Fuel mileage with 4:56's?

[babemagnet]

I'm nearly done swapping in a set of 3/4ton axles into my '83 K5 Blazer. My 350 chev engine is getting a little on the tired side and I had been toying with the idea of swapping in a 472. Heres my question though- will it suck gas like a sum bitch? I know Caddilac engines can get good mileage cause of the torque but my new axles have 4:56 gears and I'm currently running 36" tires. When plugged into a gear ratio calculator I got around 2800rpm @ 65mph. I'm not sure I'll be getting very good mileage running the big cad in that rpm range. I would be retaining the K5's SM465 4-spd trans. Any thoughts?

Thanks- John

[EZ]

I don't care what brand of engine you put in there as long as you have 4.56 gears you'll not get good gas mileage.

"EZ"

[74 Novallac]

I don't care what brand of engine you put in there as long as you have 4.56 gears you'll not get good gas mileage.

"EZ"

I gotta' second that.

[Scott]

My guess is 8 mpg.

[babemagnet]

I guess I'll have to see how it does with the 350 and go from there . I will eventually put bigger tires on, probly 40-42", and at that point it might make more sense to swap. If I'm only getting 8mpg I'll have to sell it!

-John

[Scott]

I did a google search and found this:

http://www.hardcoresledder.com/forums/l ... 22407.html

Seems you would be lucky to get 8 mpg with a 472.

[Chevlac]

I had a '79 Chevy Silverado 1-ton truck that was powered by a '79 425 Caddy engine & it got the best milage of maybe 8 mpg. And that was with a 4.10 rear end. That's why I stipped it down & swapped the body for a '95 Volvo wagon that needs repair. Any vehicle that has a gear ratio lower than 3.73:1 isn't ever going to be a fuel mixer no matter what it's got under the hood for an engine. You'll have to swap out the rear(s) to something with taller than 3.90 gears if you want something that's a little more gas friendly.

[EZ]

Well you can't really set a limit on the exact gear ratio he needs to get below (numerically) because
he is going to run taller tires than normal. The taller tires effectively chanage the gear ratio. If he goes tall
enough with the tires he wouldn't have to change the gears. But that would be some REALLY tall tires!

"EZ"

[dave brode]

John,

Mileage aside:

The innards are big and they make a much larger swing [stroke], so although 2800 cruise rpm isn't bad for a small engine, for one thing, it will beat the oil into a froth in a caddy.

The following comes from a guy that likes gear. I cringe every time I read guys saying that a caddy doesn't need gear to run the 1/4 mile. It is true that compared to less torque engines, a caddy will tug a tall gear and e.t. will not suffer as much with "less than the right" gear ratio. However, imo, they will still run better if geared to be "done" at the speed traps, just like every other engine. Note that I have 4.30s in my truck [for 1/4 mile], but even with my 21.5 x 33 street rubber, it ain't much fun on the road.

Now that I have ranted about the "they don't need gear" myth [imo], I'll say that for a ride that'll see the highway, a taller gear will be better overall. Imo, if you will run a normal convertor you can re-gear to cruise at 2100-2200 or so, and it'll be much better. You will be able to rip in back into 2nd and go like stink, then go to high gear and really roll. If you had a lock-up, or a 13" switch-pitch, I might say gear for a little less cruise rpm that that, but you do NOT want to cruise lower than the stall speed of the convertor.

Fyi - a buddy of mine replaced his '85 C-10s 305 with a 500 caddy. Stick tranny, with o.d. high and 3.08s. The combo sucked with the 305, but the big caddy liked it just fine. Used less gas than the 305.

3.42 would be what I would look at. That should bring the cruise from 2800 to apx 2100.

Dave

[Z-Dreamer]

This question is a joke, right?

[babemagnet]

This question is a joke, right?

No its not a joke. Its a question.

FWIW I remembered Dave Brode saying that about his buddies 305-500 swap a while back in someone elses post and thats what sparked the question. I too am going to use the stick trans but I remebered Dave saying the truck in question had high gears so with the ratio change being the big "change" between the two trucks I thought I'd ask. My driving is between stopped and 65, 70 means I'm in a damn hurry . I can't do a gear change but once these tires are wore out I'm going for around 40"(I was running 39.5" Boggers on the 1/2tons but they were wore out & not worth swapping to 8 lug).

So maybe once I get the 40's the cad might do better cause the gearing would be higher and the extra torque would pull the big tires around better than the small block....

Thanks guys- John

[dave brode]

This question is a joke, right?

I can't do a gear change

Thanks guys- John

Why not? Or, do you mean that you don't want to do a gear change?

