What would give better mpg? Taller gears or OD?

[DroppedZ84]

If I swapped my gears to something smaller (numerically lower; 3.42 to 3.07) would it give me the same benefits, as swapping from a Th350 to a 700R4?

[79BIG10]

Good question there. I think it matters in the way you drive. Do you drive alot of city driving or highway driving? Reason I say this is that I've had 3.07 with my TH350 and now I have 3.73 with the TH350. I get less mileage on the highway but around town running lower RPM I've gotten about the same. With a 700r4 and you're using OD then a 3.42 would be a good choice. 3.07 is awful off the line even with a 700r4 high geared 1st gear. My truck was great at highway speeds running lower RPM but taking off it took longer to get up to speed.

My suggestion is go with 3.42 and 700r4 if you have alot of highway driving. If not stick with the TH350 and 3.42 setup. I think you'll not like going with the 3.07 setup.

[Slonaker]

The advantage to the OD transmission is that you can keep your gears you have and still gain mpg. This is especially true if you tend to drive at speeds that allow the OD to kick in, like highway speeds.

If you change the gears in the rear, you will get better gas mileage, but acceleration suffers. Overdrive affects only your cruising rpm, but changing your rearend gears changes everything.

That's kind of an oversimplification, but you get the idea.

Slonaker

[Jonboy]

The 700R4 will have a lower 1st and 2nd gear ratio as well as overdrive.

[C&C Chevy]

3.73's with a 700r4 OD tranny is about as good "all around" MPG's you'll get. Pretty good in the city, pretty good on the highway.

[TEAM THC]

Im Not Sure What My Rear End Ratio Is But I Have The 700r And I Get About 13 Mpg On The Highway. I Dont Know How That Compairs To You Guys.

[DroppedZ84]

Originally Posted by TEAM THC

Im Not Sure What My Rear End Ratio Is But I Have The 700r And I Get About 13 Mpg On The Highway. I Dont Know How That Compairs To You Guys.

I get 12mpg with Th350/3.42/28". I'm wanting to get upwards near 20mpg

[jermasaurus]

Originally Posted by DroppedZ84

I get 12mpg with Th350/3.42/28". I'm wanting to get upwards near 20mpg

I would worry about the motor more than anything.
Maybe a 4.3 with a 2 barrel?

I honestly didn't see any improvement in milage going from a 350 to 700r4. Then again, I have a 2.73 rear The first gear is nicer though.
-Jeremy

[piecesparts]

The OD is a better choice over taller gears, because the mix is better in the long run. You can use the gears below the OD to get you into and out of trouble on a regular basis and then you can go on the highway and drive away with a lower RPM.

[SlapFish]

I can drive all day at highway speeds without sucking too much gas. I'd venture to say in the neighborhood of 12-15 mpg. I'm running a 4bbl 305/700R4/3.08 (GU4 on the glove box sheet). I don't know much about the rear ends GM used, but mine tells me 3.08, not 3.07.

[piecesparts]

My son's truck had 3.08 gears and a 700R4 tranny, we changed the gears to 3.73 and added a posi unit and gained 2 mPG by doing it. The 3.08 gears had the motor lugging itself on the highway and shifting everytime we hit any hills or inclines. We live in East-central kansas and we have all kinds of hills. the fact that the RPMs were around 1900 was good, but the motor had to work harder to keep up with the changes. The cruise control was no good until we changed the gears. We might have gone to far somepeople's taste but a change to 3.42 gears would make definite difference in the performance and still keep the miles Per gallon down. My truck had 3.42 gears in it and it worked fine until I started going for more horsepower. I finally built the motor to where I had to go to 3.73 gears to keep from detonating in my cylinders as I drove around in town. I now keep the RPMs up with a 385 stroker motor and can run with the best of them along with 15 MPG on the higway.

[79BIG10]

Originally Posted by SlapFish

I don't know much about the rear ends GM used, but mine tells me 3.08, not 3.07.

I believe that they changed from 3.07 to 3.08 somewhere in the years. Mine came with 3.07 printed on the glovebox spec sheet.

[piecesparts]

The scarey thing is if you count the teeth and then do the math, it comes out to be a number somewhere between 3.07 and 3.08. Actually it is real close to 3.08 so, thus the number confusion.

