I've been looking at all overdrive options for my 390 with tri-power and 8.8 rearend. All of which i think are rediculously expensive. So I'm looking at C6 options but im worried about what gear to run and how it will effect milelage and performance. What do you guys think is the right thing to do. Spend the money on overdrive or will I be happy with a built C6?????

 

Howdy,

It depends on what you want to do with your truck.

If you want to be able to tow a 5000lb trailer, you'll need fairly low gears to do it.

If you want to run on the freeway and get good mileage, you'll want "tall" gears.

If you want both, you'll either need an OD unit or a trans with OD built-in. Both are not cheap.....

All of the above will depend on your tire size, axle ratio, and final transmission drive ratio.

There's several online calculators to help you decide what to do....you just plug in the tire size, gear ratios (or trans type) and it tells you what axle gears you'll need....

With the gearing, tire size and trans I have for example, I'll turn around 2100 RPM at 60mph



Regards,


Rick

 

Mileage? MILEAGE???? You are going with a 390 tripower and you are concerned about your mileage? Let me solve you concerns with one word - yardage!

390 is going to have more than enough power to pull a more highly geared drive train. As Rick mentioned this of course depends on what you are going to use the truck for. If you want to jump off the line and eat up 10K miles of tire at a shot, or tow that 5000 pounbd trailer then something lower is better. But for general driving - both around town and on the freeway, you'll have no problems with higher gearing - and it WILL improve your mileage by about 2MPG. (even on a good day I think the best mileage I have ever gotten is 14 mpg - freeway, wind behind) going with a larger rear tire to balance out the drive rpm at mph also helped. I went from an 81" to an 88" roll out tire (245/60R15s to 235/75R15s) That change added 1MPG.

So, it also depends on what size rear wheels/tires you intend to put on the truck. Lets assume for a moment you want something that is not too big but will fill in the wheel wells. I have a 390 C6 and run 235/75R15s with a roll out distance of 88 inches.

If you want to drive at freeway speeds (65-70) at a reasonably low rpm for your 390 (about 2300-2500 rpm) with the size tires I mentioned above then you will need to gear your rear end at 2.80. 3.00 would also be good.

No overdrive will be necessary.

On my truck (again 390 w750cfm carb/C6/2.80 rear and 235/75R15s) I am travelling 65 mph at 2300 rpm, and 75 mph at 2500 rpm. I have a trailer that is the back half of an F1 in which I loaded 500 pounds of bagged concrete and could still easily accelerate up a 6% hill (ate gas like crazy, but it did accellerate fairly well).

Here's a great RPM calculator:

http://www.et-studios.com/motorsports/gears/gears.html

Your C6 ratios are 2.46/1.46/1.0/R2.18. Use roll out distance on the calculator for the most accurate computations. Mine is spot on.

 

390 C6 with 309 gears in my Uni. I have pulled loaded car dolly and auto transport trailers back and forth cross country. It has no problem holding freeway speed all day. My .02

 

My sentiments exactly! Put a built C6 behind that beast and run something like a 2:75 gear. It should burn the tires off with any gear..