I have a 1989 k5 that has a 342 axle ratio and was wondering if going to a 373 would be better. Truck is all stock with tbi 350 700/r4 265 75 16 tires it is very sluggish of the line and on the interstate in overdrive shift to drive and I can cruise 85 no problem while towing rpm runs about 2000 in overdrive 3000 in drive it shifts in and out of overdrive at hwy speed going up hills not towing but it pulls high teens for milage on interstate in overdrive drop to drive and it sucks fuel. Also wanted to know what was standard equipment for gear ratios and what is special order. Why would their be such low gear ratio on a truck that only has what 150hp.
CK5
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1989 v10 blazer axle ratio swap
- Thread starter Jamcc85
- Start date
3.42 was a common ratio (Standard? You'd hafta ask Martin). Not the most torquey gears out there, but alright for cruising.
Whether swapping them out would be beneficial depends on your intended usage, transmission, and tire size.
Whether swapping them out would be beneficial depends on your intended usage, transmission, and tire size.
My intended use is all highway and I tow my boat with it. My goal is to be able to tow in overdrive as long as the trans is not constantly shifting. I do not do much off road but I was thinking that all around going to a 373 ratio might better everything but economy. Might hurt that a little iam getting about 18 mpg at 80 mph and my tans is a 700r4 with a 265 75 16 tire I think my factory 342 ratio would better suit a sm465 trans better.
3.73:1 ratio would help. I have an 87 version of your truck in K5 body. Towing is ok on flat ground. At 7000 feet in elevation, it is anemic. The lower gears would be nice.
With a 700 I think I'd go right past 3.73 to 4.10's. Probably not going to find an 8.5" rear with 4.10's, but my opinion is that the difference between 3.42 and 3.73 is not that much, especially considering overdrive.
3.42 to 3.73 is about an 8% difference in ratio, 4.10 is about 17%. Worst case you don't let the engine scream and slow down a bit, which it seems most are loathe to do. If the trans is "hunting" between 3rd and OD all the time, you need to slow down (or speed up I suppose) to keep that from happening. The load on the truck is too great (aerodynamic drag, which is exponentially worse as you speed up) and that hunting for gears will eventually burn the trans up if it's not cooled very well. Drag is the enemy of these trucks, going from 60-65MPH is not going to require ~10% more power, it's going to need more, and it's going to need more fuel to do it.
Your present setup should run 1760RPM at 70MPH (in OD).
3.73 is 1919RPM.
4.10 is 2109RPM.
If you go to 3rd (assuming towing, decent grade, etc) 4.10 is 3014RPM, 3.73 is 2742, and 3.42 is 2500RPM.
IMO none of those with the 4.10's would be a problem. It's going to use more fuel, there is no way around that. But 2100 at 70MPH is not a bad RPM.
I don't tow much weight, but I can imagine the weight of the trailer, trailer drag, and terrain are going to be your big factors, and if you have a large/heavy trailer, and/or hills, 4.10's are going to provide much better power.
By the 80's GM had drastically reduced the options on the light duty (1/2 ton particularly) trucks, and had only two transmissions (manual and auto) and I believe the gear ratios were 3.42 and 3.73 for 1989. I just checked the brochure for 1989, and they don't specify axle ratio. 3.73 seems to be most common with the TH700.
I've never felt a properly running 1987-91 R/V series 350/3.73 truck was "sluggish" when unloaded, if the driver pushed the pedal to the floor and left it there. They aren't going to win a drag race against a Corvette, but they are infinitely more powerful than the previous 305/3.08 trucks. Just going to say that 85MPH is probably not a good idea for various reasons.
I think you would be disappointed going from 3.42 to 3.73 as you wind up spending a bunch of money for very little difference. Years ago I went from 3.73 to 4.10 (basically the same difference as 3.42 to 3.73) in my K5 and was so disappointed the first time I drove it. Honestly could not tell a difference. Since that time I have driven a lot of newer trucks that were identical besides differences in the factory option gear ratios. Cover up the tach and it's really hard to feel the difference by the seat of the pants. If you have 3.42 gears now I would suggest at least going to 4.10. For comparison there is a 30% difference between D and OD in the transmission, where as 3.42 to 4.10 gears is about a 20% difference.
^^^^
What he said
What he said
Not IMO with fuel injection.
Coupled with a tune of the PROM, having the injectors flow tested and/or replaced, and checking fuel pressure, I'd say go for it. Bigger cam is going to make less power down low, nature of the beast.
Coupled with a tune of the PROM, having the injectors flow tested and/or replaced, and checking fuel pressure, I'd say go for it. Bigger cam is going to make less power down low, nature of the beast.
They are marketing, don't buy into it. You won't make more low end torque by going to a larger cam. You will be hard pressed (if even possible) to find a dyno chart for a more aggressive cam, showing more torque starting at 2000RPM, than there is with a stock cam. Everything is a compromise, if you need torque at a specific RPM range, you buy a cam that provides that in your truck, but that will be based on what RPM you need the power.
Computer friendly is a lie. These setups cannot adjust for fueling under heavy throttle, regardless of what the sellers say. The fact that they claim they don't need re-tuning means they don't make much difference. Some of those I believe even have CARB #'s, which is another indicator they don't do much.
rayfin
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Just go 4.11. You will love them. You have overdrive... There are gear ratio vs MPH calculators on the web where you can punch in the numbers and compare.
http://www.wallaceracing.com/gear-speed.php
http://www.wallaceracing.com/gear-speed.php
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Can Can
Pusher Man
If one of your ultimate goals is to tow in OD, I'd highly recommend you install an aftermarket tranny cooler and a tranny temp gauge. 700s don't like heat- ask me how I know.
Don't tow in overdrive. It's more than just heat involved, there is a ton of stress on the overdrive parts in the 700R-4. It will fail if you tow in OD. Don't tow in overdrive, ever!
That being said, keep your 3.42's. Towing in Drive at 70 mph has the rpm's about 2700...which is fine. Don't tow in overdrive!!
Go buy the largest stacked plate cooler you can find. Don't tow in overdrive!!
@Greg Ducato
That being said, keep your 3.42's. Towing in Drive at 70 mph has the rpm's about 2700...which is fine. Don't tow in overdrive!!
Go buy the largest stacked plate cooler you can find. Don't tow in overdrive!!
@Greg Ducato
I don't know if anyone told you this yet, but towing in O/D is a no-no.
rampage
3/4 ton status
I'll be the first to recommend to tow in OD …
Only Drive
Only Drive
Can Can
Pusher Man
lol @tRustyK5. I was trying the delicate approach seeing the OP is brand new. I didn't want to be the first to tell him that his 700 will be fried after his first towing trip in OD. But since you chose the more direct approach, let me echo your statement - towing in OD with a 700 is akin to signing its death warrant, no matter how many doodads you add to the system to keep it cool.