[swervin ervin]

I believe the numbers game changed when they went from 12 bolt to 10 bolt.

[84 PUMPKIN]

Originally Posted by DroppedZ84

I get 12mpg with Th350/3.42/28". I'm wanting to get upwards near 20mpg

My daily driver is a 1984 C10 with a stock 350 engine/350 tranny/2.73 gears/28" tire. I drive 60% city and 40% highway and get about 18 mpg. My best was 21 mpg a couple of times while on the freeway but that was either down hill or with a good tail wind. I'll get 10 mpg if I do burnouts for the kids or step down to hear the Quadrajet moan. For best gas mileage I jump in the 40 mpg Honda Civic. For best performance I drag out the 5 mpg blown big block '33 Coupe.

It's very hard to get our trucks to get GOOD gas mileage but here's some numbers for you to crunch. In my experience the 2000-2300 rpm range @ 70 mph gets pretty good mileage.

TH350/3.42/28" tire @ 70 mph = 2900 rpm's
TH350/3.07/28" tire @ 70 mph = 2600 rpm's
TH350/2.73/28" tire @ 70 mph = 2300 rpm's

700R4/3.73/28" tire @ 70 mph = 2200 rpm's
700R4/3.42/28" tire @ 70 mph = 2000 rpm's
700R4/3.07/28" tire @ 70 mph = 1800 rpm's

[pedrofixlater]

Overdrive gives you more launch as well as the extra gear. Our trucks will never be gas savers. I am running a 350 with a medium cam and 3.73 with a 200-4r overdrive. One thing to remember is that if your 700-r4 has a lock up converter and your truck doesn't have a computer you can put a 4th gear only lock up switch in. I have that on mine and just flick the switch on the highway to get the full effect of the overdrive.

[piecesparts]

I have a 700 R4 tranny without a lockup converter. Yes , it does work that way. There is a slight loss without the lockup, because of the converter slip. The overdrive works well and I get a solid 15 MPG with the extra horsepower under the hood. I imagine that the lockup would give maybe another Mile per gallon. I, on the other hand, have a 2400 stall converter, due to the cam and the way the truck idles. The launch is fantastic and the shifts are hard and crisp. I also have a 91 Chevy Silverado short wide bed with a 5.7 litre, 700R4 tranny, 3.42 geared rearend (10 bolt). I average 17 to 18 MPG in everyday short trip driving and can get 20 MPG on the highway, if that is all I drive (Long Trips). It has a TBI injection and a lockup converter that is part of the reason it gets the better mileage. All of my trucks are running the same size tires and rims, so it is easy to deal with. Also my trucks have fully built 700 R4 trannys. That means the heavier duty clutches and five gear planetaries, along with the heavy duty seperator plates and throttle valves. Want great mileage ---Get out of the truck and buy a mini cooper.

[Scrubby]

Good luck getting 20 mpg with these trucks.

Before you swap anything, do some math to make sure it makes sense. The difference between 10 mpg and 20 mpg is only 10 cents a mile at $2/gallon. That means you need to drive 10,000 miles to pay off a transmission swap that cost just $1000. Maybe you drive 3 times that in a year, and that is money well spent, but it might not be either. This is stuff people don't figure out when they spring for a $2500 gear vendors overdrive to save gas on a truck they drive on the weekends.

[DroppedZ84]

Originally Posted by Scrubby

Before you swap anything, do some math to make sure it makes sense. The difference between 10 mpg and 20 mpg is only 10 cents a mile at $2/gallon. That means you need to drive 10,000 miles to pay off a transmission swap that cost just $1000. Maybe you drive 3 times that in a year, and that is money well spent, but it might not be either. This is stuff people don't figure out when they spring for a $2500 gear vendors overdrive to save gas on a truck they drive on the weekends.

Well, I guranteee I wouldn't pay more than $500 for the entire swap..so, I'm not worried about that, and yes, I drive the truck about 5000 miles a year...well, I WILL once it has overdrive. Running at 3000-3500 on the freeway is no fun, I know it's not hurting the motor, but I would rather it be running at 2000-2500, going the same mph.