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| Tire pressure is one of the most important parts of the fuel economy formula. Our tests showed that even moderately underinflated tires could cost nearly 1 mpg, not to mention the squirrelly handling and increased tire wear that goes with it. |
The biggest mpg gains come absolutely free by tightening the connection between your brain and your right foot. The "Driving Tips" below can help you improve economy 2-4 mpg, or more, in every venue. Yes, where you drive is also a vital factor. The worst case mpg scenario is the stop-and-go commute. Consider changing your route and leaving earlier or later to miss the worst traffic. In some cases, you may use less fuel taking a longer route.
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| Confused as to what will give you more mileage? You're forgiven. It's a common malady. Almost everything works but it becomes a matter of degree and cost effectiveness. The results are also variable amongst the different makes and models. It depends on what's replacing what. We're going to be doing some intense power and fuel economy testing for upcoming issues, so stay tuned. |
-Accelerate moderately with light throttle pressure but briskly enough to get into high gear ASAP.
-Analyze the traffic and stop lights ahead to keep rolling and minimize idling. Accelerating from 5 mph costs less fuel than from a dead stop.
-Cruisin'. When traffic permits, use cruise control. Even at speeds down to 45 mph, it can save a few percent. On long trips is can add 1-2 mpg over the average foot.
-Minimize idling. On long waits for traffic, shut the engine off.
-Minimize warmups. Beyond clearing the windows, just let the engine idle long enough to stabilize oil pressure then drive off slowly. Call it a "dynamic warmup."
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| Ugly, ain't it (apologies to owners of '91-93 Renegades)? Maybe but, with a hardtop, this '91 Wrangler Renegade is probably the most aerodynamic Jeep bobtail ever built. That's not saying a whole lot in a world with computer designed cars that slip through the air effortlessly but compared to a stock YJ, the fender cladding, bumper treatment and air dam, it's noticeably more fuel efficient. |
-Patience. If you feel hurried or pressured, you'll tend to drive more aggressively... and pay for it. Forgive us if this sounds too Zenlike or patently obvious but, truly, if you put on some good tunes or interesting talk radio and just resign yourself to getting there when you get there, you'll do better. Obviously, when it comes to punching the time clock on time, that may mean setting your alarm a little earlier and hitting the road sooner.
-Shift Early. With a manual trans, shift early while avoiding lugging. Get into top gear as soon and as often as possible.
-Slow down on the freeway. Most trucks and 4x4s have the aerodynamics of a cinderblock. The faster you go, the more power is required to push that cinderblock through the air and the more fuel it will use.
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| Having the miles-per-gallon available to monitor (arrow) is conducive to economical driving. It becomes a contest and that helps overcome the urge to drive aggressively. The Edge Evolution programmer for the Ford 5.4L allows you to monitor many engine and vehicle parameters, including average and instant mpg. The important factor is to get the gear ratio and tire size entered exactly right. |
-Windows and A/C. Eighties tests showed that running down the freeway with open windows cost about 1 mpg due the to loss of aerodynamics. Using A/C at low speeds around town cost 2 MPG on the same car. At freeway speeds the A/C cost about .5 MPG. More recent tests confirm these numbers and show the gap between windows open and A/C on at freeways is narrower on less aerodynamic rigs like trucks and SUVs than it is for cars.
Tire Pressure vs Fuel Economy
How much does the right tire pressure effect fuel economy? As an experiment, we took a bone-stock 2005 Ford F-150HD over a 10 mile course, five miles each way, at four tire pressures, using an Edge Evolution programmer to measure fuel economy. The course was five miles up and five miles back, run twice at each pressure.
We started the OE General Grabber TR 245/70R16D tires at their maximum pressure of 65 psi. Next we used the Ford-recommended 50 psi front, 60 psi rear. From there we went to our carefully selected empty cruising pressure, derived from tire load inflation charts. Finally we dropped to an underinflated pressure. We tried simulate a neglectful owner who is numb to the finer handling inputs of his truck.
The results (see nearby sidebar) were a 3/4 mpg difference between the underinflated point and the max sidewall pressure. The difference in mpg between max sidewall, factory recommended and our empty cruise pressure was insignificant.
Fuel Choices Count
The gas or diesel you use will have an impact on economy. Even among different fuel brands, you may see variances in BTU content (British Thermal Units, a measurement of stored energy) that may effect economy. Winter grades of gasoline, oxygenated fuels or those blended with 10-15 percent ethanol offer less economy because they have fewer BTUs. The loss could be in the neighborhood of 2-4 percent according to most sources. E85 (85 percent ethanol, 15 percent gasoline) offers even less economy, usually around 15 percent less, but as much as 20-25 percent. Theoretically, the losses are offset by the lower cost of the fuel but, unfortunately, E85 is neither common nor particularly cheap in most areas.
