4.10 axle ratio

aggiecreedman · April 4, 2003

Im thinking of swapping my 3.42 gears for 4.10 gears because of my larger tire size. I went from 245/75/r16 to my current. I was curious how it would affect my gas mileage and performance. My truck right now is not in the power band and also has no low end. Would it be a good investment?

americanmuscleman · April 4, 2003

Yes, I think you would see a nice gain from installing 4.10's. You probably know, but by installing larger tires, you've theoretically raised (numberically lowered) your axle ratio. By adding 4.10's, you'll probably get your 4.8L where it makes the best power. Your likely going to loose a little gas mileage in town, and your going to turn a few more rpm on the highway (which can be good if your truck is having to downshift quite a bit).

Assuming your truck is a 2WD, changing axle gears won't be too expensive (maybe $250 for gears, gaskets, and seals), and another $150-300 for the installation assuming your not doing it yourself. I would recommend having a pro setup your gears if you don't know how to do it yourself. If the gears are setup incorrectly, you can get a bad wear pattern, and get gear whine. I would recommend using GM Motive gears. They run a lot quieter than a lot of other gears, and there plenty strong.

Assuming your truck is a 4WD, its going to get expensive to change gears. You'll have to change your front diff gears (which are expensive) and installation is really expensive (unless you know how to do it yourself). Your probably looking at $800-1200 for changing gears in a 4WD including installation.

Your HPP3+ will let you adjust your computer for the new gears, which is nice.

:thumb:

aggiecreedman · April 4, 2003

I appreciate your help. My truck is two wheel drive. Also, What about highway mileage? If the truck is in the powerband, will it get better mileage? :thumb:

wonderl33t · April 4, 2003

Yes I am curious about this also. I would like better highway mileage and more power.

americanmuscleman · April 5, 2003

Aggiecreedman, your probably going to get a little worse mileage. Thats mainly because your going to turn more rpm on the highway at times when you don't need it (like running on flat ground). Say your truck runs 1800rpm at 65mph right now with 3.42's. With 4.10's your truck might run 2100rpm at 65mph. On flat ground, your truck won't need to spin higher than 1800rpm to maintain 65mph, but by adding 4.10 gears your engine will now have to run 2100rpm at 65mph. Your engine is consuming more fuel at 2100rpm than at 1800rpm, because it takes more energy (fuel) to spin an engine faster, even though both engines are still doing the same amount of work, pushing your truck at 65mph.

Where the gears could actually help you get better mileage is if your in a hilly area or do a lot of towing and your truck downshifts a lot out of 4th gear back to 3rd. Your truck might make enough power in 4th gear at 2100rpm to climb a hill, but it may not make enough power at 1800rpm in 4th gear to climb that same hill and has to downshift (resulting in say a downshift to 3rd gear (about 2800rpm) while you climb the hill... assuming your driving with cruise control on). Around town, you really shouldn't see a huge change in mileage. I can explain more about that if you want to know why that is.

Another thing I didn't talk about is engine efficiency, which plays a significant role in gas mileage. For the sake of not getting too boring, I tried to keep my explanation simple and not go into too much detail. Hopefully it makes sense. If anyone wants a more specific explanation than that or for me to talk about engine efficiency, I'd be happy to explain it in more detail.

aggiecreedman · April 5, 2003

Because of my larger tires I turn about 1550 rpms on the highway, it is very very low and I calculated that i would be running about 1950 rpm with 4.10 gears with my 305 tires.

SilveradoWV · April 5, 2003

Stupid Question time! Here goes. I would think GM just sticks in 3.73 gears as one is going down the assembly (that is if 3.73 is specifed) but do they compensate for tire size? For example my 2002 4x4 4.8L came with 3.73s and 245/75/16s, as you know the same "exact" truck can come with 265/75/16s. So do both trucks actually have 3.73s. If so GM must use different gears if they see that tire size is one or the other; or is one not really 3.73s after you factor tire size? You get my question? I figure when the RPO lists 3.73s they just have 3.73s on a shelf. So is it 3.73 on a 265/75/16 or a 245/75/16??? ???

americanmuscleman · April 6, 2003

Here's a little auto math.

