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Old 11-19-2010, 09:24 AM   #1 (permalink)
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vaccum road results

this is my first post. don't know why i am interested in this stuff. i had a
toyota previa; it got the same mileage whether i dogged it or flogged it: 21 mpg on the highway.

My current Car is a 95 infiniti g20 with automatic trans. obd1 so i can't use one of the fancy gauges. i ran the tube for the gauge
with the hood release cable, then under the plastic wheel well liner then down and up into the eng. compartment.

the gauge reads 20-0 in driving. i figure that is about right as i am at
5000' and one is to lose an in. vacuum for every 1000' above sea level.

i am getting about 10" at idle in gear (unfortunately i have an automatic
trans.). that increase to about 14" in N.

usually for optimal economy one is to shift up at about 2k rpm. that sure doesn't appear to be the case with this car.

while cruising at 50 mph with the cruise control on flat ground i pull about
10" in overdrive. when i shift the button to go out of overdrive the vacuum
level increases to about 14". This is also true at 65 mph and to a lesser
extent even at 75 mph!

I am informed that the gear is not locked mechanically until one is in overdrive, and that one will always have hydraulic losses in any other gear.
so it seems that overdrive would be the way to go.

so it appears that these engines have less of a load on them when revving
high. I do not want to cruise in 3rd on acct. of engine noise and wear, but
it sure looks to me that in order to save gas one should almost avoid overdrive unless going very fast. I am very surprised.

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Last edited by mor2bz; 11-19-2010 at 09:27 AM.. Reason: clarity
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Old 11-19-2010, 10:05 AM   #2 (permalink)
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Welcome to the forum.

Use your "overdrive" when cruising. You'll get better fuel economy, guaranteed. I have not encountered any vehicle that doesn't get its best highway speed fuel economy in highest gear. (Or torque converter lock-up if automatic.)

You may not be able to use an OBD-II gauge, but there is always the MPGuino option for an accurate MPG readout. It requires you make a connection to your vehicle's speed sensor, and an injector wire. It'll take the guesswork out of refining your driving techniques.
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Old 12-03-2010, 08:26 PM   #3 (permalink)
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After reading about it on these forums, I installed a cheapie vacuum gauge on my car, a 1989 Eagle Summit DL, with 1.5L engine and auto-3 transmission. The gas mileage increased from 25.9 mpg to 31 mpg (city driving) on the first tank.

Vacuum gauge: $16, sale price and with coupon from Harbor Freight.

Thanks to all for the great ideas! I especially like this site because most of the ideas are confirmed through experimental data. I look forward to reading, modding, and (hopefully) contributing in the future.
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Old 12-03-2010, 11:03 PM   #4 (permalink)
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Hey Mor2bz,

You definitely still want to keep it in overdrive as MetroMPG said. Dropping down out of overdrive does result in less load, but the higher rpm's require more fuel injector pulses per mile.

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Old 01-08-2011, 02:05 AM   #5 (permalink)
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Overdrive = good.

I should get a vacuum gauge on my Mustang...It's all I'd have.

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Old 01-08-2011, 09:26 AM   #6 (permalink)
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The engine has to work against vacuum, thats 7 pounds per square inch pushing back on the pistons. Mpg is better with overdrive, thats why its there.
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Old 01-08-2011, 12:44 PM   #7 (permalink)
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So how do you save gas with a vacuum gauge? Do you drive trying to get the most vacuum all the time?
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Old 01-08-2011, 01:02 PM   #8 (permalink)
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Never having used a vacuum gauge for anything other than diagnostics. I would like to hear from someone that is using one and what process they are using.

Highest vacuum in the highest gear possible? Keep a constant vacuum up hill? Are you only concerned with drops in pressure? What about shift points in an automatic?

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Old 01-10-2011, 01:16 AM   #9 (permalink)
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I try to keep the highest vacuum possible at speed. This becomes difficult with hills/mountains, sharp corners, ect.

Going up hills... define hills, here we have 8-10% grades... keep your speed constant. I find dropping a gear to hold your speed nets the same results as bogging down in a higher gear with your foot to the floor and slowing down necessitating a downshift anyways and using more throttle to regain the speed of traffic!
If you're in Nebraska and the wind stops or you see a tree, pull over immediately and take a nap. You're having road hallucinations.

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Old 01-10-2011, 12:06 PM   #10 (permalink)
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Originally Posted by star_deceiver View Post
I try to keep the highest vacuum possible at speed.

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