Diesel fuel just underwent a major, government-mandated reformulation to reduce sulphur content. Overall, the results are a net plus, but some owners and studies report a slight loss of fuel economy in the region of 2 percent. That loss may not show up for every diesel owner because the quality of the old diesel fuel around the country was so variable. If you lived in an area where crappy fuel ruled, you might see no loss or even a slight gain with the new fuel. You will see a mileage (and power) drop with any winter-blended diesel.
Biodiesel has fewer BTUs and thus offers lower economy. The often quoted figures are about a 2 percent drop for B20 and a 10 percent drop for B100 (the "B" and a number referring to the biofuel content). Again, in theory, the loss recovered by the lower cost of the fuel, but like ethanol, those huge price drops have not generally appeared. Someday, when the biofuel infrastructure catches up, you may see significantly lower prices.
Increasing Economy
Some fuel can be saved with modifications to your vehicle. Don't expect a few bolt-ons to counter the massive losses that come with a major trail buildup, they just reduce your losses. On a stock, or near stock rig, certain mods may offer gains over stock by a little or a lot, depending on what you do. Don't expect miracles from any single modification but each may add a skosh. The gains we listed below are averages from various sources. You could do better or worse.
Aerodynamics- Lifts and fat tires increase the frontal area of your rig and the various add-ons, such as bars, bumpers, lights and racks, decrease it's already mediocre ability to cut cleanly through the air. The adverse effects come mostly at higher speeds. You can counter at the beginning of the buildup process by moderating your lift and tire choices and minimizing the amount of stuff you bolt onto the outside. On a rig that's built already, driving more slowly and removing stuff are your only relatively easy answers. If you really wanted to be trick, you could design and build some removable cladding to smooth the airflow. The difference at freeway speeds could be 3-4 mpg between a low, slick 4x4 and a tall rough one.
Camshafts- "Economy" or "RV" cams may help with economy, though from an economy standpoint most stock cams are pretty hard to beat. A better cam becomes cost effective when you need a cam replacement anyway. If you are running a "hot" cam now, going to a milder profile can yield significant mpg benefits.
Engine Internal- Upgrading a low compression engine to a higher ratio can yield increased economy by getting more work out of the fuel. This applies mostly to "old tech" engines where the ratio is around 8:1. You can gain about 5 percent going to 9:1 but it gets harder to get an old style engine to run ping-free on pump gas with much over a 9.5:1 ratio. Roller rockers, roller lifters and light valve spring pressures all reduce internal friction and could gain you another percent. Larger exhaust valves & exhaust port modifications, thermal coatings in the combustion chamber and on piston crowns improve combustion and volumetric efficiency. How much they add is highly variable but could be significant. The problems is that the costs are fairly prohibitive and not easily recovered unless you are already rebuilding the engine.
Exhaust- Low restriction mufflers and cat-back exhaust systems with mandrill bent pipes can add 1-5 percent depending on the system being replaced. Long tube headers can add a very significant benefit, up to about 10 percent, if there are applications to fit your application. Shorty headers are much less effective but if they are replacing a particularly bad factory manifold, they can be a significant improvement.
Fuel System- Replacing a tired carburetor with a more modern one can bump you a coupla percent easily. The key trick is to think small when choosing the CFM rating. A conversion from a carburetor to a bolt-on EFI kit can often yield a coupla percent over a good carb and lots more over a tired one. Some of the gains would depend on how the new EFI is programmed. Some are strictly performance only. Chips and programmers for gas engines can sometimes offer a slight gain in mpg. You'll get the most with a programmer that offers some "tweakability," where custom tuning options allow you to play with trans shift points, timing, etc. Diesel programmers in the "Tow" or level 1 often offer a coupla mpg over stock by optimizing fuel pressure and timing. Aftermarket intake manifolds may be an improvement depending on which is chosen. Some are designed only for higher rpm efficiency, so pay attention to the specs. Free flow air cleaners and filters won't do much for stock engine mpg if the OE system is well designed. A cold air intake can add a coupla percent if it's truly delivering cooler air than stock.
Gearing
Combined with the stock tire size, the OE gearing usually offers optimal mpg. CAFƒ standard motivates the OE to make sure of that but with a tire size increase, you alter the overall gearing. There are lots of variables in how gearing changes effect mpg, including the engine's torque curve, aerodynamics and weight. Taller tires reduce engine rpm at all speeds. With no other changes, and not taken to extremes, that may help mpg on flat ground freeway cruising. It will very likely hurt in-town mileage to some degree because the engine has to work harder to accelerate and in town, you are continually slowing down and speeding up. Around town, low trail gearing actually helps by allowing you to accelerate using less throttle.
Each engine has a "sweet spot" cruising rpm range where mileage will be best. Generally speaking, that comes in the lower third of the engine's torque plateau but the engine has to have enough torque at that speed to handle the load. Trying to maintain speed while lugging the engine with a wide open throttle costs as much or more fuel than letting the engine rev. A few more rpms than stock doesn't seem to hurt economy much at all if the engine can hold the higher speed with less throttle.
