700r4 is in.

[Buckman]

I just got my truck back with the 700r4 in it. It seams that the gears in the rear end are way to hi (3.07). Going down the road I will be running 55 mph and the rpm will be 1500, at 60 mph it's turning about 1700. My question is what are most people using with the 700r4 in a auto. What is the best ratio to have to get the best gas mileage? The truck is a 71 gmc swb with a new 355 engine.

[70c10swb]

good all around gear would be 3.73.

expect about 2200 rpm @60 mph

[jhow66]

If you are wanting better mileage stay with the 3.07 or go even higher (smaller number). The less RPM you turn the better the mileage.

[ratrod67]

I was running a 3:73 ratio as well.

[BubbaGreen]

3.73 cant be beat

[hoser]

Had a 82 Blazer with the diesel, 700R4 and 3:08 got about 28 mpg on hwy 86 Suburban with 3.54 gears 350 engine 700R4 got about 13-14 hwy currently have a 90 Suburban 700R4 3:73 EFI getting 14 mpg

Dunno about the RPMs never checked have no tach, really dont matter it is what it is heh

All 3 meet my towing needs (about 3500 lbs trailer loaded) with the diesel I just took it out of OD when towing, at 70 mph with the 3.73 or the 3.54 I was in OD with the trailer..

[Senor_paco]

ok guys, here's the deal. you gotta figure your tie sizes, what you're gonna use the truck for (hauling, commuting, etc). I have 3.07 in mine as well, but got some lower profile tires, s othat effectively upped my ratio. 3.07/.9, comes out to around 3.42 or so. big difference in rpm (my .02). Use this guide to calculate tire size ,and multiply by your rear gear http://www.miata.net/garage/tirecalc.html
Remember, with the drag you're pushing, you don't want to load up your drivetrain too much, again, just my .02. with the 700r4, you've got about %30 od, but I'd go with 3.73, or even 4.10. That'll come out to about the same rpm with the same tires as 3.07. It'll be kinda loud in the back though.

[Tx Firefighter]

I've run 3.73 with overdrive and normal 28 inch tires. It's a good combo, but I like a 3.40-ish ratio better with overdrive.

A great majority of the 88-98 trucks used overdrive and 3.42 gears. It's a great combination. Only problem is that the 67-72 trucks usually had either 3.73 or 3.08 only. A 3.42 would have to be built up.

[84 PUMPKIN]

Originally Posted by Buckman

I just got my truck back with the 700r4 in it. It seams that the gears in the rear end are way to hi (3.07). Going down the road I will be running 55 mph and the rpm will be 1500, at 60 mph it's turning about 1700. My question is what are most people using with the 700r4 in a auto. What is the best ratio to have to get the best gas mileage? The truck is a 71 gmc swb with a new 355 engine.

You have the perfect combination if your daily commute includes the Bonneville Salt Flats. Most people think that installing a 700R4 in place of a TH350/400 will instantly give them great gas mileage. Tire size, engine size, driving style, and driving conditions all play a role in the end result. Most stock 350 engines like to cruise between 2150 and 2500 rpm's. If you decide to change your rear gear to a 3.73 you'll be right back where you started as far as rpm's.

Here's an example using my '84 C10 truck.

Factory setup
700R4/ 2.73 gears/ 28" tires = 1600 rpm @ 70 mph (A turd from the factory)

Today
TH350/ 2.73 gears/ 28" tires = 2300 rpm @ 70 mph (Just about right)

If I spent $2000.00 (or more) on a 700R4 rebuild and a gear swap
700R4/ 3.73 gears/ 28" tires = 2200 rpm @ 70 mph (Just about right)

[PanelDeland]

Most of the factory ratio's I've seen were in the 3.42 range.most of them with factory tire size run about 2000-2300 rpms at 65-70 that must be about right or the factory woulda done it difert.

[Buckman]

84 pumpkin, I believe your right, if I put in 3.73 gears I will be back where I first started. Can I put in the 3.42 gears in with the rear end that I have?

[guyryan100]

I have a 700r4 with 3.73 rear end and hate it. I'd trade for your grears in a second. Fulls like I could tow a camper behind me but the truck is not agressive enough for daily/fun driving.

[84 PUMPKIN]

Originally Posted by Buckman

84 pumpkin, I believe your right, if I put in 3.73 gears I will be back where I first started. Can I put in the 3.42 gears in with the rear end that I have?

Swapping gears is an option but I'd recommend having a professional do it. Another option would be to swap rear ends if you can find one with the gear ratio that you want. My dad's '72 has 3.40's from the factory so that might work for you. Here's what you'll have if you go with 3.40's.

700R4/ 3.40 gear ratio/ 28" tires @ 70 mph = 2000

The 28" tire is a 275/60-15 that I run on all four corners of my '84 C10. Tire diameter has a ton to do with the final ratio so keep that in mind. That same 700R4/3.40 with a 26" tire will give you 2200 rpm's at 70 mph.

