Engine RPMs
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Senior User
Joined: Jun 1999
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Engine RPMs
Does anyone know what the optimum rpm for a 302 is, as the most ecomomical rpm, I've got a ' 78 F150, w/C4, and 3.25.1 rear end, 27 1/2 inch tall tires, I recently changed to the 3.25, from a 2.75, to give me a little more take off, as the truck is mainly used around town with speeds under 55 mph, have I gone too far in gearing, I can't get more that 13 mpg with this stock 302 4v.
Elder User
Joined: Aug 1999
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Engine RPMs
I had a 302 4V and could only get 11mpg. Installing a Vaccuum gauge will give you insight into fuel consumption. Off coarse, with a C4 you aren't controlling when to shift. The higher the vaccuum reading the better the fuel economy. Without an overdrive gear, I don't think you will get better than 13mpg.
Fleet Owner
Joined: Jun 1999
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From: Creedmoor, North Carolina
Engine RPMs
Rick, "A little basic info"
Generaly the slower an engine is turning at 60 mph the less gas it will consume.
You stated that you removed a 2.75 gear, and installed a 3.25 gear. By doing so, you caused the engine to be turning faster at 60 mph. It will give you more power to the wheels, faster rpm increases, but lower mpg.
Did you go too far, I don't think so, but it will cost more to run the truck.
John
66 F100s
In the still cool hours of the night, you can hear chevys rusting away.
Generaly the slower an engine is turning at 60 mph the less gas it will consume.
You stated that you removed a 2.75 gear, and installed a 3.25 gear. By doing so, you caused the engine to be turning faster at 60 mph. It will give you more power to the wheels, faster rpm increases, but lower mpg.
Did you go too far, I don't think so, but it will cost more to run the truck.
John
66 F100s
In the still cool hours of the night, you can hear chevys rusting away.
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Joined: Aug 1999
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Engine RPMs
I have an 89 5/0 with 3:55s and it seems to perform better at 2600-3000rpms. I'm on my second motor but my ride has over 400k on it and I'm running a factory reman which has over 200k on it as of now and still runs great and burns very little oil. I have an aod in it(original) and still doing its job every day with over 400k. I have installed svo headers,ultra-flo muffler with 3"inlet and 2-21/2" outlets, accel hi energy ignition with 8.8 race wires and splitfire plugs. I also use synthetic engine oil(Mobil-1 5/50). After all this I still get 14.3 mpg no matter I have done. I got this with the first motor and still get the same with all the changes. I don't notice much differance between drive and overdrive. Most of my driving is highway but I only use overdrive when I am over 3000 rpms which is seldom. I can see little differance in gas mileage. Have fun with your truck and leave the gas mileage worries to the small car drivers.
Senior User
Joined: Jun 1999
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Engine RPMs
My references only show a 2v 302 in 78 and it torque peaks at a lowly 2,000 rpm; 1800 for California set-ups. This is awfully low for a small block which isn't going to give you much to work with. At 55 mph, you should be turning 2350 rpm when you really want to target your cruising rpms at 1900. So your set-up now gives you a cruising speed of only 45 mph for max engine efficiency. If you went with a 3.00 axle, this would only get upped to 48 mph. Likewise, 2 inch larger tires would do the same. Want suggestions? If the 4v was added on, I doubt it changed your torque peak-it just raised it some unknown amount. Using a vacuum guage is an excellent idea and check your rpms when driving too. Probably you are looking at a RV grind cam to flatten and extend your torque curve so you have a longer rpm range to work with and help with the mileage; 15 mpg is about where you will max out without an overdrive trans.
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bluebird_281
1980 - 1986 Bullnose F100, F150 & Larger F-Series Trucks
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Feb 1, 2004 08:53 AM
