Granny 4 speed and more top end?

[jimmydean]

I just put a new motor in my '69 LWB 1/2 ton. It has the granny 4 speed and I seem to be high in the RPM's on the freeway. A guy at work said he changed the rear end gears to make his 4 speed a 4 speed (take off in first) and that it did well on the freeway in 4th.

Is that the way to go, or should I swap the trans for a T10 or something more freeway friendly? I'm keeping it stick, so a 700r4 is out of the question. If I should swap gears, then what ratio should I go with? I'm guessing it's a 3.07 gear now, I know it's posi.

Thanks for the help, it's the last thing I need for my drivetrain.

[Russell]

How many rpms are you pulling?

With 4.10s and an SM465, I was pulling about 3200 rpm at 65 mph before.

I suppose you possibly could change the rear gears like he did, but that'd have to have been a pretty drastic change. I'd personally just find an NV4500, and have a granny gear tranny with a 5th gear with the same overdrive ratio as a 700r4. They are just as tough as the old SM-series trannys too.

[jku72]

I have a 350 4 speed in my suburban. I swapped to a 3.07 rear gear and run around 2500 rpm at full highway speed, with 31 inch tall tires. The ultra low first still gives great power when taking off, and the high rear gears give good highway manners. The trade off is the wide spacing in the SM465 gear ratios. Forget racing anyone.
But, for a decent driving truck, this combo works well. I even considered going to a 2.76 rear, even so, with a 6:1 first gear, that gives better than a 12:1 final drive in first gear for take off. Compare with other 4 speeds, and even with a 3.73 the first gear final drive is not that low. I dont know the first gear ration in a T10, but its only 3 or less. Thats why the T10 trucks need more gear.

[Palf70Step]

Unless you go with an overdrive, 4th, drive is a 1 to 1 ratio. It's mostly about the rear gearing. a 3.07 rear your engine is not going to rev high. If it winds high, then you probably have 4:10s. If it came with a granny low, my bet it also came with 4:10s. Stock on our trucks was a 3.73. If you are going to work the truck at all, but still want to keep the RPMs down on the highway, keep your granny and change the rear to a 3:08 or 3:07. I think you'll have a good combo there.

[jimmydean]

I will check the glovebox and see what it says. based on the search feature, 3.07 is what kept coming up as the rear end ratio.

I'm looking for something more street-able, but I do tow with it sometimes. I like the granny first when I need it, but the motor seems to be working too hard at 65 mph (I don't have a tach, but it sounds high). If it is 4.10, then I will for sure swap it out. I would rather do that then deal with a tranny change at this point.

Thanks.

[shelby987]

I can tell you that your best bet to come to a solution here is to gather as much information as possible, if you don't want to have a tach mounted on your dash all the time, go on ebay and buy a cheapy tach, and use it just to get some baseline numbers.
Also, with respect to the gears, as many have found, what the glovebox reads, and what it actually is, are not always the same, jack up the rear, and rotate the tire while counting the revolutions on the pinion shaft....just over 3 full pinion rotations to one full tire rotation will equal 3.07's....people are likely right though, especially if you didn't have a creeper before, you would have bogged down a bit with 3.07 in first.
To lower the RPM's on the highway, you have a couple of options, change the rear gears and keep the sm465 (this is what richmond does with their 5 spds if memory serves, 5th is still direct).
Or, you can hunt around for a tranny that gives you the best of both worlds, saginaw made a 4 spd. with a 3.50 first gear, that would probably work well with a 3.43 rear and still allow you to get a little wild if someone pushes your buttons....or a muncie close ratio trans....of course all of these mean a new trans and likely new gears........crunch the numbers for the rpm range you are looking for against your setup before buying anything....and keep in mind....your truck has to be producing the power required to move at highway speeds.....so make sure you are still running the engine in the meaty part of your powerband when you make these decisions.

[jimmydean]

Well, I know the truck is unmodified (before I got a hold of it anyway) and the glove box says 3.73 posi.

I think swaping to a 3.07 is the way to go for now since it's a common ratio and shouldn't be hard to get.