People seem to think that the 3.27 is not the best rear end for city driving and possibly not the best for 70mph and less on the highway.
I think you’re probably right about the city driving part. But a 3.27 axle should be good for highway driving.
From what I’ve read, as long as the engine isn’t too small for the vehicle, numerically lower axle ratios generally have better highway fuel economy. The exception is when the engine is under extreme load such as towing. For towing and driving in stop-and-go traffic conditions, numerically higher axle ratios can have better fuel economy. With this in mind:
Ford has announced that its 2008 F-250 and F-350 heavy duty trucks will come with new fuel saving measures. The revised trucks will come equipped with 3.55 rear axle ratio — instead of the current truck's 3.73 setup — as well as a 3.93 inch (100 mm) chin spoiler.
Despite the numerically lower ratio, Ford claims the trucks will keep their current tow ratings. However, acceleration times will likely suffer.
The improvements should net about a 1.5 mpg improvement over current fuel economy. Three-quarter and one-ton trucks are not required to carry an EPA fuel economy rating so an exact measurement of the improvements is hard to discern, but likely in the 10 to 15-percent range.
"We are making the 3.55 standard and when that's combined with new front-end aerodynamic improvements, there will be about a 1.5 mpg improvement," said Ford trucks spokesperson Wes Sherwood. He continued: "Ford is constantly looking to innovate with its trucks. Fuel economy is on people's minds right now and we're not going to wait until the next model year to make a change. We're going to implement changes like this as soon as we can."
You are correct in your thinking the 3.27 is not optimal for gas mileage.
Al, you say a 3.27 axle is not optimal. I'm not disputing it, but I'd like to know the reasoning behind your statement.
My 93 Explorer Sport has the 3.27 Limited Slip “Performance Axle” and an automatic transmission. Everything on my truck is still stock though my transmission is a new A4LD-HD (Ford’s A4LD tranny with all the updates). It has approximately 6,000 miles on it. Here are my RPMs maintaining speed on level road, driving in OD:
45 - 1500
55 - 1750
65 - 2050
70 - 2200
My Explorer is my daily driver and I use it for my work commute to and from my job downtown. So I drive a combination of highway and city miles. I also have some terrain including one fairly large hill. I just filled up last night and here’s the data from my last tank:
Fuel Used: 14.13 gallons
Distance Driven: 288.9 miles
Miles per gallon: 20.44
What do you think the optimal RPMs are for the powerband and gas mileage? At what speed do you think I should engage the OD anyway?
I know these questions were for Al. But I thought I’d share some stuff too. The biggest problem I see with the 3.27 is it’s poor acceleration. This means you may run high RPMs for longer than you would with slower gears because it takes you longer to get up to speed, burning more fuel. To try to save fuel, I used to accelerate slowly up to speed to keep the RPMs down. But I've found a firm acceleration to get up to speed quicker is more fuel efficient. Just don’t stomp on it. This really kicks up the RPMs, burning more fuel.
As for driving speeds, 55 is where I get my best mileage too. Above 67, mpg begins to drop lock a rock. As Al points out, wind resistance has a huge effect on fuel economy. According to Roger Clark, senior manager for General Motors Integration and Fuel Economy Learning Vehicles Program (FELVP), which handles EPA testing of all GM trucks and SUVs:
The best fuel economy for the typical truck or SUV is cruising right around 40mph. The EPA test cycle for the highway fuel economy number averages 48mph with a top speed of 60mph.
A good example of how drag affects fuel economy is a truck that has a 21mpg highway EPA number. Drive at a 10-percent higher average speed (53mph) and drag causes fuel economy to fall about 1.5mpg. Average 60mph and mileage will drop another 1.5mpg. Run just above70mph and now fuel economy is less than 14mpg instead of 21mpg.
Do you think that staying out of overdrive would actually get better gas mileage during speeds under 70? It seems like it would be more like having a 3.73 and using overdrive. Am I totally wrong in assuming this?
Like Al I’ve been using OD the time. But I like you, began wondering about it and just started testing the effect of staying out of OD for speeds under 55 (below where OD usually kicks in). There are two reasons for this. First, the added torque without OD which may help reduce acceleration times without unduly raising RPMs. Second, being out of OD may reduce friction (Al or any of the transmission guys, I’d like your thoughts on this). This is from
Heavy Duty Trucking’s Guide to Fuel Economy:
An overdrive top-gear ratio is usually paired with a slow (numerically high) axle ratio, and a direct-drive transmission is used with a fast (numerically low) axle.
A direct-drive top gear can save a bit of fuel. That's because it involves fewer gears in the transmission and less churning of lubricant. Controlled tests show the reduced friction of a direct-drive transmission saves up to 2% in fuel at cruising speed. Some fleet managers have measured some savings in tank mileage, and you may or may not see it at the pump. The more varied your cruising speed and operating conditions, the less actual savings you'd get from this or any other component that is most effective at relatively high speeds.
On the trip up I got 18.28 mpg with OD off, on the way back I got 20.18 with OD. I was doing about 70mph the whole way. I suspect however that I may get better mpg at lower speed with OD turned off. I will do some more testing.
I’ll be interested in your results as well.