If the engine is torquey enough, it can absorb a little extra tire with no trouble. Lower powered rigs suffer more. The answer is to use the equivalent ratio formula (see sidebar) to give your rig back the original overall gearing. If you can't afford the gear change, ask yourself if you can afford the gas.
Ignition- "Hotter" coils & ignition modules, performance plug wires, higher quality park plugs may all add a skosh but that depends on what's being replaced. It's a more effective mod on older rigs with distributors. A modified/optimized timing advance curve and timing settings may add up to 5 percent but can be tricky to achieve.
Lubricants- Going to slippery synthetic oils is proven to add fuel economy. How much depends on what you are replacing. If your truck has no synthetic anywhere, you are likely to see a small gain. If your truck has a factory fill of synthetic, you aren't likely to see much. We will be testing this effect for you in an upcoming issue. The oil viscosity you choose, regardless of oil type, can make a difference. Running a heavier oil than necessary for your climate and conditions will cost mileage.
Electric Fans- An electric fan conversion can add 5-10 percent to an engine primarily operated at freeway speed. Around town the benefits drop but are still useful. The trick is making sure the new fan has the airflow necessary to keep your rig cool.
Tires- Highway tires generate less rolling resistance than all terrains. All terrains generate less than mudders. A less aggressive mudder will generate less than a very gnarly one. A hard rubber compound creates less friction than a soft one. All that said, airing up whichever tire you have will decrease rolling resistance. Running the tires at or near the maximum pressure on the sidewall can decrease rolling resistance and offer slight gains in MPG at the cost of ride quality.
Weight- Travel light. Buy lightweight products when possible. Unload all your excess gear for your daily driving. A few hundred pounds can be worth a couple of miles per gallon in a low-powered rig. Heavy trucks with big engines don't respond quite so dramatically but the gains pay in the long run. A recent test on a gas Ford pickup showed a 0.4 mpg difference over a measured course between the empty truck and the truck loaded with 850 pounds.
Source
Edge Products
www.edgeproducts.com
888-360-3343
How to Check Your Fuel Economy
The actual calculation is easy. Divide the miles driven by the gallons used and get miles per gallon.
miles driven
__________ = miles per gallon
gallons used
The problems come with the numbers you are inputting. Here are the top problems and their solutions.
Inaccurate Odometer- A common problem because of tire and gear ratio changes without correction to the speedometer. An odo reading fewer miles than actually traveled, as with bigger tires, fuel economy will show higher than actual. An odometer reading more miles than actually traveled comes from lower gears and mileage will read worse than actual. If you do a gear and tire swap at the same time, you could be anywhere... including right on. You can do a quick-n-dirty odometer check using highway mile markers. Say the odo shows 1.1 miles over a measured mile. Divide the actual miles by the indicated miles to get your correction factor (1/1.1=0.9). Multiply the correction factor by the indicated miles driven to get the actual miles (e.g. 250 miles x .9 = 225 miles actually driven). If the number comes out the other way, say indicating 0.9 miles over an actual mile (1/0.9= 1.11) and your 250 indicated miles are actually 277 miles (250 x 1.11= 277).
Inconsistently Filled Tank- To get accurate fuel economy readings, you need to fill the tank to the same level each time. Make sure your rig is level on all planes. The automatic shutoff device on the pump will usually kick off at about the same time at most stations, but not always. If possible fill up at the same station and use the same pump for the ultimate in consistency. Diesel owners beware: Diesel fuel foams. The slower you pump diesel, the less it foams. To get accurate fill ups with diesels, you almost always have to add the last few gallons very slowly by hand and fill it right to the top of the tank.
New Tire Diameter
_______________ X Original Gear Ratio = New Ratio
Old Tire Diameter
Example:
A half ton truck owner with 265/75R-16 tires (about 31 in.) with 3.73:1 gears wants to run 36s.
36
__ X 3.73 = 4.33
31
The nearest available ratios are 4.10 or 4.56:1. To opt for highway fuel economy vs acceleration performance, pick the 4.10s. The best acceleration, trail performance and in-town mpg pick would be 4.56:1.
Do the improvements "pay" in the Long Run?
The key element to making modifications pay is to analyze the costs versus the benefits and choosing the products that offer the "mostest for the leastest." Here's how to figure it.
Let's say a product costs $500 and delivers a 2 mpg increase. You drive 15K miles per year and your current mpg is 15. Divide the annual mileage by the mpg to get your gallons per year (15,000/15= 1000 gallons). Do the same for the new 17 mpg (15,000/17= 882 gallons). Figuring a fuel price of $3.30, that $3,300/year at the old mpg and $2911 at the new. Subtract the new from the old and you get an annual savings of $389 per year. That means you pay off the modification in about a year and a quarter (500/389= 1.28). Pretty good. If it takes you some ridiculous amount of time, like 10 years, give the idea some thought. It might be worthwhile if you intend to keep your rig a long while but, if not, you're just giving the new owner a freebee.
