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838 Views 8 Replies 5 Participants Last post by  JoezGMC

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")
  • 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:
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1 - 9 of 9 Posts
Would you know what is the power band range on these vans.
Not sure where to look it up.
I think I saw it in one of the tutorial links but can not remember where.
I find that I need to change something? as at speeds around 40mph, I come up
to a hill and I need to gas it a bit to much to maintain speed? recently went from
28" tires to 30". I have looked into a unit such as this: Chevrolet Astro Van Performance Chip and asked the seller
if possibly a Silverado version w/ 4.3L V6 would work? as they have similar size tires.
no feedback yet? Otherwise I may need to change out Gears... really trying to avoid that!
as I would assume both front and rear would have to be changed?
Where do I find my current gear ratio? Any Help you can offer is greatly appreciated!
Oh, Good Tire Size Calculator: Tire Size Calculator
If you have the original build sticker (RPO codes) on the passenger door. look for either a GT4 (3.73) or a GU6 (3.42)
Rod J
Issaquah, WA
I see a GT4 in the fourth column, So 3.73 I presume. that could be a relief.
has anybody tried any of the so called "performance chips"? they supposedly
change shifting points and save fuel as well?
I’ve got the 3.73 ratio and when I changed the tire size in the pcm using hptuners to correct the speedo it changed the shift points and I noticed a big difference after.
Performance chips usually don’t work as advertised.
Sounds like I need to do more research on this?
any links or suggestions would be great...Thank You!
has anybody tried any of the so called "performance chips"? they supposedly
change shifting points and save fuel as well?
Shifting points are determined by how hard you push the throttle.
A performance chip is not going to improve your fuel economy.
I have read thru many posts, It seems I need my PCM to
be Reprogrammed for the Larger tires/Speedometer/shift points
& I also have the Catalytic converter punched out.
I do have a current Inspection sticker, w/ cat punched.
I have a Laptop and I am tech savvy. I noticed many posts by
"Godmachine81" but they seem to have stopped long ago....
Does anyone do or have access to Hptuners stuff?
1 - 9 of 9 Posts
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