305/75/16 tires are 34.01" in diameter

265/75/16 tires are 31.65"

245/75/16 tires are 30.47"

In 4th gear going down the highway, here's what your engine rpm vs. mph will look like, with each gear ratio. The RPM runs on the left, what gear your in is horizontally (1st, 2nd, 3rd, 4th. A 4L60-E transmission has 4 forward gears). The numbers in the middle are what speed your moving at (in MPH) at each RPM and in each gear.

3.42 gears, 305/75/16 tires:

Tire Diameter 1: 34.01 inches

Rear Gear Ratio 1: 3.42:1

MPH

RPM 1st 2nd 3rd 4th

1000 10 18 30 42

1500 15 27 44 63

2000 19 36 59 85

2500 24 45 74 106

3000 29 54 89 127

3500 34 64 104 148

4000 39 73 118 169

4500 44 82 133 190

5000 48 91 148 211

5500 53 100 163 232

6000 58 109 178 254

6500 63 118 192 275

7000 68 127 207 296

7500 73 136 222 317

8000 77 145 237 338

4.10 gears, 305/75/16 tires:

Tire Diameter 1: 34.01 inches

Rear Gear Ratio 1: 4.10:1

MPH

RPM 1st 2nd 3rd 4th

1000 8 15 25 35

1500 12 23 37 53

2000 16 30 49 71

2500 20 38 62 88

3000 24 45 74 106

3500 28 53 86 123

4000 32 61 99 141

4500 36 68 111 159

5000 40 76 123 176

5500 44 83 136 194

6000 48 91 148 212

6500 52 98 160 229

7000 56 106 173 247

7500 60 114 185 264

8000 65 121 197 282

So Aggiecreedman, on the highway going 70mph with 3.42 gears, your engine is running about 1600rpm (estimating off the chart). With 4.10 gears, your engine would be running about 2000rpm.

2000rpm is a whole lot better engine speed for highway driving, especially with larger tires.

americanmuscleman · April 6, 2003

For example my 2002 4x4 4.8L came with 3.73s and 245/75/16s, as you know the same "exact" truck can come with 265/75/16s. So do both trucks actually have 3.73s?

Yes. The axle ratio doesn't change. What is meant by a trucks gear ratio, is its the number of times the driveshaft spins (full 360* rotation) for every 1 time the tires make a full 360* rotation. 3.73 gears are technically written as 3.73:1. The driveshaft will spin 3.73 times for every 1 turn of the tires. So buy installing 4.10 gears, your driveshaft (and the engine) spin 4.10 times for every 1 turn of the tires.

Lower gears (numberically higher ratio, like 4.10's) will have your engine running at a higher rpm on the highway, and give you better towing power in town and on the highway. GM usually offers them as an option on their trucks. If you don't order a truck with a towing package, usually you will get 3.08 or 3.42 gears for better gas mileage (since the engine will turn less rpm on the highway). You usually can get 3.73 gears, and sometimes 4.10 gears. Most 4x4's come with 3.42's or 3.73's, and 4.10's are an option. GM usually selects which gears to install in your truck based on what the truck appears to be used for. If it isn't being equipped with a trailer tow package or four wheel drive, usually they put higher gears (numberically lower) for better gas mileage.

If so GM must use different gears if they see that tire size is one or the other; or is one not really 3.73s after you factor tire size?

You've got the right idea. The taller tires you have, the more they lower your overall drive ratio. The axle ratio never changes in a truck, but by using different tires, you effect the overall ratio.

As an example, with 3.73 gears and 245/75/16 tires, you would turn roughly 2000rpm at 69mph. With 265/75/16 tires and 3.73 gears, you would turn 72mph at 2000rpm. So the larger diameter tire (the 265) is lowering the overall drive ratio (allowing the engine to run less rpm than the shorter tire).

Hope that makes sense