I am driving my '97 F150 SuperCab Flareside XLT with 4.2L V-6, 5-speed manual and 3.55 rear end approximately 1400 miles per week. Most all of the miles are on the Interstate or 4 lane highway. Will the 4.2 L V-6 and 5 speed support a higher rear end such as a 3.05? I am currently getting a running 16.43 mpg for the last 3000 miles. The truck has 130,100 miles on it. Thoughts???

 

Engine should be able to handle a 3.08 ratio. One thing you need to consider here: what RPM are you generally cruising at and you need the spec's. on your 4.2L and determine where it (RPM) makes peak torque and horsepower. It is possible that switching from 3.55 to 3.08 may make the engine fall on its face and get worse fuel economy.

This reminds me of a former Customer that wanted to re-ratio a F-600 from 6.14 to 4.56 for better fuel mileage and performance. We did the math and found out that his model year 4.29, V-8 made horsepower at a rpm band that would have made the engine absolutely dead with the 4.56 ratio. He changed to 7.17 and increased fuel mileage and driveability horsepower.

 

Gearmanx,
Thanks for the information! I will check for sure but I do believe that max torque and horsepower both occur above or around 4200 rpm. I am generally cruising at 65 - 70 mph and running 2250 - 2500 rpm. At 60 mph, the tach indicates 2000 rpm.

 

I'm inclined to say no. You have many more miles on that truck than I had on mine, especially on the freeway. I would say that you'll never get that truck into 5th OD with the 3.08. It's important to keep it above 1500 rpm or the torque and hp are not there.

I've driven a 4.6l with the 3.08 and that will give you just what you are looking for. Even that engine with an auto had a tendency to shift out of OD because the rpm was too low. The fuel economy was great though. Best I've seen.

That's better milage than I got on mine, but I drove very little freeway, and when I did test it on a trip my wife drove a while and she thought there were only 4 gears. It was under 16 mpg.

Maybe better to look into things that improve gas mileage. The 235 tires should do better than the 255s. A tonau (sp) will reduce wind drag. Then there are maintance items, clean the MAF, new fuel filter, I ran some Scheaffer's Neutra 131 in my gas to clean out varnish so I could run reformulated gas. That improved milage on my 5.4.

Just my thougts. I don't drive that far in a month and a week.

 

Greencrew,
Good thoughts. I do have a hard tonneau cover on the bed. That may account for something. I just went through and fed it some injector cleaner and HEET to get rid of any possible water contamination and I have moved up to 89 octane fuel. The results are good. For the past 1000 miles, I have averaged 17.72 mpg with one run averaging 18.2 for just under 300 miles. I will check my tire sizes. Now that you mention it, I believe I may have moved up to the next size taller than stock. I'll check for sure. I'll check on the torque and power curves for this engine as gearmanx suggested too. Thanks for the ideas

 

I'd like to get a tonneau myself, but I first need to learn how to spell it.

 

The one I have, I bought shortly after purchasing the truck. It has been on and off a few times and I have had to replace the hinge and gas tubes on it once. I had to go to the Product Directory here on the site to find out for sure how to spell it. I had it painted to match the truck and I do have to say I have really liked having it.

 

Tell me about Schaefers Neutra 131 and reformulated gas???