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Old 10-22-2005, 07:50 PM
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Originally Posted by Jmark
I need some opinions from all you tranny experts please.
My truck is a '77 1/2 ton with a metal shell camper. Its not a tank but it is an older heavy truck.
Current engine is a mild 350 with a th350 tranny. I built both about a year ago. Both are fine. My gears are 3.41 and my tires are 31" tall. My current rpm at 65 is 2600. My driving is probably split between in town (40-45) and in town freeway at 55-60. On trips, I try to keep my speed at or below 65.

The problem is my fuel milage stinks. I"ve spent a lot of time tuning and re-jetting the carb and have managed to increase my milage about 1.5 mpg overall. I still only get 9.8 in town with the A/C on and highway is 13+ with A/C and no load in the truck. I sometimes pull a Coleman tent trailer or my boat, neither weigh that much and I don't use them much either.

I"m wondering about finding a good 700 core and building another (my second) tranny for my truck. I currently have the 9" tail shaft so I won't have to make any drive shaft mods and because i'm running a stock Qjet carb, I "should" be able to hook up the TV cable easily.
Do you think it would be worth it from a milage standpoint?
I like the lower first gear for sure, that would be great with my 3.41 gears, but i'm concerned about lugging down the engine with the 4th gear OD and the lock-up converter. I know I can lock out the converter until I hit the freeways, but I selected my cam for optimum hp and torque at cruise speed, which is right at 2600-2700 rpm, although it pulls real good down to about 2000rpm.

If I have to "get into it" more to keep it going, I'll end up in the bottom step of the metering rods and kill any milage increase from the lower rpms. Any thoughts are appreciated.

My chart shows you should be at approx 2430 rpm at 65 mph. With varibles , I can see the 2600 rpm you post

you have thought of one area most folks do not know is there...

If the engine RPM drops low enough or you have your foot on the gas pedal a bit harder in OD fuel mileage suffers. Vacuum falls off and the carb will richen up.

If it is worth the swap is a number you will need to cypher on. Cost of 700 tranny installed and the resulting fuel mileage increase if any.

you already know it is a bolt in.

Years ago I swapped my brothers t-350 out of his 1977 GMC truck with 4.10 rear gears. With the 700 tranny his mileage jumped UP 4.5 mpg on the hiway. He was happy till his wife told him to sell the truck 2 years later.

I spent a chunk of money to get better fuel mileage a month ago... Sold my F-150 Super Crew Ford and bought a 2006 VW Jetta TDI car. Current fuel mileage on the Jetta after 2k miles is 40 mpg in town.

Dont worry how old you are... it is the mileage on your body that adds up.
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