, you’ll likely need to re-gear. Lower (again, numerically higher) gearing will allow your truck to get loads moving easier at low rpm. Just be careful here (i.e. don’t go running for 4.88’s), as going too short will force the engine into seeing excessive rpm at highway speeds. There is always a fine-balance when re-gearing a diesel and it’s best to do your homework before swapping ring and pinions or leave it to a professional.
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