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Looking for advice on gear ratios

Timmy2tone

Timmy2tone

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I'm looking for some advice on what gears to run in my '90 Blazer. Stock 350 TBI, 700R4 not sure of the stock factory gears I have now. I have 35" KO2's. She's a little lazy from a stop, shifts around 2000 RPM the torque converter locks up and runs out at around 55-60 mph at 900-1000 RPM. I don't want her screaming going down the interstate at 60 mph. Any advice is appreciated. Thanks

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Good looking K5!

Have you checked the speedometer with a GPS? I am wondering if it ever got corrected for the larger tires.

Out of curiosity, does it still have the option code list in the glovebox? If it does, AND no one has swapped the axles, the axle ratio hopefully is a G code, and could have a G80 next in the list behind the ratio.

With 35s, 4:56 in 1/2 ton axles should be fair. I personally would go to 4:88 in a 14 bolt, semi-float 6 lug axle, but that requires cutting and welding new spring perches on, AND regear after you find the axle.
 
6872xtc said:
Good looking K5!

Have you checked the speedometer with a GPS? I am wondering if it ever got corrected for the larger tires.

Out of curiosity, does it still have the option code list in the glovebox? If it does, AND no one has swapped the axles, the axle ratio hopefully is a G code, and could have a G80 next in the list behind the ratio.

With 35s, 4:56 in 1/2 ton axles should be fair. I personally would go to 4:88 in a 14 bolt, semi-float 6 lug axle, but that requires cutting and welding new spring perches on, AND regear after you find the axle.
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Thx. I do have the option code list in the glove box. What would the G code designate?
 
Timmy2tone said:
Thx. I do have the option code list in the glove box. What would the G code designate?
Click to expand...
It will tell what ratio it left the factory with.

Mine had GT4, which is 3:73. A GU4 is a 3:08. There are others, as well.
 
6872xtc said:
It will tell what ratio it left the factory with.

Mine had GT4, which is 3:73. A GU4 is a 3:08. There are others, as well.
Click to expand...
I thought I had a 3.73 but a friend said he thought it was a 3.42. Do you know what code for the 3.42?
 
Timmy2tone said:
Also the speedometer is reading 5 mph slower than GPS
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At least you know that, which is helpful. I wouldn't worry about correcting it until you decide about the gear ratio change.
BUT! I would consider making it adjustable with DIP switches, either yourself, or get ahold of @sweetk30

Having it read off could possibly make the torque convertor lock-up work improperly if it is still controlled by the ECM, but it kinda sounds like yours may be all hydraulic if you are seeing it at 900 to 1000 rpm at 60 mph.
 
6872xtc said:
At least you know that, which is helpful. I wouldn't worry about correcting it until you decide about the gear ratio change.
BUT! I would consider making it adjustable with DIP switches, either yourself, or get ahold of @sweetk30

Having it read off could possibly make the torque convertor lock-up work improperly if it is still controlled by the ECM, but it kinda sounds like yours may be all hydraulic if you are seeing it at 900 to 1000 rpm at 60 mph.
Click to expand...
I'm going to take her down the road tomorrow just to verify that is what I saw. What is a DIP switch?
 
with 35 at least 4.10, 4.56 will better. 4.56x .70 = 3.19 final gear ratio, 1865 rpm at 60mph. I personally have 700r4 and am running 4.88 on 33's. 3.42 in 4th. I think maybe I should have done 5.33 =3.73 in 4th
 
Timmy2tone said:
I'm going to take her down the road tomorrow just to verify that is what I saw. What is a DIP switch?
Click to expand...
It's a small 2 position switch that solders onto a circuit board.
The factory DRAC module has jumpers soldered on. The switches make it easier to change the digital signal to the ECM and speedometer.
 
Timmy2tone said:
I just went out and checked. GU6
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If it still has 3:42 gears, which it probably does, based on what you have observed, then 4:56 will make it a whole new truck!
I have 4:56 behind a big block with a 700R4, in my '70 K10. When it had 33's, it was more fun than now with the 35's. It wasn't a problem IMO with the smaller tires, due to what the terrain is like around here.
If you have more flat roads and such, 4:56 should be good.
Be prepared for an expensive gear swap for both axles. And the higher driveline speed will probably give you vibrations if the U joint angles are right.
 
Wes Harden said:
with 35 at least 4.10, 4.56 will better. 4.56x .70 = 3.19 final gear ratio, 1865 rpm at 60mph. I personally have 700r4 and am running 4.88 on 33's. 3.42 in 4th. I think maybe I should have done 5.33 =3.73 in 4th
Click to expand...
What would a final gear drive be with a 4.88?
 
6872xtc said:
If it still has 3:42 gears, which it probably does, based on what you have observed, then 4:56 will make it a whole new truck!
I have 4:56 behind a big block with a 700R4, in my '70 K10. When it had 33's, it was more fun than now with the 35's. It wasn't a problem IMO with the smaller tires, due to what the terrain is like around here.
If you have more flat roads and such, 4:56 should be good.
Be prepared for an expensive gear swap for both axles. And the higher driveline speed will probably give you vibrations if the U joint angles are right.
Click to expand...
I'm also considering a Tom Woods Slip Yoke Eliminator with a double cardon drive shaft
 
6872xtc said:
If it still has 3:42 gears, which it probably does, based on what you have observed, then 4:56 will make it a whole new truck!
I have 4:56 behind a big block with a 700R4, in my '70 K10. When it had 33's, it was more fun than now with the 35's. It wasn't a problem IMO with the smaller tires, due to what the terrain is like around here.
If you have more flat roads and such, 4:56 should be good.
Be prepared for an expensive gear swap for both axles. And the higher driveline speed will probably give you vibrations if the U joint angles are right.
Click to expand...
Thanks for the help
 
spicerparts.com

Spicer Engine RPM Calculator

Measure your engine’s RPM based on the ring gear and pinion gear ratio, speed, and tire height, with the Spicer Engine RPM Calculator.
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This handy calculator will tell you what your rpm will be based on tire size and the gear ratio you select.
 
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