Another fuel mileage debate...

[propanemudtruck]

Hello guys,

My 90 Chevy is a shortbox regular cab, 350 TBI, 700R4 (lockup not hooked up yet) and 3.42 gears. I've got a front and back plow on it and drive 50-55 mph to work. The only way to really tell is to try this out, but I have a theory yet to be proven.

http://www.fueleconomy.gov/feg/driveHabits.shtml

This link says overdrive always helps because it lowers engine rpms. I agree, in a passenger vehicle. BUT if the engine drops below a certain rpm with so much weight, would it be better for me to leave it in drive instead of overdrive? It seems more responsive and the truck isn't lagging as much.

Any input?

I assume the plows add another 600-900 lbs with all the heavy brackets, pump, valves, hoses and whatnot.

[Josh]

Most engines at higher RPMs while cruising produce more power than is being consumed by the vehicle to move it along. A V8 will almost always get better gas mileage the lower you can keep engine speed, however, that is assuming there is enough torque output at that rpm to maintain vehicle speed. I'd try it both ways for a tank or two and see what works. What you wanna do is try to keep it in it's sweet spot of volumetric efficiency/peak torque.

[propanemudtruck]

Right. I know a stock TBI 350 is no power monster but it runs pretty good. I still don't understand why my TBI blazer had so much more pickup and response with the same gears.

I figured next tank I'll try a run with just drive and see how it goes. W/O any extra weight of plows, the truck usually gets around 15 mpg. W/ the wind wall and weight added on, I figure I'll be about half that. So anything to get it any better will yield higher profit at the end of the season.

[Prerunner1982]

Is is necessary to drive 50-55mph to work?

I was able to get 20mpg out of my Jeep Cherokee just by driving 40mph to and from work instead of the posted 45-50mph. And of course ease in and off the gas as well. It isn't fun to drive mileage conscious.

[Fursphere]

Originally Posted by Prerunner1982

Is is necessary to drive 50-55mph to work?

I was able to get 20mpg out of my Jeep Cherokee just by driving 40mph to and from work instead of the posted 45-50mph. And of course ease in and off the gas as well. It isn't fun to drive mileage conscious.

Here in California you'd get run off the road for driving under the posted limit.

[special-K]

Your truck should be able to pull 1,000# w/o affecting it's power so drastically that it can't run at hiway speed in o/d within a proper rpm range.In my '92 I use 50 mph as the divide between using o/d or straight drive.I'd say I use more fuel running 45 in 4th that 55 in 5th.I'm sure I use less fuel at 50 in 5th than in 4th.

[propanemudtruck]

I drive about 11 miles to work, but I've got many miles of driving in my plow route once snow hits. The weight isn't so bad, but keep in mind I loose all aerodynamics if there was any to begin with a scooped wall in front of the grille.

40 mph isn't possible on a 55 mph road and I keep in mind if someone's behind and I'm under the limit I'll kick it up a bit.

But thanks for all the input

[raycow]

There are two reasons why taller gearing saves fuel. One is the expected saving that results from reduced engine and accessory frictional losses. The other is that engine operation is more efficient at larger throttle openings. You save power on the intake stroke because you are pulling the piston down against less vacuum. I know this sounds trivial, but laboratory tests have shown that the saving is measurable.

Naturally, your fuel delivery system has to be calibrated properly for this to work. If the mixture goes too rich you will lose what you gain. Computer controlled injection in modern engines makes it easier of course, but it can still be made to work even with a carburetor.

Ray

[propanemudtruck]

I read something similar in one of the last few diesel magazines I read, there was a column like 50 reasons a diesel is better than a gasser and one reason is there is no such thing as running lean (it'll just stall) because there is no air flapper or butterfly on the intake just fuel modulation. The gasser has to fight to get air on every stroke and overcome that butterfly (creating vaccum)...

[Prerunner1982]

Originally Posted by propanemudtruck

40 mph isn't possible on a 55 mph road and I keep in mind if someone's behind and I'm under the limit I'll kick it up a bit.

It's called a speed limit not a speed requirement. If they are in so much of a damn hurry, I will give them ample opportunity to go around but until they pay for my gas I will be damned if someone else is going to dictate how I drive.

I get what you are saying though....

[Psycho71]

Originally Posted by Prerunner1982

It's called a speed limit not a speed requirement. If they are in so much of a damn hurry, I will give them ample opportunity to go around but until they pay for my gas I will be damned if someone else is going to dictate how I drive.