What would cruise rpm be with 40s?

Dave

[babemagnet]

I am on a "find a parts truck and gut it" kinda budget Like when I get those future 40" tires I can guarentee they won't be new hahaha. It's not that I'm cheap or anything, I just have the 3 kids & a wife thing going on and thats the only way I can(or she will let me) build anything, plus, I kinda like "scrounging"for good parts. Through almost strictly budget/traded for used parts I got a 99.5% rust free K5 with 8+" of lift, SM465 4-spd,NP205 t-case, hyd clutch, hydroboost brakes, and 3/4 ton axles. Hell I even got my flowmasters used on Ebay for $40 + shipping lmao

So I'm stuck with the 4:56 gears for the time being (though I do have a dana 70 with a locker sitting beside the garage, it has 3:50 gears...hmmmmm maybe I can find a matching front axle for that.)

Maybe it sounded like a stupid question but I'm not off my rocker I've been around cars & trucks, I wasn't really trying to say "I want a lifted fullsize 5000lb 4x4 with a HUGE engine that gets great gas mileage so I can park my Honda" hahahaha It was more like "I have this lifted fullsize 5000lb 4x4 with a medium sized engine that I want to swap for a bigger engine but can't really see going from 13-14mpg to 5mpg, so what do you guys think..." One thing is I might have been jumping the gun as I haven't got to drive my truck around with the new axles and the 350 yet, I may be getting ALOT worse mileage already.

I plugged in the 40" tires and looks like the rpms go to 2490 @ 65mph, about 310 rpm less. Maybe I'll have to run 44's, that puts it at 2263rpm....but at some point the added bulk of the tires will start to outweigh the "higher" gearing and mileage will start to go down.

Thanks guys- John

[Chevlac]

Hey "Babemagnet", I like your explanation on why you are doing things on the "cheap". I too have other "life" things that get in the way of going all out on my car project. Since i own my own home, I'm the one why has to fund both the money & labor to get things repaired or remodeled. Plus I too have a woman in the house that controls the "purse" so to speak. I had to do some "trickery" just to get the money to buy the '68 Chevy car that I'm (slowly) turning into "Project Impaville". I did try the "big ol' Caddy" in a 1-ton Chevy truck thing & it didn't work out like I had hoped it would. I didn't have trouble driving the truck, it just was way too hard on gas.

[babemagnet]

^^^Yep, one day we'll be able to just throw money at our rigs left & right but for now its all OTC hahahaha I can't wait for the day when a front & rear gear change won't sound so daunting to the pocketbook!

-John

[OldSub]

^^^Yep, one day we'll be able to just throw money at our rigs left & right

Any idea when that day will come for me?

[74 Novallac]

Getting the rpm in the sweet spot is the key to good gas mileage. Changing the gears may cost a lot but will save gas and money in the long run.

[Bigtim44]

I put a 500 in my 94 1 ton gmc,put a 5 speed behind it with 4.10's and regular sized tires,it was getting 9.5 -10mpg . If you put 40's or 42's on there ,I'd bet youll get high teens for mileage. I pulled the caddy motor and stuck a cummins in there,now has a 2005 12 valve cummins 6bt out of a bus,gets 20.2mpg highway.

[83fleetwood507]

My daily driven '87 Silverado Short Box 1/2 ton, 4x2 TBI 305 has the stock 8.5' 3.08:1 rear. The truck can pass another vehicle up a steep grade in OD at 80mph much easier than it could on level ground at 60mph in OD. It always seems to be straining on the street, it really only seems to be comfortable at 55+mph in 3rd gear, & 70+ mph in OD. I'd love to swap out the 3.08's for a set of 3.73's, or at least 3.23's. I wonder if when GM designed it, did they discover that the 3.08's will give me the best overall mpg? It seems to me that the truck would get better mileage with lower gears. I average 12mpg on the street now, which is where 90% of my driving is done. I'd hate to mess with it & wind up getting less than that. The kicker is that I had an '86 Suburban, 1/2 ton 4x2 with the Rochester 4bbl, 3.73:1 posi rear & I religiously got 14-14.5 mpg on the street. The Suburban was at least 1,500lbs heavier than my p/u. Makes me wanna go to the yard & pick up a set of 3.73's. -Steve

[EZ]

If you do let me know. I think I have 3.43 or 3.73 gears in my '85 GMC 1/2 ton PU. I'd love to have a 3.08 for when I put the 500 in there. It's either that or a 700R4. I can't afford 10 MPG in a truck I plan to drive every day.