Tech Q and A


I have read somewhere factory 20”s on a Dodge Ram cuts the towing capacity by 1,000 pounds. Is this true? I have looked at the brochure and it says nothing about tire size. It says the CC 4×4 w/3.92’s tows 9,950 pounds. So with 20-inch wheels would that be 8,950? Is it the tires that might affect the load rating? Maybe replace the factory 20s with an 8-ply version? – Fit2BTowed

Wheel (and tire) selection can affect maximum towing capacity by as much as 1,100 pounds according to Dodge’s tow rating information. The heavy-duty models only come with 17s, so this isn’t a factory issue. However, if you do upgrade tires and wheels, make sure both replacement tire1) The optional tire’s diameter is so large it effectively reduces the truck’s final-drive ratio to the point it equates to an axle-ratio reduction, which decreases towing capacity, or 2) the optional tire’s tire load-carrying capability is so low it reduces Rear Gross Axle Weight Rating (RGAWR/wheel load-ratings are equal to, or greater than, the factory versions being replaced.

In general, vehicle manufacturers usually change a pickup’s or SUV’s Trailer Weight Rating (TWR) for one of two reasons: 1) The optional tire’s diameter is so large it effectively reduces the truck’s final-drive ratio to the point it equates to an axle-ratio reduction, which decreases towing capacity, or 2) the optional tire’s tire load-carrying capability is so low it reduces Rear Gross Axle Weight Rating (RGAWR, which reduces the ability to carry necessary tongue weight for a calculated TWR.)

If you have a 2010 Ram, you might want to check out the tow-rating charts ( Read the footnotes carefully on all these tow-rating charts; they spell out the caveats. For example, towing those maximum trailer weights requires the use of a weight-distributing hitch as Dodge limits all of their pickups to 5,000 pounds “on-the-ball” (weight-carrying mode) with the factory Class IV hitch.


My pickup’s speedometer is apparently reading slow, at least that’s what an officer said when I got ticketed for speeding in a construction zone. I’d recently had a set of 35s put on my 2008 Silverado 2500 HD and now need to figure out how to correct the speedo. Any suggestions? – Hot In Phoenix

Welcome to remedial math class: Actual speed = new tire diameter x indicated speed /old tire diameter. Or, nix the iPhone calculator and go on the web where you’ll find any number of calculators to plug in your tire sizes (old/new) and get the speedometer reading difference. One we have linked to on our web site: Plug in your tire information and indicated speed, it pops out the actual speed. For example, if your stock tires were 32 inches tall and the new ones are 35, the speedo error is about 5 mph slow. Once you have that info, log it in your head—or put a piece of tape on the dash as a reminder. The faster method: Call Hypertech ( and order one of their plug-in modules ($229) that do it all electronically.


Are there any issues switching from the OEM engine oil in our new Ram 3500 Cummins to synthetic oil? We’re coming up on the first oil-change interval and want to go to synthetics for better fuel economy and longer duty cycles. – Pete Austin, San Antonio

 In most instances we’d say go for it. But according to our technical source at Ram, you wouldn’t see the extended-service interval switching to a true synthetic. “The engine oil in the new Cummins is a high-quality, energy-conserving-type lubricant and contains additives to aid in the proper break-in of the engine. If desired, a synthetic oil of the proper weight, API compliance and viscosity may be substituted. However, the oil and filter change schedule must remain the same as for petroleum based oil [to stay within warranty.]”


We are looking into several heavy-duty hitches we could use in our various pickups with the 2-inch receivers so we can tow our bigger equipment trailers. Can you recommend one pintle/ball combination that’d work best? – Steve Wickenburg, Racine, WI

Curt Manufacturing, Draw-Tite, Reese, Wallace Forge, B&H, and others make the pintle/ball combinations with adapters to go into the 2-inch Class III/IV receiver. What you choose is just a matter of looks/style on your part. However, be aware changing the hitch and/or the receiver doesn’t increase a pickup’s towing capacity. Even if the hitch and receiver (ball-type or pintle) are upgraded to ones of higher capacity, it’s the limitation placed by the vehicle maker that determines the maximum tow rating, not the hitch. (See “Towing The Line,” Spring 2010.) Exceeding the vehicle’s towing limits – and/or towing with the vehicle improperly equipped as indicated by the vehicle manufacturer – places you at risk from both vehicle warranty and personal/corporate liability standpoints.


I’m in a debate with a co-worker about tire pressures. What is the proper tire pressure to run, the one listed on the sidewall or the one shown on the pickup’s door? – K. B. Guthrie, San Diego

Hope you’re the one who is betting on the tire pressures listed on the vehicle’s door tag! The inflation pressure listed on a tire’s sidewall is the tire manufacturer’s recommendation for it to carry the maximum load for which it’s rated. The vehicle manufacturer’s tire pressures are for a specific size tire and for that specific vehicle. In fact, on many newer HD pickups there are two tire pressures indicated – one for light (unloaded) and one for heavy (towing/hauling). Inflate to the lower tire pressures for the best ride when running light, and air up when a trailer is in-tow or the bed loaded.



What’s this about bed-mounted fuel tanks being illegal? We’ve built our own 80-gallon tanks for our pickups here on the ranch for years and have never had an issue. – Steve Pence, Modesto, CA

The biggest legality issue, especially in California, comes into play if the fuel tank is connected directly to your pickup’s fuel system. Such an “auxiliary fuel tank” MUST be both Federal Motor Vehicle Safety Standards (FMVSS) and DOT-approved, such as those tanks offered by Transfer Flow and American Tank. If for some reason you are caught running a non-compliant fuel system, you face thousands of dollars in fines in accordance to the 1990 Clean air Act. If the tank is not connected to the factory fuel system, i.e., it’s a refueling tank with a separate hose and pump, then it still must comply with DOT standards for Intermediate Bulk Containers. With either fuel tank application, think first about potential liability and insurance red flags.



Send them to Bruce Smith at [email protected]