I used to have a GMC K-2500 with a 6.2 NA (later had a Banks turbo) that had a GV overdrive. Some observations:
It flat-out improved my MPG from 20 to 22 MPG. The 6.2 was wimpy but economical.
The GV is a well-made piece of machinery. The electrical lines are exposed so you must protect them or the thing will drive you nuts. Mechanically...bulletproof. It uses planetary spur gears for reduction and has a slight spur gear whine when engaged.
The overdrive ratio is 0.8:1. This isn't quite as deep as the 0.72 built into a ZF-6-650.
I will flat guarantee you it will destroy automatics like Godzilla destroyed Tokyo. Spectacular failures...scattering parts and ATF down the road. Almost always in Interstate driving. I never though I'd live to see the day a TH400 could be broken but the combo of 6.2 diesel (one of the wimpiest ever) and the GV trashed one every 12,000 miles or so. Even an Allison torque converter didn't help. Ford, Dodge, or even the Allison 1000 behind Duramaxes will be wrecked by these units. Maybe a Brian or a Suncoast can make a special that will stand up, but I gotta see it to believe it. The GV slows down engine RPM at a given road speed and the automatic cannot pump enough ATF to the cooler and they get hot and scatter parts.
A GV works fine with a manual if you can live with two OD gear meshes.
Reducing engine RPM at a given road speed does improve mileage by reducing engine frictional HP. This is why my 3.08 gears work. I've done a bunch of research on this and I'll post another thread on the subject.
Optimal RPM for MPG? It will undoubtedly vary from engine to engine but I'm of theb opinion that 1200 RPM @70MPH is about optimal. Five feet forward per engine revolution. My 7.3 is an unmitigated torque monster and can handle this. I'll be posting more on the subject.
I guess it's the combination of torque and low rpms.
I just don't see how lowering the RPMS by that amount could hurt my truck cruising empty on the road.
In my 80 econo program I hit O/D at 40mph with the tc locked, and can cruise there all day long.
If this doesn't hurt the auto trans, why would the GV running on the freeway?
Towing may be a different story, but empty cruising at 70 the GV would take 3:73's down to 1600 rpm, from 2000 rpms stock, right?
Seems like it's still turning enough to lubricate the trans.
"...empty cruising at 70 the GV would take 3:73's down to 1600 rpm, from 2000 rpms stock, right?
Seems like it's still turning enough to lubricate the trans."
That's what I thought when I put the GV on my GMC. Practical experience showed that to be in error.
Maybe you need to consult an automatic specialist like Brian or Suncoast. I will defer to them on automatics. Frankly, I have completely given up on frail, expensive, inefficient automatics and don't waste much in the way of brain cells on them. Keep in mind, with the GV engaged, your actual ratio becomes 2.98:1, and you pick up some additional load from the additional gear mesh.
I hear ya dave. Although my auto isn't frail and weak.
Fortified with a better torque converter and valve body it is holding an amazing amount of HP and TQ from an otherwise stock tranny.
The aftermarket modded ones are even more amazing. Lot of the top guys running 500-600-700+ HP and god knows at least double that torque through automatics and they are performing well when built by a reputable builder.
IMO John Wood and BTS being among the best out there.
I had a 94 Chevy dually with a 454 and a 4L80E that towed it's whole life with a GV and never had any issue mechanically. The GV will not destroy any transmission if it is operated correctly. The way you drive it will dictate how long it will last. I had one on a 6.9L C6 truck as well. 278K miles on the C6 and never had it apart.
The GV is quite aggressive when it shifts and will shock load the trans if you let it shift under load. I just back out of the throttle and shift the GV. I do the same thing on my non-GV equipped SD when towing. When coming up to a grade I know I can't pull in O/D I back off the throttle, lockout O/D and continue on.
The more gears you have the better. If I could squeeze an RTLO low inertia 18speed in the truck I'd have one in a heartbeat.
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