3.08 vs 4.10

[DTrain]

Im about to swap my axles on my truck from 3.08 gearing to 4.10. I really have no idea what to expect. So I'm wondering about gas mileage, power, top speed, etc. If anyone can shed some light on the differences between the two, I would greatly appreciate it.

[ehjorten]

Your RPMs at any given speed are going to jump up around 33%.  So if your cruising RPM was 1,800 it will be about 2,400 RPM.  It is really hard to say what your gas mileage will do without knowing your combo, but it will go down.  Power will not change, but torque will go up the same 33%.  Your top speed will go down accordingly.  If we knew what your redline was, transmission you were running, then we could tell you more accurate numbers.

You should search online for RPM and Speed calculators as there are lots of them out there.

[frotosride]

From experience it is possible that your milage can stay close to the same or even increase a little if you do a lot of highway driving. Milage is primarly based on load on the engine and obviousely your engine's design and specs bit keeping your foot out of it in long trips seem to help milage a lot on long trips because the engine can turn the tires a little easier, despite the slightly higher rpm's. A lot of people disagree with this soley based on engine rpm alone but it's not impossible.

[DTrain]

My truck has 350 4barrel and a sm465.

[zieg85]

If you do a lot of highway driving your engine is going to be screaming at 70.  Your gas mileage will go down as well.  If you tow a lot with it and do mostly in town stuff, you will probably be about the same but your clutch will like it better...

[Lt.Del]

i concur with mileage may get better for town driving because you dont have the mash the accelerator constantly...you will get better off line acceleration.
Interstate driving?  your engine will be taching considerably higher.  Depending on your cam and such, you will be able to accelerate very well to pass someone below 70mph or there abouts. She'll be taching high above that.
tire/wheel size is important to know as well.  with 4.10 you can put bigger wheels on it to compensate for the lower gears.

[Irish_Alley]

i have a sm465 with 4.10 and 38s. at 65 i hit above 2k

[Stewart G Griffin]

What size tires you got?

Here's a chart that tells how much power you can increase with a change:

http://www.wallaceracing.com/reargear.htm

i personally wouldn't do it for the street, but it depends what you are going to use if for?

[gto109]

In the past I had a ford that had 4.11 gears the engine was 300 hp 351with a c6.  great work truck. I think the tires where 285/75/16s.  at 65 it would get about 12 mpg.  In town driving it would get about 10th mpg.  on the highway at 65 I was around 3500 rpms.  but on the highway at 65 with a car trailer pulling my 67 le mans and a ton of fertilizer the rpms was around 4k at the same speed. 

[Irish_Alley]

slipping? but about my mpgs if i keep it under 60 i can get in the high 9s. if i go above 60 it go down to 7. i wana say at 65 im hitting 2500. the sm465 is a good trans for hauling but if you want better millage then go with a 700 or something with od

[ehjorten]

So not knowing what size tire you have...if I assume you have a stockish tire at about 31" diameter, and your 350 is topped-out at about 5,500 RPM (again...not knowing your engine specifics); then first gear is going to get you to a max of about 19 MPH.  It won't be good for anything but starting on a steep incline, getting going with a real heavy load, or crawling.  4th gear will be topped-out at around 120 MPH and a 60 MPH cruise speed will get you around 2,650 RPM.

Irish with his 38's should hit around 2,200 RPM at the 60 MPH cruise speed.

In case you don't know, your SM465 has the following gears: 6.55/3.58/1.70/1.00/6.09R

I've got 3.73s in my '77 with the 350 and 31" diameter tires and I cruise at 60 around 2,400 RPM.  It gets old on the highway, especially around here where the speed limit is 70 MPH, but cruising around town and up in the woods it is perfect.  I average about 10 MPG around town in it.

[Irish_Alley]

i have a sm465 with 4.10 and 38s. at 65 i hit above 2k

this is why i put this up before. its been awaile since i been going 60 in my 79 but its about 2300 going 65
i only use l for taking off up a hill with a load. other than that i never use it and if you have 3.73 then you could use L and still have 3rd as more of a poor mans od

[heavychevy91]

I've got 3.73s in my '77 with the 350 and 31" diameter tires and I cruise at 60 around 2,400 RPM.  It gets old on the highway, especially around here where the speed limit is 70 MPH, but cruising around town and up in the woods it is perfect.  I average about 10 MPG around town in it.

I have 4.11's in my K20 with 33's and feel the same way. Trans swap is in my future...

[roadrage41]

Try highway cruising at 2400 when your Turbo is all in under any kind of a load at that rpm. Took me along time to get to where I could pass people without either burying the speedometer  instantly or breaking the tires loose. The drone of the motor gets annoying after 30 miles or so, really gotta get overdrive.

[ehjorten]

This is why I love my '91 Crew Cab!!!  I've got OD and 4.10 rear gears.  My 60 MPH cruise RPMs are actually to low at about 1,650.  Thinking of going to 4.56s and then my 60 MPH cruise will still be around 1,800 RPM.  Right now though cruising on our highways at 75 MPH is a meager 2,100.