With 3.42s I could feel the 31s. Switched to 3.73 and it feels better. To me it's not worth the investment for taller gears than that, 3.73s are readily available in Astros and Safaris already.

I did tow a 3500 lb trailer with the 31s and 3.42s, was a real dog doing that.

 

I'm only on 29's IIRC ( LT 235/75R 15), and running the 3.73 ratio, and a Torsen style diff. I could feel a slight change going from the stock 215/75 to the 29's, but I figured some part of it was also due to the much more aggressive tread.
I think the 3.73 ratio is pretty much ideal for most needs, until you go really big and tall lift, cut fenders, etc.
Unless somebody handed me a complete free rear end with a 3.90 and a limited slip, I would not bother to change it out.
Rod J

 

With 3.42s I could feel the 31s. Switched to 3.73 and it feels better. To me it's not worth the investment for taller gears than that, 3.73s are readily available in Astros and Safaris already.

I did tow a 3500 lb trailer with the 31s and 3.42s, was a real dog doing that.

I'm only on 29's IIRC ( LT 235/75R 15), and running the 3.73 ratio, and a Torsen style diff. I could feel a slight change going from the stock 215/75 to the 29's, but I figured some part of it was also due to the much more aggressive tread.
I think the 3.73 ratio is pretty much ideal for most needs, until you go really big and tall lift, cut fenders, etc.
Unless somebody handed me a complete free rear end with a 3.90 and a limited slip, I would not bother to change it out.
Rod J

probably will end up running 3.73 because I’ll have them but wouldn’t mind getting a little more low end. We'll see.

 

3.73 is fine with 31" tires there are charts and calculators online if needed, but lower gears than that aren't needed at 31"

I've got 245/75r16 with factory 3.73 it works well

 

yeah the one I used put me a at 3.90 but haven’t seen anyone on this site actually use them. Figured 3.73 would be just fine but I figured there’s a ton of knowledgeable people here why not ask. Thank you

 

I do not see an RPO code listed for a 3.90 ratio; only 3.73 (GT4), and the 4.10 (GT5)
Maybe there is an aftermarket R&P for that ratio in the correct size
Rod J
Issaquah, WA

 

I do not see an RPO code listed for a 3.90 ratio; only 3.73 (GT4), and the 4.10 (GT5)
Maybe there is an aftermarket R&P for that ratio in the correct size
Rod J
Issaquah, WA

The ones I was looking at were aftermarket

 

MMusicman had posted this previously to know the RPM's to the usable power band is related to tire size.

* * AMAZING "TIRE>GEAR>RPM>SPEED" CALCULATOR! * *

This has got to be hand's down the absolute BEST calculator anywhere on the web!

Completely interactive!
Change one number and they all change interactively immediately.
Compare A>B too!
Also fills in the unknown numbers for you with transmission, transfer case, tire size, etc

STOCK ASTRO TRANS: (both have same overdrive)

• 85-92 - 700R4 trans (0.70 overdrive)
• 93-05 - 4L60E trans (0.70 overdrive)

TIRES (speed differences negligible):

• 85-02 - 215/75-15 (27.7")
• 03-05 - 215/70-16 (27.8")

TRANSFER CASE:

• Stock AWD not listed (1:1 ratio)
• NP231/NP233 (2.72:1 ratio)

(this is only needed if you need low-range calculations)

---------------------------------------------------

QUICK REFERENCE: (here's a few gear/tire=rpm statistics @70mph)

• 3.42 - 27.7" = 2033
• 3.73 - 27.7" = 2217
• 3.42 - 30.0" = 1877 (below power band)
• 3.73 - 30.0" = 2047
• 4.10 - 30.0" = 2250

Note: each tire/gear change is good for about 10%

Here's the INCREDIBLE calculator:
http://www.grimm

 

omg that is awesome. Really allows you to get a feel for any changes you want to make and with all the trans and tcase ratios in there already. Hope someone pinned this in the drivetrain sub

 

have axles with 3.73s. Thinking I should step up to 3.90

For offroading u need transfer case with low range
Without low range no matter .73 or .90 wasn't enough to go back in deep sand or thick mud
At highway .73 will be more effective

I always look to bigger brother -tahoe
Their stock tires 31.5 and for roads they have 3.42 and with offroad package 3.73

 

For offroading u need transfer case with low range
Without low range no matter .73 or .90 wasn't enough to go back in deep sand or thick mud
At highway .73 will be more effective

I always look to bigger brother -tahoe
Their stock tires 31.5 and for roads they have 3.42 and with offroad package 3.73

got a 233 for the low range. Just Want to try to maintain my mpgs as well as driving characteristics as much as possible

 

ya run the 3.73, buying new 3.90 probably not worth the expense, 3.73 are good middle ground for all uses at tire size you're looking at. Run skinnier rather than wider tires of similar diameter if you can, an make sure you are aired up good

 

ya run the 3.73, buying new 3.90 probably not worth the expense

One day I calculated how much time(mileage) I will be needed if I swap transfer and front diff to make my Astro true 2wd at hwy and lower consumption (I have true AWD -not on-demand)
The result was - I need 5 years of driving
And it was 9 years ago
Same shit with my home -After buying it I wanted to insulated it better -but it was expensive option
Now I have kinda big spends on heating
So in one hand -it looks better for paying much and saves after
But in another -it easy and less stressful to pay little more but in prolonged period

 

This thread may have got a little off topic on mpg's (which doesn't appear to be what the OP is asking). But regardless.. mpg's are part of the equation too... even if you're will to accept less.

