Lowest most efficient low load rpm w/ 350

[original condor]

I'm in no hurry usually so I run @ 60 most times, lots on 2 lane roads, plenty of time & shortage of $$$.
I've got 308's and 33"ers, with a mild build 350 & SM465, rolling on flat ground most times. This calcs out to approx 1900rpm, with 35's it drops to 1774. Is this too low, or will it get less thirsty down there?
I'm considering 35's for longer freeway legs if in a hurry, and more clearance under the diffs.......Thankyou...... Steve Rieck

[bmchevy1979]

I would think it would start using more fuel at that point if you want 35s and you like the rpms or how it drives now I would jump up to 3.43 gears they are pretty common in late 80s blazers. I personally run 3.73 with 33s and for what I do they are not low enough 3.90 would be ideal but I will likely go with 4.11 when I jump up to 35s

[RIDE-RED 350r]

I know it's apples and watermelons, but my 06 Silverado with the 5.3 is turning about 1550 at 60 mph. I had an 01 with the same engine but 4.10 gears that turned 1850 at the same speed. I have not noticed a jump in fuel economy since getting the current truck.

My '78 with SM465, 3.73 gears and 35" tires runs right about 2400 at 60 mph. I would be very happy with your 1900 at 60 if I were looking for better compromise of fuel economy and power. But better power lays my hair back in my circumstances so my truck running as it does is OK by me.

I would just be careful of getting your gearing too high though, you can end up lugging it to the point it does suck more fuel and sacrificing alot of giddy-up-go to boot. In other words, you can end up stuffing your foot deeper in the 4bbl to maintain highway speed taking your MPG's the other way.....

[bmchevy1979]

Yeah Im not sure what rpms I'm turning my tach has been getting more and more off but I often tow or drive on winding mountain roads with heavy uphill grades so I don't need top speed my truck rarely sees 50mph when I'm in the mountain it will do 110mph on a good stretch of road but all the vehicles I usually end up racing or ****ing with are buddy's and most of them top out at 80-90mph so I could stand to gear it a little lower it would only make me loose them quicker

[special-K]

All I know is I had a '90 Blazer 350/4spd/3.08s with 33/12.50s and it rolled on down the hiway just great. Never felt the lack of overdrive. We could roll right along effortlessly with interstate traffic. It did fine at 75, although I also prefer 70 or less. The gears felt a bit tall for starting off, but there's always granny when you're starting off on a hill or with a load. I think 33s are as tall as I would go with 3.08s. Without any calculating you will have something like 2.73s which is awful lanky for a truck as it is. With 35s it will be even worse. You'll have Bonneville gears. I guess that works for flatlands ok, but the only gain would be rolling faster because at around 65-70 I think it won't be into it's sweet spot yet.

[original condor]

After looking at the overall picture, I think when I get the correct cam in this thing, that 33's with the 3.08's and 1900 rpm seem pretty sweet for right now. It is already a little high geared for 4 wheelin', but that will be fixed when I put the doubler in eventually as funds permit. Thank you all for your input & help...............Steve

[snj8198]

Hey Steve, install a vacuum gauge in the cab and drive by that. The higher the vacuum signal while cruising the more timing advance the can will offer, providing better economy.

I have a 5 speed in my truck and at 60 I usually get better economy in 4th gear because it allows the motor to stay in it's power band better and maintain a higher vacuum signal. If i grab 5th, i lower my rpm too much which makes more load on the motor, creating less vacuum signal, then the can starts dropping out vacuum timing advance, thus, there goes economy and responsiveness.

[special-K]

A vacuum gauge is a great suggestion. I have one in my '72 also. That tells the true story