383/700r4/3.08........and 10.5MPG!!!!!!

[nightmare]

Originally Posted by smoky02

you also said that your tranny doesn't lock up, that takes alot away from you. And the higher gear ratio, would more than likely increase your mileage in town, but not out on the highway at all.

Hmmm. PATC's website states that you want your rpms to be at at least 2000 @ 70 mph. All the websites that have the calculators on them show that I should be turning ~ 1675 (so roughly 1700 rpms) with the lockup. If I'm only 200 rpms off, how much is that taking away? My guess is that with the 3.42's, it should be turning around 2100, which should be "optimal" for my setup to increase mpg.

So there's a possibility that my in town mpg could be more than my highway mpg? I don't see how that could happen.

[mclairmo]

Hook up the vacuum advance. Also, make sure the vacuum secondaries are not opening too soon. You may have to go to a stiffer spring in the vacuum advance.

[69GMCLonghorn]

As stated above, if you want to see what lower gears (numerically higher) will do for you, keep it in 3rd for an entire tank of gas and don't change any other driving habits, etc. If your mileage drops, don't change the rear end.

['72customdeluxe]

You either a-have 3.73's (which I think you do), or B-have a WAAAYY off speedometer. Borrow a gps from someone. I figured out the other day I had 3.73 gears, not 3.08's with the gps. You should be running 1636rpm in lockup at 70 with 3.08's and 31's. With 3.73's you'll run 1982rpm. This is on the bowtieoverdrives.com calculator. They also have a calc. for selecting speedometer gears and how fast you can go in each gear with different transmissions. Another thing is, you built a cammed 388 without a quadrajet and expected to get decent mileage?? That cam is a 224/230@.050 on a 110lsa. You'll never get mileage with that and especially with an Edelbrock. I'm not arguing that 10.5 mpg is normal though. That seems a little low. And 3.73's are probably your best bet for mileage on smaller tires, you might want to bump it up to 4.10's with those 31's. My 350 running 2900 rpm at 70 gets better mileage than you but I suspect this will fade with an xe262 and an Edelbrock in my future.
edit-just saw you had a holley, still not any better for mileage

[70duramax]

Getting a bigger carb at least a 750cfm would be a great starting point a 650 is fine for a 350 but too small for that stroker and your rpms are right where you want them. if your after milage do not change your rearend. look at a dyno sheet for just about any 383 stroker and you'll see at 1900rpm you already have more torque than most stock 350s have throughout there entire power band. you have a tuning issue get a bigger carb and get it tuned. bring it to a dyno shop its amazing what they can do in just a hour or two. plus it'll be cheaper than changing out rearends! i do like 3.73 its what i installed in my 70 4x4 with a 388 stroker/t400 combo i only swapped out the 3.07s for playing in mud and snow and to teach them pesky mustangs and rice bunners whats up! and p.s. it didn't improve my milage at all! haha JUST MY 2c good luck

[70duramax]

forgot i ran a 750 speed demon w/vac sec. exelent right out of the box very little tuning perfect for a 383

[capev86]

84 suburban 1/2 ton 2wd, 350 quadra-jet, 700r4. 3.73FD
=17-18mpg (once got 19mpg w/ 100,000 miles and low compression)
w/ 406sbc swapped in place of 350 (stock intake and exhaust)
=16-17mpg

87 suburban 1/2 ton 2wd, 350tbi, 700r4, 3.08FD
swapped in the 84 rear axle and didn't lose any mpg.

trucks have tall tires and that's why a somewhat aggressive gear AND an OD are the best combination unless all you do is drive 90mph all day long and don't use your truck as a truck. on a car, a 3.08 gear is a good idea if you don't have OD and the tires are on the shorter side.

my 72 burb 3/4 ton 2wd, 307, 3 on the tree, 4.57FD,
=10.75mpg on this last tank full. i did not exceeed 60mph
saginaw OD unit, 4.10FD swap should get me well over 15mpg as the truck has taller tires than a 1/2 ton truck w/ a 3.73

[americanmusc1e]

I ordered a rebuild kit for the Q-jet. and I'm gonna order an ignition tune-up kit.

I think I'll buy some rear-end fluid and change the fluid in the rear-end and check the gears at the same time.

[rscam]

Creative math will get you any MPG you want. It works for alot of people on here.

[70duramax]

lol! thats funny!

[Jim_PA]

Originally Posted by 69GMCLonghorn

Trying to get the carb to be on the idle jets when cruising will not happen as the carb is OFF the idle jets by the time you get just over 1000 - 1200 RPM. Maybe he could put a Gear Vendors behind the 700R and get those R's down a little more???

Seriously, just to prove me wrong or everybody else wrong, drive it in 3rd for an entire tank of gas and see what your mileage does. If it's worse, then gearing isn't your problem and tuning IS.

The idle circuit is not a function of RPM's. It has everything to do with the amount your throttle blades are open. Modify the idle circuit properly which will allow you to close the throttle blades more at idle. (I'm still betting he's almost pulling from the main jets at idle...) Do a little research on idle circuits before posting something that sounds way off base.

Use full manifold vacuum -

[Jim_PA]

Originally Posted by Longhorn Man

your RPMs are too low, you are under the torque curve. While you aren't ligginh it, you are under the curve and it takes too much throttle to try and maintain the speed.
9 times out of 10, a 3.07 and overdrive are not a good combination.

With his combo, I doubt torque at 1900 RPM's is a problem.

['72customdeluxe]

1900rpm sounds peachy to me. You'd only be running 2300 rpm if you had 3.08's and 31's with no overdrive. With all that torque you should be gaining mileage. I still think you might have 3.73's because if it's running 1900 rpm like you said with 3.08's in overdrive locked up you'd be running 85 mph according to the bowtieoverdrives.com calculator. I think it'd be pretty easy to tell if you were going 85 in one of these trucks. But 3.73's match the description perfectly. You'll just have to gps it or count gears like you said when you pull the diff cover. You can also do the whole put it on jackstands and count driveshaft to tire revs thing without opening it

[1969 GMC]

Originally Posted by 69GMCLonghorn

As stated above, if you want to see what lower gears (numerically higher) will do for you, keep it in 3rd for an entire tank of gas and don't change any other driving habits, etc. If your mileage drops, don't change the rear end.

Kind of apples and oranges, but I used to have an '88 Buick Lesabre. It had the 3800 V6, 2.73 gears in the transaxle (4T60) plus the overdrive in 4th gear. I would consistently get the same fuel mileage (between 27-29) in 3rd gear or 4th on the highway. That car didn't have a tach, though.