This article is going to be about proper final ratio to the ground and the effects.
OPTIMAL FINAL RATIO TO THE GROUND
3.55:1 to 3.73:1
Remember when you have 3.55 axle gears you cannot swap to a larger tire without a huge impact on EGT's and loss of power. Cummins engines are not designed to be cruising at highway speeds at low RPMs. True 3.73 final ratio will be roughly 2,100 RPMs at about 65 MPH. Even what I'm set up for with my own truck with 245/75 R16 is a 3.69:1 final ratio. I'll be using the stock tire size of most trucks which was 31-inch tires. I highly suggest you take a moment and plug in the tires size and gear ratio into this gear ratio calculator.
Lower than 3.55 final ratio you find that EGT's are much much higher and the cruising RPM is too low. The engine will struggle to make power and accelerate without pushing EGTs higher yet. When you fall into the 3.55 to 3.73 realm the truck runs cooler EGT, coolant and, transmission temperature-wise.
Being most of my local highway speeds out here is 65 MPH I opted to reach down towards 3.73 for towing and highway use. This places me at roughly 2,000 RPMs at 66 MPH. This was done by using 245/75 R16 tires on 3.55 axle gears which will produce a 3.69:1 final ratio. If your want to run any oversized tires larger than a 31-inch tire which is either 235/85 R16 or 265/75 R16 then you'll be required to have 4.10 gears. There is no way to get around this problem.
You have to remember I'm not just basing this on my personal vehicle but all the vehicles I've ever worked on. For example, I had a member from the forum here come to my place asking for me to build him a Quadzilla tune. He's running 35-inch tires on 3.55 gears which produced a final ratio of 3.38:1. His EGT's were high with 100 HP injectors and even an updated turbo. After even building the tune and managing to cut about 150-200*F of EGT's out it still was barely cruising at 1,600 at 65 MPH. Way too low you should never be that low cruising at highway speeds. Anytime this member rolled into the throttle a little bit the EGT's jumped fast past 1,000*F no problem. Again the final ratio is just too tall and the engine is struggling. This was even seen in the engine load number which is almost 30% to 40% at 65 MPH.
Now to compare this member to my own truck now with +150 HP injectors, Quadzilla, smallish HX35/40 Turbo, 245/75 R16 tires. EGT's are at least another 200 to 250*F lower compared to this member. Engine load is 13% to 18%, cruising at 65 MPH will be right about 1,950 RPM. My 65 MPH cruising EGT's is roughly 550*F. Which proper gear ratio this combo has even allowed me to exceed my old high mark of 27.2 MPG and surpass it at 28.04 MPG. A proper ratio will reduce rolling resistance on the engine. Even running the interstate here in Idaho at 80 MPH will just the RPM to 2,450 roughly. Still optimal and will not impact MPG's much.
So if you really want to keep your oversized tire and that off-road look you going to find out that gear swaps are not cheap. I've had another member ask me to help him get the power up and EGT's down on his truck but he had 37-inch tires and 3.55 gears. This produced a final ratio of 3.00:1. Again the member had installed twin turbos hoping to fix the high EGT's. Didn't help. Now after talking to him and finding out he was not going to give up his 37-inch tires this means that he needed to swap both front and rear gears for 4.10 gears. This cost him close to $4,000 to correct this issue. After swapping to 4.10 gears this brought him back to 3.55:1 to final ratio. So be aware if you really want to keep your tire size or look then it might be rather expensive to correct.
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