FAQ: Improving Bad Gas Mileage

Quote :

I didn't even know 400 was possible. ~320 was my all time best.

I was like 2mm left of E with E light on and I fill up 14.5 gallons. That's why I assumed a 15 gallon tank.

400 miles per tank really isn't too difficult. Our engines sip the gas at low RPM. If you are driving 100% highway, 600+ miles is possible on a single tank. I've driven 250 miles on less than 7 gallons before I lost TC lock-up.

I just put 18.3 gal in my last fill-up. You can drive much further on E than most think. You need a way to monitor exact fuel level if you do. I use the AeroForce to track capacity in %. It's way more accurate than the OEM gauge.

This is all really hard for me to believe...

I do about 10-15% highway driving. I am lucky to ever hit 300 miles on a fill before the E light turns on.

Only time I get >300 miles on a fill is when I travel on the highway, but even then I don't get much farther than ~330 miles.

I would love to consistently hit 300+ miles on a fill! Any components I need to look at in the car to investigate this? Believe it or not, I am all about hypermiling and my lead foot only comes out rarely. I am all about coasting to lights and using momentum to get around.

How can I approve my MPG?

Time for the hypermiling thread: https://rivperformance.editboard.com/t3481-faq-hypermiling-modifications-techniques

I have been getting pretty bad gas mileage, even on highway. I set my cruise control at 72mph and my engine is revving at 2,000 rpms, and I am averaging 16-17 mpg. I don't seem to have any misfires, or bad idle, I also don't have flat tires or excessive weight in my trunk. What kinds of things could cause this? The sparkplug wires have less than 30,000 miles and the plugs have less than 10,000 miles.

RidzRiv wrote:

I have been getting pretty bad gas mileage, even on highway. I set my cruise control at 72mph and my engine is revving at 2,000 rpms, and I am averaging 16-17 mpg. I don't seem to have any misfires, or bad idle, I also don't have flat tires or excessive weight in my trunk. What kinds of things could cause this? The sparkplug wires have less than 30,000 miles and the plugs have less than 10,000 miles.

72 MPH should be about 1800-1900 RPM. Maybe the converter isn't locking up. An exhaust restriction like a clogged cat would do it also.

If you have a 95 like I do,most likely the injectors are fouled.they can get pretty ineficient over time.
I would check out mrinjector.com and find out what your options are.i think he only charges $15 bucks or so to clean and bluprint them.catch is,you would need another set to tide you over until the cleaned set comes back. you can pull a set from the pick-a-part for around $20.

LARRY70GS wrote:

RidzRiv wrote:

I have been getting pretty bad gas mileage, even on highway. I set my cruise control at 72mph and my engine is revving at 2,000 rpms, and I am averaging 16-17 mpg. I don't seem to have any misfires, or bad idle, I also don't have flat tires or excessive weight in my trunk. What kinds of things could cause this? The sparkplug wires have less than 30,000 miles and the plugs have less than 10,000 miles.

72 MPH should be about 1800-1900 RPM. Maybe the converter isn't locking up. An exhaust restriction like a clogged cat would do it also.

...yup,that too.

If I remember right, I was ODing at 75mph @ 2000 RPM, last time i locked cruise in.

At 80 MPH, my tach reads between 2000 and 2100. That is right where it should read.

336/tire height X final drive X MPH = RPM (with lock up)

Tire height of P225/60R-16 = 26.6", Final Drive is 2.93 X .70 = 2.05

336/26.6 X 2.05 X 80 = 2071 RPM

I just had my transmission rebuilt but they never mentioned anything wrong with the torque converter. Is it possible that I don't have my vacuum modulator adjusted correctly?

RidzRiv wrote:

I just had my transmission rebuilt but they never mentioned anything wrong with the torque converter. Is it possible that I don't have my vacuum modulator adjusted correctly?

No, not possible. They are either good or bad. They don't get adjusted. If the modulator was bad, you would have obvious shifting problems and the transmission could slip. That is NOT your problem.

Your problem is either a restricted exhaust, a vacuum leak, or dragging brakes. You should check the engine with a vacuum gauge. Fully warmed up, it should pull 20" at idle in Park. Raising the engine speed to 1500-2000 RPM in Park, and holding it steady, the vacuum should be at least the same (20"), and it should remain steady. If it slowly begins to fall, suspect a clogged Cat, or collapsed pipe/muffler.

