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Old October 1st, 2017, 02:43 PM
POWERSTROKE POWERSTROKE is offline
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Default Re: Topping out at 55 mph?

I have to agree with BigBlockV6, swapping in a newer 5-speed or 6-speed overdrive trans would provide best highway cruising MPG and at lower rpm and best lower gears for starting out heavy loads.

When I ordered out my 1987 pickup I had a choice of two manual 4-speeds, NP-435 and a Mazda over-drive trans. 2nd gear starts even when towing were the rule. MPG suffered if trying to cruise over 65 mph. I had 3.55 gears, and 31 inch tall tires, 4th gear was direct drive.

Finally when I ordered out my '96 I had a good 5-speed manual overdrive available. Same 3.55 gears, 31 to 32 inch tires, but the .77:1 overdrive lets me easily cruise at 75 mph. I 've even pulled brush in 4wd Hi in low gear, and was able to spin my tires at idle on blacktop. So I have the best of both ends of the operating range. Took me a while to find a truck with the manual 6-speed, but my dealer finally got one in. One Saturday morning Son and I and the salesman took it for a ride. Same close ratio My 5-speed is, just smaller splits between gears. But 1st and 5th gear ratios the exact same as my 5-speed. I just had to shift one more time to get to my cruising gear. With my 5-speed and 3.55 gears I'm good towing to 20,000#, if I occasionally towed heavier I'd either go 4.10 or 3.73 gears or the 6-speed. If I towed heavy a lot, I'd go 3.73's and the 6-speed.

I think you will find your running your 305 V-6 around 2400-2500 rpm, which is about where it's torque peak is at, and it's horsepower peak is 3600-3800, I've run everything from 3-on-the-tree to 13 speed Road-Rangers and 5+4 twin stick transmissions, and everything in between. More torque and a wider engine rpm range is always better, but more closer spaced gears makes up for a shortage of horsepower in lots of situations. Like my little commuter car, 218 hp, flat torque curve from 1700 to 5000 rpm, and manual 6-speed. Most times I skip a gear or two when not in a hurry.

I missed my calling decades ago, I wanted to be a fleet buyer for a major trucking company. Ended up buying parts for manufacturing for 30+ years. I wanted to match transmissions with engines to provide drivers the best performance with the best mpg and durability for the company.
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