Rear gear ratio

[BigBlueIron]

I need to start planning a complete rear end swap for the future. 1950 F1 twin turbo 390 manual trans with OD splitter. Current rear is Dana 41 or 44  w/4.27 ratio has somehow held in this long but it only a matter of time and I would rather replace it before failure. I try not to be to rough on it at slow speeds in fear of snapping an axle. Runs 2050rpm @ 55.

Mainly street driven with most 2 lane roads having a 55 or 60 mph limit, which I try to stay close to.. I do plan on visiting the track at some point but not much. I would also like to take it down the interstate without being run over, 70mph limit here. But again not much.

Axle will either be an 8.8 out of a late model explorer or a 9".

Curious what anyone might think.

[e philpott]

what tire size are you going to be running ??

[BattlestarGalactic]

I would gear it so your RPM is comfortable at your typical highway speed would be in OD.   Too fast of gear and you'll be wanting to downshift too much on hills or such.   With boost it might not be such a problem?  My thought is cruise the truck in OD at your slowest rpm that it maintains easily.  Then do the math for 70 mph at that RPM.

Since it's not a full blown race car, optimal gearing for the track is a moot point. 

[cjshaker]

Something doesn't add up? If you're running 55 @ 2050 with a 4.27 ratio, you'd have to have a 38"+ tire. Is that what you're running? Those are off-road mudding and rock crawling size tires. Banging gears with a toploader and a mud truck with 38" tires is not exactly ideal, and probably would not last long.

[BigBlueIron]

what tire size are you going to be running ??

31x11

Something doesn't add up? If you're running 55 @ 2050 with a 4.27 ratio, you'd have to have a 38"+ tire. Is that what you're running? Those are off-road mudding and rock crawling size tires. Banging gears with a toploader and a mud truck with 38" tires is not exactly ideal, and probably would not last long.

Advance adaptors Ranger overdrive. I believe it's a .76 ratio.

[cjshaker]

Well duh, I forgot about the overdrive 
Larry gave you good advice, but I would think that going up hills with too low a gear and boost would likely bring issues of detonation, unless some sort of boost control or timing control were in the mix. I don't know much (anything actually) about the Advance adaptors Ranger overdrive, but if it's easy to drop in and out of, then that may not be an issue.

I don't think I'd go with the 8.8, unless you're planning on seriously beefing it up. They're a bit weak to begin with, and big tires and boost, coupled with a heavy vehicle weight. is a killer combo....as in 8.8 killer.

[57 lima bean]

Mainly street driven with most 2 lane roads having a 55 or 60 mph limit, which I try to stay close to..

       
                     
                                Thats Funny

[BigBlueIron]

I don't think I'd go with the 8.8, unless you're planning on seriously beefing it up. They're a bit weak to begin with, and big tires and boost, coupled with a heavy vehicle weight. is a killer combo....as in 8.8 killer.

Well to be honest to begin with I didn't even consider it. But seems a lot of people are running them, apparently the Explorers came with 31 spline axles in a 3.73 or 4.10 with limited slip and disc brakes. That gets fairly attractive at $200.  Probably need another $300 for a girdle and odds and ends.

[BattlestarGalactic]

With a lack of traction I think the 8.8 would be fine.   I have friends drag racing stick shift Fox bodies into the low 10' with an 8.8.  Yes, the tubes were welded and braced, plus a good rear cover.  Wheel hop would likely kill it, but so would any other rearend.

I've braced a few, running tubes from the pinion area over to the control arm mounting area on the tube.  Keeps it from bowing in the middle and letting the axle tube twist in the center section.  Welding the tube at the casting works to a point, but with dissimilar metals it will fracture.

[chris401]

With a 9" I prefer the 3.70 gear. 275/60 R15 truck turned around 2500 at 60 mph without OD.

EDIT: In the 90's it was possible to find a 9" 3.70:1 Equalock in an 65-72 I-6 3 speed manual F-100. Those were the years I found them in. Not sure how the old units would hold up behind turbo'd 390.

[jayb]

I voted 3.73, but I think I'd actually run a 3.89 or 3.91, if that was an option.

[BigBlueIron]

I would gear it so your RPM is comfortable at your typical highway speed would be in OD.   Too fast of gear and you'll be wanting to downshift too much on hills or such.   With boost it might not be such a problem?  My thought is cruise the truck in OD at your slowest rpm that it maintains easily.  Then do the math for 70 mph at that RPM.

So this is good and practical advise. And certainly something I have been thinking about, just helps when someone says it to you. The area around me is hilly and I'm used to driving trucks geared low enough that you never have to shift down at speed.

I also did some more research on the actual ratio of the OD, my math was a little off and I'm fairly confident on the tire size and accuracy of the tach.  The OD is listed as 27% over which truly translates to 0.80, I had been using .76. I have weighed the truck in the past but can not remember the actual number, I want to say just under 4000lbs full of fuel and my portly butt in the seat.
Here are some comparisons for viewing, might change your opinion. I'm worried about lugging to much.

[BigBlueIron]

And for comparison in 1 to 1 no OD