I as well as some other members of the forum are using adjustable vacuum modulators. You can turn a screw to adjust line pressures to some degree.

Anyways, I am glad that that is not the problem. The first thing I will check will be the emergency brake line. I'll also have to buy a vacuum gauge. Thank you for your help!

The only time an adjustable modulator is of use is when you are using a camshaft that alters the vacuum characteristics of an engine. You can adjust the minimum and to some degree, the part throttle shift points. That adjustment can be abused and lower line pressure to the point that the transmission slips, and that ain't good. I don't see the purpose in adjusting the modulator for our cars. What was it, 95 and 96? At some point, we went to electronic control.

Well a new non-adjustable replacement was somewhere in the 100 dollar range. This adjustable one was 12 dollars.

RidzRiv wrote:

Well a new non-adjustable replacement was somewhere in the 100 dollar range. This adjustable one was 12 dollars.

That's crazy. Why did you replace your original?

I had read on here that they tend to wear out so I figured for 12 dollars why not see if it makes an improvement

The only thing that fails on them is the diaphragm, and that is relatively rare. Then it can pull ATF into the engine where it is burned. At the same time, shifting problems and or slippage develops, so there is no doubt that there is a problem. They do not wear out.

Check how accurate is your gauge, and the amount of stopping you do as a % of hours (not miles) drive time. If you only do 15% or so highway driving your miles per tank might be right in there. Riv does not get good m ileage in stop-and-go driving.

charlieRobinson wrote:

This is all really hard for me to believe...

I do about 10-15% highway driving. I am lucky to ever hit 300 miles on a fill before the E light turns on.

Only time I get >300 miles on a fill is when I travel on the highway, but even then I don't get much farther than ~330 miles.

I would love to consistently hit 300+ miles on a fill! Any components I need to look at in the car to investigate this? Believe it or not, I am all about hypermiling and my lead foot only comes out rarely. I am all about coasting to lights and using momentum to get around.

How can I approve my MPG?

O I'm good. It was the F/R weight distribution that was messing up my MPGs. I am back in the game now.

Thanks, tho.

Very unlikely your ride-height issues hindered your MPG. Aside from tires and brakes, there's very little you can do to the suspension or weight distribution to significantly impact economy. There was another factor, or it was the main reason for MPG variance: driving style and route selection.

No, not ride height. Weight distribution. Before I was like this / now I am like this \.

 I am most definitely seeing an improvement in MPGs, power, and traction. Before the new springs, my weight was dumped in the back. Now I have it back where it needs to be up front.

I never got MPGs like this before. My driving style has only become more aggressive. So figure that one out. It is for sure the weight distribution.

Power and traction, sure. Handling, sure. Weight distribution can make differences there. But MPG doesn't care where your weight is at, it only cares how much weight you have. Shedding weight is what matters; moving weight - not so much for MPG.

To compare, I've spent most of the time I've owned the Riv moving weight to the rear, because I believe that's the ideal way to carry weight in a car. For example, removing weight from the engine bay and front suspension, and adding heavier things like stereo equipment and trailer hitch receiver to the rear. All of this moves away from the OEM 70/30ish distribution and closer to 60/40. I've kept very (very) accurate fuel logs for many years now, and can see what makes a difference and what doesn't. The weight shift didn't matter even a slight bit, and I've shifted about 100 lb so far.

Your aggressive driving style creating better MPG could be real, or it could be imaginary. If by "aggressive" you are taking lots of hard corners so the need to brake and accelerate is less, then yes I can buy it. If by aggressive you mean more acceleration and braking than usual, then no it's not giving you any better MPG. Even if the change in weight distribution from your new springs were helping a little, it would be so miniscule compared to the huge loss from accelerating and braking that you would see effectively lower MPG in the end. There must be some other factor at work here. Need to do some controlled tests to know anything beyond a guess.

That's all fine and dandy. But this is real.


This is how you get poor MPGs


This is how you get better MPGs

I track my MPGs and miles per fill. Numbers dont lie.

Drive around without your center caps? I've never tried that

Abaddon wrote:

Drive around without your center caps? I've never tried that

I knew you were going to say that..


AA, think about it. If it affects traction, acceleration, etc.. its going to affect MPGs. The power is being put to use and not being wasted like it was when the weight was tipped in the back.

It's not even something I need to wonder why. The numbers done lie. My fuel economy has increased. The end.