[Longhorn Man]

I'll bet a punch in the gut that your MPG will not go up by much... if at all. Lower RPMs do NOT always mean better MPG. your freeway RPM there is simply too low and it is under the curve.
If lower is better was true, then all the new stuff would have 2.50 or so gears.... or a straight one to one and overdrive.
It simply wouldn't work.


Try the 3.42, that is probably your absolut best bet.



Onless you have a motor with gorlla low end torque like a 455, any caddy big block, or a diesel.

[Boog]

I can offer some experience with the highgeared rears in a daily driver.
My 88 GMC
305, 700r4, all stock
2.73 gear
28" tires
I guess the thinking behind my truck being spec'd with this gear was to lower the rpm to increase the mpg. That can be true to a point. It lowered the rpm to the point the engine is lugging and the tranny hunts for the right gear when the slightest headwind or incline is encountered. My first thought was that the engine is just that weak but not so. It is running well below the torque peak.
I started running the truck in drive instead of overdrive and the mileage picked up. Running it at 70 or more in drive wasn't a strain either. So for the past 2 years I have been running my truck full time in D, not OD. It still runs cool, has a bit more pickup and is still getting more mpg.
The 350 engines will not be as adversely affected by the higher gear due to more available torque and hp.
Using the gear calculators I believe the 3.73 would be a great gear for mostly around town and towing things. The 3.42 may be a better choice for mostly highway or more all around driving. The target engine rpm should probably be in the 2,000 to 2,300 rpm range at cruise speed for optimal mileage with the small V8s.
I have my eye out for a replacement rear end for my truck. I plan to keep it a while longer so it's worth the investment.

Buckman. In your case I don't think the 67-72 trucks came with 3.42 but you can most likely buy aftermarket and have them installed. Or adapt a later model rear end to your truck. You will be happier with lower gears. Until then try running it in Drive instead of Overdrive and see the difference.

[Sonny]

I just did the swap from a TH400 to a 700R4 (3.73 gears) and I highly recommend the swap. The only problem I have encountered is the performance serpentine pulley kit has slowed down the rpm of the alternator enough where I have to rev the motor over 2000rpm to get it to engage. With the tires I have on I run 2000 rpm and right at 68-70 mph. I am still computing MPG change. I am figuring right now I have gone from 8 mpg to about 14 mpg if I keep my foot out of it. But it's so fun to put my foot in it now. It will roast em all the way to 4th! What a rush!

guyryan100, I don't know what your setup is but I enjoy my truck tons more since the swap. Stoplight to stoplight fun and now it has highway legs too!

[Longhorn Man]

Sonny, if you don't charge below 2000 RPM... you need to start looking at your charging system...something isn't right.

[davidh]

It seems like a lot of the 88-93GMs or so 700r trucks had 3.42 gears so I think that would be a good one.

[FormerMember]

Originally Posted by Longhorn Man

Sonny, if you don't charge below 2000 RPM... you need to start looking at your charging system...something isn't right.

Sonny, sounds like you need to hook up the "exciter" wire on your altenator to a circuit that can actually sense a voltage drop and turn on your altenator before 2K rpm.

[72MARIO]

I have a 700R with 3:73's 28"tire with my driving style the 3:42ish would be keeping the rpms around 2000-2200 I drive a little fast. Lots of open road's and highway mainly. I like the lock up on a toggle switch that save alot of rpms too.

[Striker169]

side question do you have to have a high hump for the trans?

[72MARIO]

No high hump needed. Just make sure you cab mounts are not old and saggy. As long as the cab mounts are good you should be fine.

[Richard]

I have 3.42's in my 94. I bought it that way knowing I was going to lower it and run 275/40/17's in the back. Think it is about a 25.5" tire. The combo was perfect. The low thing got old after a few years and I raised it back up to stock with 275/60/17's. As said it has no power and is always hunting for the right gear, needs 3.73's. One of the things I hate about the 700R4 or 4L60E (mine) is the larger gap between 1st and 2nd. It does not transfer good power for a driver without good tire and gear selection.

Wanted to add that my milage actually decreased with the larger tire as well.

[Buckman]

Thangs alot guys, It looks like I'll do like Boog did, I'll run it in drive and change to 3.42 gears when I save up for some cash. This old truck is getting a little expenses put I'm having fun.

[67ChevyRedneck]

My 67 has the 350/700R4/3.73 rear. I don't know my tire size, but I turn about 2400 rpm at 75 mph. I get a hair over 15 mpg going that fast. I did a long trip on a slower, mostly no stops highway going 55-60 the whole way, I got almost 16.5mpg that trip. I get about 13 around town. I used to get 11-12 town or highway with my I6, 3spd, 3.73. My 00 Z71 has the 5.3 and 4l80??? which is a .67 vs the 700r4 .70 OD, and the truck has a 3.73. 31" tires and 2300RPMs @ 75 mph and 18.2 mpg. This thread answered my question I was going to ask.... is 3.08 too high.... guess the 3.73 stays....