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12 hours ago, Moses Ludel said:

Terry...What is the vehicle year and chassis type?  Glad to share...

Moses

I have a 2015 ram 2500 megacab diesel, tires are 35x12.50R20 and it has a 3" lift. I find that I have to push the break pedal a little harder than expected to stop.

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Terry C...I have a similar layout with our 2005 3500 Ram/Cummins, Quad-Cab 4x4 with single rear wheels.  My wheelbase is shorter at 140.5 inches with the short box and Quad-Cab...Your brakes are similar. 

I have run 35" tires and now 37".  While the stock brakes (rotor size, pad composition and calipers) are ample, running oversized tires reduces your engine/compression braking.  Unless you simultaneously changed your axle ring-and-pinion ratios to restore the original/equivalent gearing when adding the 35" tires, you will be using more braking effort when coasting down to a stop.  This is not an illusion, it's physics.

When new, our truck had 31.6" diameter tires and 3.73 gears.  When I decided to change the gearing (after changing to 35" diameter tires), I had the option of either 4.10 or 4.56 gear sets to compensate for the "overdriving" effect of the 35" tires.  Since we pull our travel and car hauling trailers at a maximum of 65 mph, I opted for 4.56s.  Beyond trailer pulling, which was great with the 4.56 gears, the gearing proved too low for the 35" tires when running unloaded.  

I like the diesel to run between 1600-1900 rpm for strong torque and decent fuel efficiency.  Pushing over 68-69 mph dropped fuel mileage considerably with the 4.56 gears and 35" tires.  (The engine spun 2114 rpm at 70 mph in overdrive.)  I compensated with 37"x12.50x17 tires, which gives me 1972 rpm at 69 mph or 2000 rpm at 70 mph—each rpm in overdrive with the four-speed 48RE's overdrive ratio of 0.69:1.  For normal or milder 6% downgrades, I use a BD lockout of the overdrive and coast down in direct 1:1 (3rd gear).  With the trailer in tow, I brake then drop to 2nd gear if the grade is steeper.  I always slow the truck down when trailering and hold within the 1-2-3 gears (no overdrive) when climbing or descending grades.  I am not hard on brakes, which means the gears and engine compression do their job.

I'm currently planning to change gears to 4.10:1 and drop my tire diameter back to 35".   I'm not a big fan of large unsprung weight mass, and the 37" tire/rim combination is 102 pounds per wheel.  Braking is not impacted severely due to the 4.56 gearing.  To maintain normal compression braking and reasonable fuel efficiency with 35" tires, I'll want 4.10 gears.  This will provide 1901 rpm at 70 mph. 

Ideal fuel efficiency for my 5.9L Cummins is between 1600-1900 rpm.  I do have a Hypertech MaxEnergy software tune, which bumped up the torque peak to 2,100 rpm.  For maximum fuel mileage without a load, however, 1600 rpm works best with the 5.9L.  The closer to 1600 rpm, the better.

Regarding brake wear, I changed the front pads one time in 182,000 miles.  I'm still on the original rear pads.  Rotors are original, though I did glaze break the front rotors during the pad change.  I rely on compression braking, do not overheat the brakes and make efficient use of the trailer brakes.

Moses

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  • Moses Ludel changed the title to Oversized Tires and Brake Needs
  • 4 months later...
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joshkalfelt...As I described in my first reply, the gearing is more important than brake size.  Gearing determines engine compression braking ability.

I still run the OEM brake rotor size and stock calipers (stock drums at the rear of the XJ) on both our XJ Cherokee and the Ram 3500.  The XJ Cherokee has 33" diameter oversized tires.  I changed from the OEM 3.55:1 axle ratios to 4.10 ratio front and rear.  This restored the engine compression braking.  On the Ram, as I described, the 37" tires require 4.56 gears to restore the compression braking.  I am changing to 4.10 gears front and rear on the Ram while dropping back to 35" diameter tires.

In each example, the compression braking is restored to the OEM level.  The Ram originally had 31.9" diameter tires with 3.73 axle gears.  This is nearly identical to 35" tires and 4.10 gears.  The 4.10 change in the XJ Cherokee also restored the engine speed and compression braking, equivalent to the original tire size and 3.55 gears.

There is a simple way to determine what gear ratio will restore the compression braking.  I use engine rpm at a fixed road speed as a guide.  There are many calculators online now.  I've used http://www.csgnetwork.com/multirpmcalc.html for years, there are others.  Take your pick.  When using any of these calculators, be sure to use the overdrive gear ratio for your vehicle at highway speeds.  I check and compare engine rpm at 60, 65, 70 and 75 mph, looking for the gear ratio that places engine rpm closest to that with the original tire size and factory gear ratio.

Make sense?  As far as going to "bigger brakes", that might be an alternative but can be costly.  In some cases, there is no room for a bigger rotor or brake drums.  I don't fiddle with the factory brake size on modern vehicles with four-wheel disc brakes or disc front/drum rear brakes.  Disc brake conversions are often sensible for older 4x4 vehicles.  For better performance and braking, I concentrate on correcting the axle gear ratios to match the new tire size.

Moses

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