Any Noticeable change going 3:42 to 3:07?

[kelsfine69]

Well the old Gov-lock 3:42 Posi packed her in last night. So now I'm on the hunt for a another diff. I plan on getting another diff and re-bearing it and doing a full axle housing swap. The problem is finding a 3:42 non posi is almost impossible around here. I don't really need a posi for the truck, it is just a daily driver/hobby. Its a 1987 Silverado 350 Fuel Injected with a 700r4 trans. I found a 3:07 rear end that is in pretty good shape. Will I have a problem with power? mileage? Did the 350/700 combo come with any other diff ratios other than 3:42? Or should I be looking for something else? Thanks Guys

[Clyde65]

I would keep the 3.42, the 3.07 change wont get you much MPG if anything, since you have the 700, keep the gears you have.

[jesdude7789]

well my 1987 has a 2.73 gear. I was told that this was the only year they put that ratio in these trucks and for good reason! I'm looking to get a 3.42 to swap into it. I have lousy off the line acceleration because i don't have enough torque getting to the wheels. I had an 1989 with a 3.07 and it was okay but that rearend broke a couple teeth on me and i did put a 3.42 gear in it and it was much better off the line and towing things so i liked it alot better. gas mileage for me wasn't that much different since i mainly drive it around the city and maybe 30% on the highway and now i do the same thing with my 1987. I would rather have more fun with it than get the best mileage possible out of it. I probably get around 14 city, 17-18 hwy now and maybe 12 city, 15-16 hwy when i did the swap to the 3.42 in my 1989. I also had the holley 502-6 trottle body on it and no other mods. stock other than that. oh and they are both 5.7, 700R4 TBI K ENGINE 2wd long beds that i'm talking about.

OH and what wheels are those? If i'm not mistaken, these trucks have a narrow width between the back wheels than the front wheels. Are your rear wheels backspaced to make them even with the front set?? could you tell me what you have?

[C-10 simplex]

There would be a 10% decrease in power going from 3:42 to 3:07 ( i didn't know they made 3:07, i thought it was 3:08?)

i don't know of the exact formula of why/how degree increase/decrease when changing gear ratios, but i have a chart here that lists all the possible combinations, so if anyone is curious i can answer any proposals.

For example, going from 2.41 to 4.10 gives a 70% increase in power to the rear wheels.


Personally, i wouldn't worry too much about going higher, but that's just me---i don't think i would tow/haul anything too heavy in a 1/2 ton right now coupled with time constraints is why i feel this way if i was in this situation.

Also, if you go with the 3.07 and need "help" with the speedo gears, i have info on that as well.

[jesdude7789]

you're right simplex. it was a 3.08 It's been a long time since i owned that other truck haha. I would be interested in knowing what your chart says about going from my 2.73 to the 3.42? And i would like the info on the speedo gear too. My speedo isn't exactly accurate as it is.
Thanks!

[chevymotocross]

My 87 305/700 combo with 3.08 gear..... wish it had 3.42. Great gas mileage at 18mpg (driving slow), but the truck struggles on hills.

[INSIDIOUS '86]

wouldnt go with 3.08 if u can help it sometimes a low gear gives better gas mileage than a high gear ie 2.73 not to mention more fun

[Goldfinger]

High gears aren't very bad at all. Mine has a 2.73 but I've never had a hill problem i can cruise along at 70+ on the interstate with or without trailer I rarely go over 2.5k rpm. I'm getting about 20-23 with my 305/700 combo. Which i couldn't do without the 2.73. for dd it's great. only drawback is added stress on the power train and low gawr.

[f.monroe]

Originally Posted by C-10 simplex

There would be a 10% decrease in power going from 3:42 to 3:07 ( i didn't know they made 3:07, i thought it was 3:08?)

The ratios from GM for 12 bolt truck rears is 3.07 and I think when they went to the 10 bolt they are listed as 3.08's. Same result but both are correst depending on which rear you have .

Originally Posted by INSIDIOUS '86

wouldnt go with 3.08 if u can help it sometimes a low gear gives better gas mileage than a high gear ie 2.73 not to mention more fun

I agree about the mileage. I had an '87 with the 2.76 gear and found overdrive almost useless on the highway and the truck was not that great on gas mileage. The truck was OK in town since the 700 has a lower first gear. Your tires are short so the 3.07's will probably work for you but will probably lose a little throttle responce and maybe mileage with the 3.08's.
Frank

[INSIDIOUS '86]

Originally Posted by f.monroe

The ratios from GM for 12 bolt truck rears is 3.07 and I think when they went to the 10 bolt they are listed as 3.08's. Same result but both are correst depending on which rear you have .



I agree about the mileage. I had an '87 with the 2.76 gear and found overdrive almost useless on the highway and the truck was not that great on gas mileage. The truck was OK in town since the 700 has a lower first gear. Your tires are short so the 3.07's will probably work for you but will probably lose a little throttle responce and maybe mileage with the 3.08's.
Frank

i found it fun idling just about at 70mph and the 305 could get me any were on the highway in the state on 10 dollars of gas, that was with the 700r4, now with 3.08s and a th350 the power is way better and city mileage is pretty decent for a large bore sbc and big cam that eats fuel

wish i can afford the 3.73 dodge 8,3/4 rear for it

[kelsfine69]

OH and what wheels are those? If i'm not mistaken, these trucks have a narrow width between the back wheels than the front wheels. Are your rear wheels backspaced to make them even with the front set?? could you tell me what you have?[/QUOTE]


Those are the old Slots that I found used and polished up. 10's on the back 7.5" on the front

Thanks for the input guys.

