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I'm looking for ideas for the best rear end gearing for cruising around town and still being able to do about 75-80 on the highway comfortably with a 300 and 3 speed. Keep in mind I have no idea about this topic and its for future reference when I get closer to finishing my project.
A bit more information about your truck would be helpful. F-100 or F-250? 2wd or 4wd? Tire size comes into play also. Is your town driving Nebraska flat or San Francisco hilly?
There are a lot of variables. Without knowing at least some of the particulars it is really difficult to offer a decent suggestion.
There are on-line calculators that can help but they need the same information.
In my opinion, if your truck is a F-100 2wd with a 235-15 tire, a 3.25 rear gear is a good starting point. A F-250 4wd with a 30"-31" tire might work better with something in the 3.73 gear range.
I have a 300 six, a four speed (no overdrive) and 33" tires. I LOVE my 3.50 gears. They allow me to do everything I want. Without a creeper gear, if you ever plan on hauling anything, you want to consider 3.73 gears. I WOULD NOT go up to 3.90s or 4.11s though. They will kill your ability to cruise on the interstate. As it is, the 3.73's will turn the engine up to cruise at 60 MPH., but it'll be the happiest medium I think. If you don't plan on pulling a trailer or hauling heavy loads, 3.50s or even 3.23s will be fine.