Rear end question.

[shrtbx]

Seems like i read a thread once that said better mileage if you get gears that match up better with your engine. I have an 86 swb with an edlebrock 1904 on top of a 350 with a 700 r4 trans. The truck has 3:08s in the back. I read a few posts that said you can waste gas with the 3:08s because it takes to long to get up to speed and maintain a certain speed without having to give it gas. I hope this isnt too confusing but could i get some input on this.

[LONGHAIR]

Yes. You should get better mileage and driveability with lower gears. That is way too high for a 700r4. The factory actually went as high as 2.73 in those trucks.

[shrtbx]

Thats inaccurate the truck had a 305 in it. and it is infact a 3:08 from the factory. I still have the little credit card looking thing that came from the dealer that has the options on it. The options are also in the glove box too. I think 2:73 is cutting it close. 2:73 or lower the 700 r4 doesnt work properly from what i have seen.

[shrtbx]

LOL You are right..My brain is toast today and i mis read your reply.....So i proboably will get better gas mileage with a set of 3:73s.

[LONGHAIR]

I have personally taken 2.73s out of 4X4 trucks. They were bad enough on stock sized tires, lifted and larger tires just kills them. You would think that the factory would have done some testing with this combo? Apparently not...LOL

[C-10volvo]

Depending on the tire size, a 3.42 gear is what you want.

[shrtbx]

Yeah i have heard that the 2:73s and on some occasions the 3:08 gears dont let the 700 r4 operate the way its suppose to. Im figuring 3:73 gears will put me in the powerband that i want to be in. Im just curious what others think about that. Im got 16 mpg with the 1406 carb i had on it. I have been driving back and forth to san diego from los angeles on the weekends. All tuned up new plugs wires dist etc. I think alot of the time its your driving style that makes a difference, but i guess thats to be seen.

[LONGHAIR]

Some of it depends on the speed you are traveling too. Back then the Feds had the speed down to 55MPH. Now with higher speed, and mostly highway driving, the 3.08 may not be so bad. Does your transmission shift in and out of over-drive a lot? RPM at cruise speed?
A gear change can be expensive, it may take some time to get that back unless you get a big improvement.

[olivedrab]

the gears you want depends on alot of things..for instance. where do you do most of your driving? is this a 4wd or 2wd? what size tires? and yes keeping it under 65 mph on the interstate would def help because gas mileage only decreases the faster you go most of the time....and depends on how you drive it...

[johnboyon20s]

i dunno if it makes a difference with fuel injection but i had 87 stock motor long length headers dual exhaust no cats with 700r4 and shift kit(shifted hard and fast to 2nd gear) and i had 3.08 gears , it got 16-18 mpg ,

[piecesparts]

Originally Posted by shrtbx

Seems like i read a thread once that said better mileage if you get gears that match up better with your engine. I have an 86 swb with an edlebrock 1904 on top of a 350 with a 700 r4 trans. The truck has 3:08s in the back. I read a few posts that said you can waste gas with the 3:08s because it takes to long to get up to speed and maintain a certain speed without having to give it gas. I hope this isnt too confusing but could i get some input on this.

Been there and done that. The son's truck had a 350C tranny and we stepped up to a 700R4 and with the 3.08 gears he got lousy mileage. We could have gone to a 3.42 gear set and probably done quite well, but I had a chance for 12 bolt posi unit that had 3.73 gears in it and that is what we put in. The truck gained almost 3 MPG and the acceleration is outstanding. We could gain a little more with a change in tire size. With the gearing that we now have the cruise control works well and is not constantly pulling down the shift to get back up to speed and the RPMs o n the highway are around 2500 RPM at 70 MPH. If I changed tires, I would not let it go below 2200 to 2300
RPM.

My 84 GMC has a 3,73 gear set and a 700 R4 tranny (with a 2400 RPM stall converter) attached to a 385 stroker motor, I get around 15 MPG on the highway. I am working the idea of changing tires on it some this summer.

[Pyrotechnic]

I just can't understand why some of these trucks got 2.73's and 700R4's from the factory. I seriously just don't get it.

Almost as bad as newer trucks coming with 3.23's and 4L60E's.

[8T3GMC]

I have 3.08s or higher but I am going to rebuilt the 10 bolt and but a 3.73 gear in. I My truck has a 350 with 700r4. I am puting shift kit in the trans and the 10 bolt.

[SBTork]

Originally Posted by Pyrotechnic

I just can't understand why some of these trucks got 2.73's and 700R4's from the factory. I seriously just don't get it.

Almost as bad as newer trucks coming with 3.23's and 4L60E's.

the '87 I used to have had a 305/700r4/2.73 combination. The truck did not know what it was doing going 65 with cruise control on.

[LEEVON]

I just swapped my TH375 / 2.41 set-up for 700R4 / 3.73 and what a difference! I could see 3.42's being good too depending on tire size (up to 28" maybe).

[8T3GMC]

I am runing 275/60/15 tires. I want a 3.73 But what do you all think would be best.

[piecesparts]

The 275/60/15 a 27.8" diameter tire and would work quite well on a 3.73 geared rear end. Taking an assumption in two different trannys and the RPMs at 70 MPH would be 2209 RPMs for a OD tranny and 3155 RPMs for a NON OD tranny. It is just what are you going to use and what do you want for acceleration, not to mention a top end on your truck.

I would go with the OD tranny if you don't have one already.

[spudz]

Well my 04 z71 came from the factory with 3:73 so I am thinking that anything less then that would suck with an overdrive.

[gchemist]

Keep your cruise speed at 2100-2300 RPMs. I have 3.08's but use 255/60R15 tires. Short tires for a truck. Cruise speed at 70 MPH is 1950 RPMs. A stock engine has max torque sweet spots at 2000-2300 RPMs.

I need 3.42 or 3.73's now. High HP and high gears don't work very well.