This Tech Tip from Carli’s Blog is so good that we had to share it! This is something that we talk to customers all the time about and wanted to write something up about. But the guys over at Carli did such a good job spelling it all out that they just told us to use it as we pleased!
Carli’s original blog post: Original Post
One of the first questions we ask when a customer calls with question pertaining to ride quality is “What wheels & tires are on the truck and to what pressures are the tires inflated?” Why, you ask? Simply put, not all wheel/tire combinations are created equal. Load Ratings and pressures, number of plies, radial vs. bias, tread design (A/T, M/T, Highway), and profile all affect one’s ride quality. Suffice is to say, to choose a tire based on price and size alone won’t necessarily net a customer the proper tire for their application.
This guide will provide insight into the more important specifications assigned to a tire by the manufacture and to what heavy-duty truck owners should pay attention when selecting an aftermarket tire. There is an abundance of information on the web pertaining to each tire specification in as much detail as a person could want to read so we’re going to focus on load rating and tire pressure; the most important factors in our opinion.
Specific to heavy duty trucks, let’s take a look at the door tag on our 2012 Ram 2500 Megacab. It lists the stock Michelin’s Spec:
– Front: 265/70R17E as 5500 lbs total (2750 each tire) at 60 PSI.
– Rear: 265/70R17E is listed as 6010 lbs. (3005 each tire) at 70 PSI.
– Total Tire Capacity: 11,510lbs.
We drove our 2012 Ram 2500 Mega cab (6” Performance 2.65 kit and 37 inch Toyo M/T’s) to the scales to get actual weight. We found that with ¼ tank of fuel, normal cab clutter (2 car seats, kid’s toys, race radio) and a 37” spare, we weighed in at 8,480lbs. She’s no featherweight. Further detail revealed a front weight of 5200 lbs. and 3260 lbs. in the rear. This nets us a difference of 1,120lbs less than the GVWR.
The gross vehicle weight rating (GVWR) is the maximum operating weight of a vehicle as specified by the manufacturer including the vehicle’s chassis, body, engine, engine fluids, fuel, accessories, driver, passengers and cargo but excluding that of any trailers. Basically, the maximum weight the vehicle is built to withstand in bolt on accessories plus the load carried in the cab and bed combined. Below is a chart for our particular 2012 Mega Cab 4WD Truck.
So we’ve got a factory Tire capacity of 11,510 lbs. and Truck capacity of 9,600 lbs. We also see the axles are rated to handle the same load to the factory tires, 11,510lbs meaning they’re likely the limiting factor in the GAWR (Gross Axle Weight Rating). This means the GVWR is the true limiting factor to loading our truck, thus it’s the baseline against which we’ll compare the tires.
After purchasing a Carli Suspension, most customers purchase an aftermarket 35” or 37” tire, or a metric equivalent. These aftermarket, all terrain/mud terrain tires are often more robust in both sidewall and ply construction commensurate to their intended application leading to higher load capacities at lower pressures.
We’ve been through more than a few tires in search of the Holy Grail, do it all tire. For the sake of this article, we’ll compare two well regarded brands with which we have extensive experience, Toyo and Falken.
Toyo is our comfort zone; the bar against which all tires are measured. Becoming the Carli go-to tire without any working relationship between the two companies (we pay what you pay for Toyos) was not an easy feat. It took us years upon years of pounding tires everywhere from Southern California to the tip of Baja on the road and on the most rigorous dirt trails and yes, even cacti fields, for these tires to earn their reputation. The tires proved their merit again and again. We recently struck a deal with Falken Tire so a few of the shop rigs are now running the Wild-Peak A/T. The construction of the tire is pretty incredible. The weights and sidewall construction are on par with a Toyo M/T in their equivalent size. They’re D-Rated in the sizes we’re running and haven’t been subjected to near the abuse to which we’ve subjected the Toyo’s. Time will tell if they’re a contender but we thought they’d make a great comparison given they’re a D-Rated tire often recommended to the budget-minded consumer where the Toyo’s are the more expensive, E-Rated option.
Back to tire pressures… one of the most prevalent mistakes encountered is failure to adjust tire pressure on a tire well suited to the heavy-duty platform. Shops commonly inflate tires to the MAX cold PSI per the sidewall before sending customers on their way. In an OEM style tire, this provides enough capacity to comfortably carry the GVWR. In an aftermarket tire, however, this means the customer is now unknowingly rolling around with up to 17,200lbs. of tire capacity in an unloaded truck weighing less than half the new tire’s load capability.
The stock Michelins are rated for 3,005 lbs per tire at 70 psi. For sake of comparison, we’ll keep to our recommended tire size, 37” tires for a 17” wheel, as this document pertains to tire recommendations for Carli Suspensions.
The 37×13.50r17 Toyo M/T is rated for 4300 lbs. at 65 psi.; that’s 1,295 lbs MORE at 5 psi LESS per tire, when comparing to the factory Michelins. Referring to Toyo’s weight rating chart, at 35 psi , the tires hold 2,835 lbs. per tire and 40 psi supports 3,110lbs. The pressures recommended by our product developers are 40 psi front, 35 psi rear as that is the pressure at which the suspensions are tuned. These pressures support a total of 6,220lbs. front and 5,670 in the rear, according to the Toyo load chart. This exceeds the GVWR (again, max loaded weight, not unloaded weight) of the truck and we’ve not yet broken 62% of the tire’s maximum capacity.
Again, our truck weighed: 5,200lbs. Front, 3,260lbs. in the rear. Referencing Toyo’s load inflation tables, it’s not only perfectly safe to run 40F/35R, but highly recommended you run the aforementioned tire pressures to ensure maximum ride quality and proper tire wear without the need to constantly adjust for a load. For the Falkens, we found 40F/35R to be a bit “squishy” in the sidewall. We attribute this to the “D” load rating and recommend you raise a D-Rated Tire about 5psi over the pressure at which you’d run your E-Rated Tires. 45F/40R eliminated all handling issues and wears evenly. The Falkens being rated to 3,525 at 50psi still give customers plenty of adjustment up and down to achieve ride quality and maintain load capacity despite their “D” load rating.
The final note is on wheel diameter. Simply put, the larger the wheel and shorter the side wall (assuming an E-Load Rating), the stiffer the wheel and tire will ride. We’ve found that many combinations will significantly reduce small bump compliance, the most popular being 35” tires on 20” wheels on our leveling suspensions. The question most often asked is why new trucks come with 20” wheels if they ride poorly and the answer is simple; to sell trucks. Dealerships want to offer trucks that are aesthetically appealing and customers want larger wheels. If a customer were to inquire about a base model truck (built to work, not look good) they’d find them spec’d with 17”-18” wheels while the Lariats/Laramies will have paint accented 20” wheels. Both can achieve the necessary capacity of the vehicle, but one does it more efficiently and rides better.
Given the spring rate required to suspend these 8,500lb. vehicles, wheel and tire selection remains a very important factor in improving small bump compliance. If we were to adjust the spring rates/shock valving enough to compensate for the stiffer running gear, you’d give up substantial control of the vehicle in all other scenarios other than cruising the expansion joint ridden highway. Carli Suspensions are designed with utilitarian purpose in mind; thus, we’re not willing to make that compromise. We want the truck to be as composed down rutted-out back-roads as it is on the highway; this goal is easily attainable with the right combination of suspension and wheels/tires. We’re a performance based suspension manufacturer so all systems are tuned unloaded with 17”-18” wheels on 35”-37”, E-Rated Tires, inflated to 40F/35R. Running any combination outside of this will result in deviation from our desired ride quality.