Axles Article - Proper Final Ratio

[Mopar1973Man]

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. 

View full Cummins article

[Silverwolf2691]

 If may weigh in on this, because hopefully I'll be swapping tires soon, trucks that have 4.10s from the factory are in an interesting spot.

 

From the factory, our tires are undersized for economy, bar none. At 60 mph, with 285/70r17s, I'm turning 2000 rpm. It sounds like the engine is screaming when doing 70-75 down the highway. While I haven't gone through my truck to ease up on weight or drag like @Mopar1973Man has (yet), 15 mpg is my average.

 

I have tried messing with the quadzilla but cant seem to match the theories that Mike has brought forth and found to be true.

 

I think there is a crossover point with the rpms, vehicle speed, fueling, timing, and oil temp that trucks with 4.10s on smaller tires (sub 34") cant hit without changing things around.  

 

If you plan on going to larger tires ever, start at 4.10s.

 

The calculator we like to use is from tiresize.com - https://tiresize.com/gear-ratio-calculator/

 

Warning for using this calculator: you need to know the factory size tires to use this right I believe. I know one size is 265/75r16, I'm not sure of the rest.

 

35s with 4.10s will give a 3.71 effective ratio

37s with 4.10s will give a 3.51 effective ratio.

 

34s give a 3.82

33s are 3.92

 

So for the person who wants to run a 33 inch tire would need 3.73s to not have to put pizza cutters on like Mikes truck, (no offense).

 

So there are two drawbacks with some of the stuff above. Re-gearing to 3.73s is expensive, and putting 35s on is going to raise the truck.

 

This increases frontal area and drag plus 35s are getting heavy. 70+lbs for just the tire. Also depending on the rims you will rub with 35s and possibly at a minimum have to level the truck, further increasing frontal area and drag.

 

 

[Mopar1973Man]

There is more to it even rotational mass. Yeah you might call them pizza cutters but less rotational mass is easier to accelerate.

 

Most oversized tires are aggressive tread patterns which have the highest rolling resistance compared to A/T tire like I'm running.

 

I'm setup to be right in the middle of my torque curve at 65 MPH which produces the least amount of heat and fuel requirement.

 

Truthful I'm looking for a wider 30 inch tire so I can keep my required height but get a bit of width for summer use but run the narrow for winter time.

Edited October 29, 2021 by Mopar1973Man

[Silverwolf2691]

Tiresize.com has a similar size area for tires might look there for some direction

 

https://tiresize.com/tiresizes/245-75R16.htm go to alternate sizes and see what you like. 

 

215/85r16 would be real pizza cutters for winter lol. Almost the same size but around an inch narrower.

Edited October 29, 2021 by Silverwolf2691

[Mopar1973Man]

Little Red had the 215/85 R16s and did really good in a foot of snow. Even gave small boost in MPG being they were slightly lighter and also a 30 inch tire. Even on a V8 5.9L gasser it works really good.

[Doubletrouble]

 Do the smaller tires break loose a lot easier in winter when you don't want the to? Just curious, with the less mass and the torque of these trucks it seems like you could spin out pretty easy.

 Mine is a dually and I notice by having the rear weight spread over 4 tires rather than 2 creates a traction issue sometimes in slick conditions.

[Mopar1973Man]

1 hour ago, Doubletrouble said:

 Do the smaller tires break loose a lot easier in winter when you don't want the to?

No. Being ive got more pounds per square inch so it push down to the asphalt quite well. The dually and wide tire truck tend to spin easy because of flotation. They have less pounds per square inch of tire face.

 

Another secret I up shift (manual) early keeping the torque low where a auto wants to run higher RPMs making it easier to spin.

 

Kind of like walking into deep snow your foot will go to the bottom, now throw on snowshoes your spreading your weight allowing you to walk on top of the snow. Dually and wide tires are like running snowshoes and floating on top.

Edited October 30, 2021 by Mopar1973Man

[Doubletrouble]

 I short shift as well. Usually around 2000 rpm when just daily driving with the truck empty. 

 I have seen guys in the past remove one of the rear wheels on each side for better winter traction but it looks ridiculous.  

[Silverwolf2691]

Set of 255/85r17 would help with the dually if you have 4.10s and 17s.

 

3.73 gears allow the usual 33"/275/285 sized tires to be run.

 

Looks like im getting a set of 35s for my truck. My birthday was on the 22nd and my parents are getting them for me as a gift.

 

@Mopar1973Man, whats the rotational weight equivalent per pound that you found? That was factored in against supply, tire weight, and size.. 

[Mopar1973Man]

Rule of thumb... For every 1 pound of rotational mass removed is like 8 pounds off the frame.