I get what you are saying though....

Another reason I don't like going to Oklahoma. And probably why you cuss us Texans.

[raycow]

Originally Posted by propanemudtruck

one reason is there is no such thing as running lean (it'll just stall) because there is no air flapper or butterfly on the intake just fuel modulation. The gasser has to fight to get air on every stroke and overcome that butterfly (creating vaccum)...

You are absolutely right. The working fluid in any internal combustion engine is mostly air. The fuel contributes relatvely little mass or volume. A diesel gets a full charge of air on every intake stroke, while a gas engine gets a full charge only at wide open throttle.

Ray

[Prerunner1982]

Originally Posted by Psycho71

Another reason I don't like going to Oklahoma. And probably why you cuss us Texans.

Yeah.. we are smart enough to allow more than enough time to get to where we are goin'....

[USSkoval]

Two things, the first being very important.

1. Get the lock up working!!! You will overheat and kill the tranny with it disabled. Overdrive also isn't as effective without the lock up.

2. If you are going slow enough that the engine is lugging, it will likely help to drop down a gear. Next time you are driving 55mph in OD, without moving/changing the accelerator, drop down to drive. If the truck starts to go faster, then it is more efficient to run in D.

[propanemudtruck]

1. The truck used to be a 5 speed and someone threw the auto in before I got it. Do I need to get a standalone harness/controller for it or should I look for a computer out of an automatic truck?

2. Thanks for the input. I'll try that tomorrow when I'm cruising around.

[USSkoval]

You can get a simple lock up kit from someone like B&M or TCI. Or you can even wire it up to a toggle switch... you just have to remember to turn it off when you slow down.

[propanemudtruck]

Wonder if there's a relay I could use a push button to engage and hitting the brakes would cancel it. Is that similar to how the aftermarket kits go?

[raycow]

Originally Posted by propanemudtruck

Wonder if there's a relay I could use a push button to engage and hitting the brakes would cancel it. Is that similar to how the aftermarket kits go?

A push button and a latching relay would do the job, but you don't really need that much extra hardware unless you want the lockup to work in all gears (not 1st gear though, because a stock transmission won't allow it - this is prevented hydraulically, not electrically).

All of these transmissions I have looked at already had a 4th gear switch installed from the factory, so you don't really need a kit. Just wire it so the solenoid power goes through the 4th gear switch. That's basically what the kits do. You can still add a manual switch for when you DON'T want the lockup, but there is no need to turn it off when you slow down, because the converter will unlock as soon as the transmission shifts out of 4th gear anyway.

Ray

[USSkoval]

The kits I've seen also have a vacuum switch that only allows lock up during lower engine loads.

[propanemudtruck]

at what amount of vaccum? Do engines measure in inches water column like gas pressure? I work in a technical support team for a water heater manufacturer.

Thanks again to all for the input and ideas

[red71cheyenne]

Originally Posted by raycow

A push button and a latching relay would do the job, but you don't really need that much extra hardware unless you want the lockup to work in all gears (not 1st gear though, because a stock transmission won't allow it - this is prevented hydraulically, not electrically).

All of these transmissions I have looked at already had a 4th gear switch installed from the factory, so you don't really need a kit. Just wire it so the solenoid power goes through the 4th gear switch. That's basically what the kits do. You can still add a manual switch for when you DON'T want the lockup, but there is no need to turn it off when you slow down, because the converter will unlock as soon as the transmission shifts out of 4th gear anyway.

Ray

You can also wire them from the brake switch in the cab to perform the same function. Using the normally closed side of the switch allows the converter to unlock when you step on the brakes. I originally wired my 700r4 this way, but went with a hydraulic lock up on the rebuild. It automatically locks up around 35 when in 3rd or 4th. I always think its 3rd.

[raycow]

Originally Posted by propanemudtruck

at what amount of vaccum? Do engines measure in inches water column like gas pressure?

Inches of mercury. Most stock engines with a mild cam will be able to pull about 18-20" at idle. I think the transmission vacuum switches are usually set to open somewhere around 4-6", but some of the aftermarket switches are adjustable.

Ray

[special-K]

Plows will kill your aerodynamics and effect mileage...true.But,when I plowed I only had the plow mounted when out to plow.The fuel consumption going to between jobs was excellent compared to while plowing.