4.56 or 4.88 on a TBI blazer with 33's, maybe 35's in the future?

[351FUN]

My truck has the 3.42 gears and with 33's and a basically stock TBI it's a dog. The next mod I want to do is gears, but with it parked next to my F250 on 35's I keep thinking about going with those in the future. The truck is never used for towing, does see some off road use, very little highway (which may change in the future but I'd never cruise much past 70 anyways) and mostly does daily driver duties. I'm trying to decide if I want to go with 4.56 or 4.88, but I know it's a pretty small difference anyways.

 

[1998_K1500_Sub]

My truck has the 3.42 gears and with 33's and a basically stock TBI it's a dog. The next mod I want to do is gears, I'm trying to decide if I want to go with 4.56 or 4.88, but I know it's a pretty small difference anyways.

Pick a rear ratio to provide a cruise engine RPM at 70 MPH in the 1900-2200 range for mileage / comfort, higher (2200-2500) for more performance or if your driving habits warrant.

What’s your RPM now at actual, GPS measured 70MPH? Change the rear proportionately from today’s 3.42.

FYI my father’s stock TBI 5.7 1995 K1500 4L60E had 3.73 gears and ran ~2100-2200 RPM (tach indicated) at 70. That truck drove nicely. Mileage was around 15 mpg as I remember, maybe a tad higher.

BTW, don't trivialize the difference b/t 4.56 and 4.88, that's a 7% change... that's akin to taking 300# off a 4800# vehicle, akin to increasing your engine from a 350 to a 375. You'll feel a 7% difference.

 

[Supercharged111]

I went from a 3.73 to a 4.10. Lots of folks would say it's a waste, but I'm happy with it. For reference, my truck hits 2000 RPM at 58mph IIRC. That's 285/70/17 tires, a 4.10 gear, and a 4L80E overdrive. Your highway RPM will be about 175 less in that range. Are you talking metric 33/35s or 'Murica sizes?

 

[Silverado1994]

One thing you've gotta know about TBI engines and their computers is that they have a 3200 cutoff built in. That's it...that's the whole show and I couldn't believe it when I first found it out. If you're running that steep a gear you're not gonna have a whole lot of usable RPM left. With 4:10's you'll be cruising at around 2000 rpm @ 60mph and as was mentioned by 1998_K1500_Sub 2200, RPM @70 mph. Given these facts....man.....I just don't know about 4.56 gears given the 3200rpm limit.

There are a few ways around the rpm limitation but.....that's another story.

 

[Supercharged111]

One thing you've gotta know about TBI engines and their computers is that they have a 3200 cutoff built in. That's it...that's the whole show and I couldn't believe it when I first found it out. If you're running that steep a gear you're not gonna have a whole lot of usable RPM left. With 4:10's you'll be cruising at around 2000 rpm @ 60mph and as was mentioned by 1998_K1500_Sub 2200, RPM @70 mph. Given these facts....man.....I just don't know about 4.56 gears given the 3200rpm limit.

There are a few ways around the rpm limitation but.....that's another story.

What?

 

[1998_K1500_Sub]

One thing you've gotta know about TBI engines and their computers is that they have a 3200 cutoff built in.

Are you *sure* about that???

I've "tuned" three or four of those 1995 $0D code base ECU p/n 16197427 vehicles (I need to say that $0D was only used 1994-1995 AFAIK, maybe you're taking about a different year) and I don't remember experiencing a cutoff that low, or seeing a configuration for such at that RPM.

 

[Stringer]

4.11

 

[Silverado1994]

What?

Are you *sure* about that???

I've "tuned" three or four of those 1995 $0D code base ECU p/n 16197427 vehicles (I need to say that $0D was only used 1994-1995 AFAIK, maybe you're taking about a different year) and I don't remember experiencing a cutoff that low, or seeing a configuration for such at that RPM.

Welp...after digging some it looks like I've put some misinformation out there

You must be registered for see images attach

and I most certainly stand corrected here. I do, however, remember reading about the 3200 rpm limitation which is with the factory cam. The factory cam is what the hold up is. Specs on Stock TBI cam is .050″ tappet lift (intake/exhaust) is 165/175; and maximum lift with 1.5:1 rocker ratio (intake/exhaust) is .390/.410. So there's still no real usable powerband beyond 3200rpm for the stock cam. So to correct myself its the cam, not the computer that limits a stock LO5 to a usable 3200 rpm. Sorry for the foul up fellas.

 

[Supercharged111]

Welp...after digging some it looks like I've put some misinformation out there

You must be registered for see images attach

and I most certainly stand corrected here. I do, however, remember reading about the 3200 rpm limitation which is with the factory cam. The factory cam is what the hold up is. Specs on Stock TBI cam is .050″ tappet lift (intake/exhaust) is 165/175; and maximum lift with 1.5:1 rocker ratio (intake/exhaust) is .390/.410. So there's still no real usable powerband beyond 3200rpm for the stock cam. So to correct myself its the cam, not the computer that limits a stock LO5 to a usable 3200 rpm. Sorry for the foul up fellas.

WOT upshifts still occur around 4000-4500 RPMs in those trucks.

 

[351FUN]

I went from a 3.73 to a 4.10. Lots of folks would say it's a waste, but I'm happy with it. For reference, my truck hits 2000 RPM at 58mph IIRC. That's 285/70/17 tires, a 4.10 gear, and a 4L80E overdrive. Your highway RPM will be about 175 less in that range. Are you talking metric 33/35s or 'Murica sizes?

Metrics, 285/75R16 is on it now but my 250 is on 315/75R16 and I keep being tempted by them.

One thing you've gotta know about TBI engines and their computers is that they have a 3200 cutoff built in. That's it...that's the whole show and I couldn't believe it when I first found it out. If you're running that steep a gear you're not gonna have a whole lot of usable RPM left. With 4:10's you'll be cruising at around 2000 rpm @ 60mph and as was mentioned by 1998_K1500_Sub 2200, RPM @70 mph. Given these facts....man.....I just don't know about 4.56 gears given the 3200rpm limit.

There are a few ways around the rpm limitation but.....that's another story.

It definitely spins past that. It may not do much but make noise, but I wanna say 4500?

4.11

Lol no

BTW, don't trivialize the difference b/t 4.56 and 4.88, that's a 7% change... that's akin to taking 300# off a 4800# vehicle, akin to increasing your engine from a 350 to a 375. You'll feel a 7% difference.

That is a fair point.