69 3/4 ton highway speed

[8bolt]

What kind of highway speed can I expect with a 350 motor 400 turbo auto trans and factory 3/4 ton rear end. Are there any better straight bolt on option. Thank you.

[68gmsee]

No reason you can't cruise at 70. The problem is going to be on gas mileage that you expect to get. The faster you go the less mpg.

Some years ago, I borrowed a big block 3/4 ton from a buddy to haul some stuff. I could actually see the gas gauge moving as I was going down the highway. I think the sucker maybe got 8-10 mpg on the highway at best.

[69GMCLonghorn]

You will be able to do approximately 120 MPH!!! Of course, you'll be taching 8000 RPM, but with a mild cam it shouldn't be a problem...

Ok, seriously, if you have a 4.10 rear end and average 31/32" tires, you're looking at taching right around 3000 RPM at 70 MPH, give or take a little. I ran mine between 65 and 70 and it seemed pretty comfortable.

Also, make sure you calibrate your speedo and tach! So many of them don't read accurate for a variety of reasons... Case in point: When I bought my truck, it said I was doing 3700 RPM at 60 MPH!!! The tach was off by 30% and the speedo was off by 20%!!

[Sinister]

I agree with the above. In my '69 C-20, 350, Th400, 4:10 gears cruise at 70 mph, right around 3000 rpm.
If you want better then that, time for a Gear Vendors OD, 700R4 or a new rear end.

[69GMCLonghorn]

Originally Posted by Sinister

I agree with the above. In my '69 C-20, 350, Th400, 4:10 gears cruise at 70 mph, right around 3000 rpm.
If you want better then that, time for a Gear Vendors OD, 700R4 or a new rear end.

Or all of the above! I was so tired of taching 3K on the highway, I would have settled for a little lugging at 70 MPH... Imagine... 3.42's, 700R4, AND a gear vendors... Ahhh... sweet! I could always downshift, 7 TIMES!!

[capev86]

yeah.....perfect combination if you have a 1/2 ton that doesn't pull anything, you generally drive in flat country and you have a serious big block or preferably a diesel that makes the torque to compensate for absolutely no mechanical advantage.

3.42 x .7 x . 7 = 1.67.

compare that to the FD in my 89 lesabred fwd 3.8 in overdrive is 2.0:1.....which is very extremely soft.

don't forget that trucks have taller tires than cars.......that is an overdrive in itself!

[69GMCLonghorn]

Originally Posted by capev86

yeah.....perfect combination if you have a 1/2 ton that doesn't pull anything, you generally drive in flat country and you have a serious big block or preferably a diesel that makes the torque to compensate for absolutely no mechanical advantage.

3.42 x .7 x . 7 = 1.67.

compare that to the FD in my 89 lesabred fwd 3.8 in overdrive is 2.0:1.....which is very extremely soft.

don't forget that trucks have taller tires than cars.......that is an overdrive in itself!

I was exaggerating. Too much of a good thing (OD) can be a bad thing, I realize.

[falvarez12001]

don't have a tach on mine but you can hit 130 with mild cam upgrades, high flow air filter, but the gas mileage really sucks

[Longhorn Man]

tall tires is nothing like an overdrive, it only adds to the total gear ratio. The trans is still a one to one.

[2tons of fun]

uh, taller tires at 1:1 will get you farther per revolution.

they will also increase motor load and consume more fuel.



my new project is a 3.21 14bff with the tallest d/e rated highway tire i can find, and a 402/400..........

[2tons of fun]

Originally Posted by 8bolt

What kind of highway speed can I expect with a 350 motor 400 turbo auto trans and factory 3/4 ton rear end. Are there any better straight bolt on option. Thank you.

depnding on which rear end you have, d60 or eaton you can swap in a 3.33 ish gears.

[Devil Pup]

Originally Posted by 8bolt

What kind of highway speed can I expect with a 350 motor 400 turbo auto trans and factory 3/4 ton rear end. Are there any better straight bolt on option. Thank you.

I have a 69 3/4 ton that used to have a four speed with granny gear and 4.10 in the rear. Then i changed out for a turbo 350 with a shift kit and mildly high stall torque converter. it moves well but on the free way it just seems "to me" that it is pushing itself a bit too much. I have 31's on it and im upgrading to some 33's to see a bit of improvement. if it still cant run on the free way im gonna go with a gear vendors or 700r4

[special-K]

The GMC Dana60s were available in 3.54 ratio.I still hold it at 65 on the hiway.It doesn`t hurt to stay at the speed limit in an old truck.I used to run my 4.10 trucks all day long at 75-80.It never hurt a thing and sure sounded sweet!I`m slowed up a bit in all my stuff.My Harley is falling apart from to much speed on the hiway.I keep my speed spurts for a quick thrill these days.

[69GMCLonghorn]

Originally Posted by Devil Pup

I have a 69 3/4 ton that used to have a four speed with granny gear and 4.10 in the rear. Then i changed out for a turbo 350 with a shift kit and mildly high stall torque converter. it moves well but on the free way it just seems "to me" that it is pushing itself a bit too much. I have 31's on it and im upgrading to some 33's to see a bit of improvement. if it still cant run on the free way im gonna go with a gear vendors or 700r4

By going with 33's, you'll only see about a 200 RPM drop. Keep in mind you will have to recalibrate your speedo to realize that drop. In other words, the speedo and tach will read the same as when you had 31's, but you'll actually be going slightly faster... and I mean SLIGHTLY. Also, consider that you will probably lose more in MPG than you'll gain in drivability. Go with the O/D if you have that option.

[Longhorn Man]

the danas with the 3.54 werent only in GMC trucks... but all big block automatic 3/4 ton trucks unless otherwise optioned.

[2tons of fun]

there are more d60s than just gm

why did you go from a 465 to a 350? you went from a 1:1 tranny to a 1:1 tranny, zero gain.

then add a stall onto it? now you are always spinning at least xxxx[what ever your stall is] when you could only be spinning 2/3 or half that.

going to a 700r4, a 200r4, a 4l60, or a 4l80 will get you an overdrive and then you will actualy see an improvement in mpg.

big tires will get you more speed for th esame rpm but will take more fuel to push.

[special-K]

Changing tire diameter has no effect on how your tach will read,only the speedo is effected.The tach reads engine speed off of the engine.
It`s true Dana60s were available in Chevys and came in other makes.I tend to call them GMC rears since they were standard equipment and we were only talking about GM rears.

[capev86]

the tach will read the correct engine speed regardless of tire size, however....bigger tires WILL result in a slower engine speed at a given road speed/gear etc. any reduction in engine speed will save gas, reduce engine wear and noise. with the aggressive rear gears our trucks have, taller tires are not going to hurt gas mileage but improve it. and you would have to have go with really big tires to put too much load on the engine with even a 3.08 gear. a good rear gear coupled with an overdrive is the best bang for the buck....dual purpose!

when i swap from a 4.57 to a 4.10 gear in my 3/4 ton burb, it will be a 10% drop in gear ratio.....and hopefully drop engine speed by 300rpm or so. i will take what i can get. an OD unit on the saginaw 3OT will make it comfortable on the highway. i'm hoping to get the current best gas mileage of 11.1 up closer to 15 and then around 17-18 with the overdrive.