Next question... If I swap in a 12 bolt will the driveshaft bolt up or is the u-joint a differnt size? Might have a line on one

[hatzie]

Originally Posted by jesdude7789

well my 1987 has a 2.73 gear. I was told that this was the only year they put that ratio in these trucks and for good reason!

I had a stripped down base model 85 C10 with the SM465 and 2.73:1 rear. The 2.73 gears, 305 V8, and delay wipers were about the only options on her. That combo got me 14mpg hwy so I'm not complaining.

[INSIDIOUS '86]

[QUOTE=

Next question... If I swap in a 12 bolt will the driveshaft bolt up or is the u-joint a differnt size? Might have a line on one [/QUOTE]

just did this on a friends..... no they wont, you need a new u joint pressed in

[kelsfine69]

Just the U-joint? or the driveshaft too? Whats different about the U-joint?

[C-10 simplex]

Originally Posted by jesdude7789

you're right simplex. it was a 3.08 It's been a long time since i owned that other truck haha. I would be interested in knowing what your chart says about going from my 2.73 to the 3.42? And i would like the info on the speedo gear too. My speedo isn't exactly accurate as it is.
Thanks!

25% torque increase. Give me a day or 2 on the speedo info.

Originally Posted by INSIDIOUS '86

wouldnt go with 3.08 if u can help it sometimes a low gear gives better gas mileage than a high gear ie 2.73 not to mention more fun

That's true sometimes too high is not good. i think they did that mostly for emissions reasons.
i think riding a 10-speed bicycle alot could be very educational regarding gearing/gear selection.

[INSIDIOUS '86]

Originally Posted by kelsfine69

Just the U-joint? or the driveshaft too? Whats different about the U-joint?

the u joint size is different on the 12 bolt i belive. you can have an adapter u joint pressed in though

[woodwright]

Kelsfine, I have 2 3.07 gear sets for a 12 bolt. I'm right below you in the Idaho panhandle.

[kelsfine69]

Thanks guys..The hunt is on..Need to find something close..Hope to find a 3:42 but If I find a 3:07 I have a set of old GM gears here(GM Waranty) to swap in...

[beamn7]

My 85 had factory 305/700r4 w/3:08's in the rear. Trans was too "busy" for my liking and almost idled down the freeway which made for a downshift if I even breathed on the gas pedal. Yanked 'em and put in Richmond 3:73's with a Auburn limited slip carrier and am very satisfied. If I recall it was about $600 bucks for the parts and an afternoon in the driveway.

[grs]

How can I find out what my gear radio is? I have a 305 with 700R4, and tbi. I get about 24 on the highway. It is a 1987.

[piecesparts]

Originally Posted by kelsfine69

Well the old Gov-lock 3:42 Posi packed her in last night. So now I'm on the hunt for a another diff. I plan on getting another diff and re-bearing it and doing a full axle housing swap. The problem is finding a 3:42 non posi is almost impossible around here. I don't really need a posi for the truck, it is just a daily driver/hobby. Its a 1987 Silverado 350 Fuel Injected with a 700r4 trans. I found a 3:07 rear end that is in pretty good shape. Will I have a problem with power? mileage? Did the 350/700 combo come with any other diff ratios other than 3:42? Or should I be looking for something else? Thanks Guys

If you take your truck to a 3.08 gear ratio, the RPMs will drop significantly and you will loose gas mileage. My son and I put a 700 in his 86 truck, while it had 3.08 gears and to cruise down the highway, the RPMs were so low that the truck had to work harder to get up and down hills. Also cruising around cities was hard on the fuel. We then took his truck to a 3.73 gear set and the fuel mileage went back up 3 1/2 to 4 MPG and the truck actually is enjoyable to drive. The 3.42 combination is a good one, but it is just on th ebottom edge of efficient and too high of a gear to be practical.

I found that the mid 80s GM V8 engines like a RPM band of 2000 to 2500 as the optimum power band. Outside of that, then you pay in fuel mileage.

[81ShrtBox]

Buddy of mine still has his 79" Malibu. All it has is a 305 crate motor, quadrajet, stock manifolds, flowmasters, 700r4 and 3.73 gears. He runs 20" wheels and loves it. He can torch the tires at will with that gear ratio.

When I change mine up I'm going with a 4.10 ratio and 24" wheels in the rear

[Tx79chevy]

My 79 has a factory 2.73 in it.

[Tx79chevy]

Originally Posted by jesdude7789

well my 1987 has a 2.73 gear. I was told that this was the only year they put that ratio in these trucks and for good reason! I'm looking to get a 3.42 to swap into it. I have lousy off the line acceleration because i don't have enough torque getting to the wheels. I had an 1989 with a 3.07 and it was okay but that rearend broke a couple teeth on me and i did put a 3.42 gear in it and it was much better off the line and towing things so i liked it alot better. gas mileage for me wasn't that much different since i mainly drive it around the city and maybe 30% on the highway and now i do the same thing with my 1987. I would rather have more fun with it than get the best mileage possible out of it. I probably get around 14 city, 17-18 hwy now and maybe 12 city, 15-16 hwy when i did the swap to the 3.42 in my 1989. I also had the holley 502-6 trottle body on it and no other mods. stock other than that. oh and they are both 5.7, 700R4 TBI K ENGINE 2wd long beds that i'm talking about.

OH and what wheels are those? If i'm not mistaken, these trucks have a narrow width between the back wheels than the front wheels. Are your rear wheels backspaced to make them even with the front set?? could you tell me what you have?

My 79 has a factory 2.73 in it also

[BIG10]

Since its not your daily, I would probably wait for a 3.42 to show up. But if you go another route let us know. I have a 3.42 in my 78 now, but I always wondered if I could get a little better mileage with a taller gear.