Runs fine but no power & awful MPG. HELP!

[Heavy B]

I have a 1994 Ford Ranger that I purchased to replace a 1993 Ranger. Both had/have the 2.3l motor/standard trans. The ONLY difference between them is the old one had 14" rims and standard tires and the new one has custom 15" rims with much bigger tires. The problem I have is twofold. My mileage in the new truck is TERRIBLE (almost half what the 1993 got and almost all my driving is highway miles!) and the new truck will only go 60mph or more in 4th gear and then loses speed in 5th and can't even stay at 60mph. Even on a level road. The 1993 never had problems like this! My first instinct was that it was because of the tires. However, my father (who is usually right when talking about car stuff) insists that the tires are not to blame. He says once the truck is rolling, the tire size shouldn't affect it (other than the speedometer) and with a bigger diameter tire doing highway miles, I should get BETTER mileage out of it. I changed the spark plugs because I had no idea how long the old ones were in but they looked fine, the motor runs smoothly, blows no black or blue smoke out the exhaust and no engine warning lights are ever on. I suspected a clogged CAT but the other night after a long run on the highway, I got out and had a look, expecting it to be glowing red, but it wasn't. Ideas?!? Could my dad be wrong???! Thanks!

 

[UrbanRedneckKid]

Sounds like a rear gear issue.

How big are the tires, and what gear is the rear axle?

 

[Lil-Pony]

whats your rpms on the highway in 5th

 

[Insanejughead]

Larger tires take more force to move, not because of their sheer size and weight, but because of the amount of rubber that creates friction between the vehicle and road surface. They also lower your engine RPMs because they rotate less as compared to a low diameter tire going at the same highway speed. Lowering your RPMs enough will cause a HUGE difference in the power felt, especially since the four-cylinders depend on RPM to get their power. IIRC, my ranger (stock) did best at highway mileage in the 2700-2800 RPM range. That's with a 3.73 differential and 225mm/75%/14" wheel/tires.

Also, the inline four-cylinder vehicles do better on fuel mileage at the higher RPM range. If you want to keep the bigger sized tire/wheel combo, then you'll need to lower the gear ratio in your differential. That will offset the circumferential difference created by the larger tires.


Btw, what are the exact sizes of the tires that you have on each ranger?

 

[Heavy B]

Seems everyone is looking in the same direction, so I guess the answer is pretty obvious. Thanks for your replies!

 

[zekew64]

By the way, Heavy B, some clarification:

First, tires/wheels encounter what's called "rolling resistance." In essence, the wider the tire you have, the more friction is produced as you drive down the road, and the more the tire robs the motor of the power it produces. If you want to decrease rolling resistance, go to a narrower tire. Think of it this way: A road-style 10-speed bike, with extremely skinny tires, will always roll better than a mountain bike with fat, wide tires. This is because the 10-speed bike has less rolling resistance to overcome.

Next, there is some difference when you have bigger wheels/tires. Bigger wheels and tires weigh more; because of this, using the laws of physics, the inertia is larger, which means it takes more power to get going. This will also affect your gas mileage. Also along those lines, what kind of tread do your tires have? An offroad tire, with a deep cleat (such as mudding tires or trail tires) will get less MPG than a road tire, simply because the deep tread doesn't have as much rubber that actually contacts the road. Now, this may seem like that the mudding tire would actually get better MPG; the difference is that the road tire doesn't have as much air resistance to deal with while it rotates, while any gains the mudding tire gets are offset by the air resistance it encounters while rolling at highway speeds.

By the way, +1 to the comments about the rear gearing. A taller (lower) gear will increase your torque, but actually hurt your MPGs at highway speeds. What you need to do is look at what gears you have now, and find gearing that will give you both torque and decent highway mileage. Most Ranger users have either 3.73 or 4.10 gearing. Personally, I have 3.08 gearing, with a 2.3. I have a feeling that once I go to 15" wheels, I should get more torque out of my truck while keeping a ballpark MPG figure.

Just my $0.02.

 

[alwaysFlOoReD]

Another point to look at is the height of the truck. Taller tires = taller truck = more drag @ highway speeds.

Richard

 

[Finnigan]

Could it be a clogged cat converter? Does it bog down earlier than your old one in all the gears at high RPM's? You can check if the cat converter is clogged with a vacuum gauge.

Also to think about, I had an '87 Toyota that would loose speed and bog down at highway speeds. It turned out to be a clogged fuel filter. It wouldn't bother me around town, but spend a little time on the highway and I would loose speed and power. I changed the filter and it fixed the problem.

just a couple other things to consider.

 

[RonD]

Yes, replace fuel filter.

And yes exhaust blockage can effect higher RPM, have someone rev the engine while you check the exhaust pipe flow, if flow reaches a certain point and then doesn't increase even when RPM increases you found your problem.


A leaking fuel regulator diaphragm can decrease MPG, remove the regulators vacuum line and smell it for gas.
This would usually cause a rough idle because raw fuel is being sucked into the intake.

 

[Finnigan]

You can also just use a vacuum gauge. Connect it to your vacuum tree via an unused port. run the engine. It should idle at a certain vacuum (mine idles at about 23 by my gauge) with the car in neutral give the car gas to raise the rpms and hold it steady. The gauge will drop at first and then should level off at about the same as idol. If the vacuum continues to drop while you maintain rpms then cat or elsewhere in the exhaust is clogged.

See Scenario 14 in the link below.
http://www.secondchancegarage.com/public/186.cfm