I have an 85 Jimmy with a 350/700r4 combo in it with 4.56 gears and 35 inch tires. I don't use it as a DAILY driver but I do drive it to work when the weather is nice, not to mention it's a long drive to places I like to off road. It kills me to drive at highway speeds though. I hate feeling like I'm having to smash my foot through the firewall to maintain 70 mph, plus the engine noise from the high rpms is a bit much to handle on my 45 minute commute to work and/or longer drives not to mention I don't enjoy watching the gas gauge fall rapidly. Don't get me wrong, I know that I can't "have my cake and eat it too" and that different engine, gears, exhaust, etc... would all make a difference. But I'm just interested in transmissions. My Jimmy has a newly rebuilt 700r4 with a shift kit in it. I have been looking around the net and it seems like a 4l80e six speed maybe an option for a transplant. I don't do any CRAZY off-roading. Mostly mudding or 10-15% grades or ledges less than 2 feet high. I do more highway driving than anything but I do like to take it out and play when I have the opportunity and I want it to be able to handle that.

As far as the 4l80e goes I believe I read somewhere there are mods you can make to eliminate the need for a controller? I can't find the forum again and I may have read it wrong. How much difference would the sixth gear make? What's the likely-hood I could do 70 around or even under 2000 rpms if I make no other drive train changes?

Is there a better transmission for the job? I'm only interested in an automatic.

Transmissions aren't my specialty and my knowledge is basically limited to what I have read on the internet over the past few days so please be gentle. Thanks in advance!

 

From your description, the problem isn't your tranny, it is the gears. 4.56 gears and 35's aren't the best combo. Having to mash the skinny pedal to maintain 70 on the highway is a gear problem. Add more gears to the tranny will only make matters worse trying to maintain speed. You need to look at the final gear ratio to keep yourself in the torque band.

 

Is the 6 speed a double overdrive? If so, it might help. If not, there wont be any difference because the final drive ratios will be the same

There are calculators online where you can put in tire height, rear end ratio, and transmission ratio, and it will tell you what speed the vehicle is traveling at each rpm point. Input your setup how it is now, and compare it to how it looks with the six speed's ratios.

 

I do plan to lower the gears to 4.11's in the hopes that will help. I don't want to go to low. The torque is nice to have. How do go about figuring out the final gear ratio?

 

I believe it is a double overdrive according to what I have read. How do I find out what the transmission ratio is? Is it standard for say all 700r4's or whatever type of transmission you have?

 

A google search will yield the transmission ratios. The overdrive ratio for the 700r4 is 0.70:1. A T56 has a sixth gear of 0.63:1. While it is a "taller" gear, it's not really much of an improvement over what you have. You want a higher gear ratio (numerically lower) to spin the engine slower. For example, a 0.50:1 overdrive ratio will spin the engine slower than the 0.70:1 the 700r4 has.

It's all a relationship. The 35" tires and overdrive are helping to tame the 4.56 gears, but those are still really deep gears to have. Do a search for "rpm calculators". It will let you change the tire height, transmission, and rear gears and show you what you rpm will be at different speed. You need to find which will yield the balance of torque and highway manners up need.

Edit: you said 4L80e, right? It's not a double overdrive. It's overdrive ratio is 0.75:1. So the 700r4 you have actually has a taller overdrive ratio. The six speed would make your problem worse.

I did a quick calculation using your 4.56's, the overdrive ratio on the 700r4, and a 35" tire. At 65mph, it's only 1,992 rpm. That seems pretty reasonable to me. What rpm are you seeing at highway speeds?

 

Awesome, thank you! I will search for the calculator. Just to make sure I understand correctly all 700r4's have the same gear ratios? It's not like a rear diff where you can pick what you want i.e. 3.63's, 4.11's, 4.56's etc...?

 

As far as I know, all 700r4's will be the same. Same goes for TH-350, 400, 200-4r, etc. Now, different transmission models will be different, i.e. a 700 will different than a turbo 350. But all 700's will be the same to each other.

Although, it might be possible that some custom built, or aftermarket transmissions might have different ratios put in. But all factory units should be the same.

 

Have you considered and over/underdrive unit or auxilery overdrive?

I'm looking at a chart that shows the 700r4 has the same ratios as my 4l60e. I have 4:10's and 30" tires and run about 2200 RPM at 70mph. You have a steeper gear but a LOT more rollout with a 35" tire. Unless we are missing something your turning a very reasonable RPM.

 

Not familiar with underdrive or an auxiliary overdrive. I'll have to do some reading but no, I haven't considered them.

My Jimmy doesn't have a tach so I'm not certain what rpms I'm running at but this is my third vehicle with the 5.7l/700r4 combo and it is certainly running higher than the 2000 rpm range. If the tire/gear combo should have it running around 2000 rpms at 70 mph I'm thinking maybe the overdrive isn't working?