When I got my 2000 (over 10 years ago), it had 8 lift, 31.5" AT tires, and a stock 3.42 gear.
It was a TOTAL DOG on the highway! Around town too.
Also lacked power miserably when towing anything as well.
The AWD (single high-range) was practically useless to me, and failed me pulling off-road on several occasions.

No power, no pep, very slow acceleration... just terribly weak. ALL THE TIME.
Driving a different stock van felt like a sports car compared to my weak van.
Worse yet.. with even the slightest incline, my van would constantly need to downshift to 3rd.
My van struggled terribly to maintain highway speeds with even the slightest headwind too.
While cruising, I always had the throttle pushed hard just before the point of downshifting.
There were times (with headwinds) I literally could NOT maintain upper highway speeds.

Beside lack of power (with mine), this also badly sucked fuel and resulted in some pretty poor fuel mileage. Sometimes on the highway in a headwind (with A/C on), beside being unable to maintain highway speed, my mileage would suffer as low as 9-10 mpg. No power, no speed, no fuel economy. Everything was HORRIBLE!!

Believe it or not, I solved some of my lack of power troubles by simply downsizing 1 inch from a 31" AT tire to a 30' HT (highway touring) tire. This small difference made a HUGE world of difference for me! A slightly smaller tire equaled slightly more torque (just enough to make a difference), and the "highway touring" tires have far less rolling resistance than the AT. My new tires are still a rugged LT tire... but beside exceptional improvement on the highway, it pulls just fine off-road too. These two slight improvements combined made a HUGE difference for me. Better power, now able maintain upper highway speeds, and much better fuel economy (suddenly went from 10-12 to 13-15mpg). I've even seen 16-18 now (under right conditions).. which was previously IMPOSSIBLE. It was the previous combination of all the various EXTRA loads adding up to more than the engine could handle.. and now I'm just below that threshold.

I calculated that I need somewhere between 3.73-4.11 (about 3.90) gear to compensate for tire size. Even with this taller gear, it would still only feel as good as a 3.42. The stock 3.42 IS a good ratio for light typical use (stock tires stock van).. but if you tow (or simply want more pep).. then more gear is the solution. Taller tires require this compensation adjustment just to bring you back to where you should be.

You NEVER want less gear (lower rpms).. which is what taller tires do for you.
This forces the engine to run too low of rpms BELOW it's optimal sweet spot range (for power and economy).

So is changing to a 3.73 gear "worth it"? (or even a 3.90)
That question really depends on what you consider "worth it".

Do you have lift and taller tires?
Do you push driving over 65?
Do you drive hilly roads? Mountains?
Do you drive a lot?
Do you ever tow?
Do you appreciate pep and improved performance?
Can you afford to do the upgrade? (or able to do it yourself)?

If you have an AWD.. then that DOUBLES the cost and labor!
With AWD... you have to match and do BOTH front and rear.
So far.. this has been the deal breaker for me.

I bought a 3.73 G80 locker for mine (years ago).. but haven't installed it yet because I need the front to match. Either I have re-gear it (huge amount of work).. or find a matching 3.73 front differential, buy, and install. They don't exist in Florida, so that affects my search. I've even considered just changing the rear only (I'm a "rule breaker") and temporarily living with mismatched gears (this can be done with a converted 4x4, but would not work with AWD). For now... I just live with what I got, since it's not as bad as it once was.

The 3.73 vs 3.90 won't really be much noticeable.. and if you already have 3.73's, then I'd go with that. It's a common gear.. perfect for any Astro, and perfect if you're running taller tires.

 

I bought a 3.73 G80 locker for mine (years ago).. but haven't installed it yet because I need the front to match. Either I have re-gear it (huge amount of work).. or find a matching 3.73 front differential, buy, and install. They don't exist in Florida, so that affects my search.

I was facing the same problem here in southern California, I ended up buying a 3.73 front diff from an S10 on eBay. It was sold by a auto wrecking business in the midwest. Delivered to my door for less than $150. I just swapped the stub shafts and all associated stuff to convert from "selectable 4x4" to the Astro/Safari/Bravada AWD setup. Just food for thought.

 

I know one Astro with 29 and 4.10
I can't say that it accelerates much better than mine with 29 and 3.73 (but it heavier than mine cuz its conversion and mine stock passenger )
But his AT kicks harder at upshift due to overall reduction ratio
So its better to test (and reprogram ur TCM to different tires and ratio)