Miles Per Gallon versus Set-up

[luvbowties]

Originally Posted by truckster

You really should figure out your rear end ratio - it's a good place to possibly pick up a couple of MPG.

You could always pull the cover; that gives you the opportunity to change some very old gear oil. Otherwise, lift one wheel off the ground and put a mark on the tire. Also mark the driveshaft. Turn the wheel and count how many times it rotates to make one complete rotation of the driveshaft. That's your gear ratio. For example, if the wheel turns 3 3/4 turns for one turn of the driveshaft, your gear ratio is 3.73.

Earlier u stated: "Turn the wheel and count how many times it rotates to make one complete rotation of the driveshaft. That's your gear ratio. For example, if the wheel turns 3 3/4 turns for one turn of the driveshaft, your gear ratio is 3.73."

R u sure?

[blasher]

It's the other way around. Turn wheel once for a posi and twice for open diff. Count the driveshaft revs.

[ubtripn]

Thanks to everybody. I am definitely going to go with a Quadrajet and have it tuned plus change the rearend.

If I bought the 190 HP GM crate engine and mated it to my 700R4 and then put the best rearend in, what kind of MPG could I expect?

[ubtripn]

Thanks all, especially GMCJoe and CC69rat.

[ubtripn]

Oh yea, I forgot to mention, it had a powerslide in it stock before I got the 700R4.

[GRX]

I started an MPG thread here a few weeks back. 10-14 MPG seems to be the rough average no matter the engine. Hot rods exempt of course.

[BlaC/K99]

To the OP, late model GM half ton 2wd trucks commonly have 3.42 gears with a 4l60 transmission (same as 700r4). For example, my '99 OBS 2wd with a 4l60, 255/70-15 tires and a 3.42 gear turns 2000 rpm @ 70mph. Also my '05 Avalanche 2wd with 3.42s is around 72 mph @ 2000 rpm due to taller tires.

If you are turning 2000 rpm @ 60 mph in overdrive you might have a higher numerical gear ratio in it than you think.

[bonnieclyde100]

going down in hp wont increase mpg in fact it could go lower if the reason your at 190 hp is 72cc or more combustion chamber heads cause it will put your compression ratio to low.you should aim for an engine with 9.1 to 9.5 to1 ratio

[ubtripn]

I see, when I was talking about the 190 crate I was just thinking about what engine/carb/rearend combo would get the best MPG with my 700R4. (I mean the 290hp crate.)

[derrickmanx1]

I'm hoping My jakes 4l80e will help my Fast injected bbc 408 get close to 12-15 with the 3.73's. I get to dyno the engine mid december. Hoping for 460-500hp.

[skorpioskorpio]

Something I've always wondered, why overdrive? Is there some reason why overdrive with a certain differential gear is preferred to 1:1 with an equivalent overall ratio? In other words if your total gearing to the wheel in 1st, 2nd, 3rd, and 4th (in a vehicle with a hypothetical non-overdrive transmission) was identical to an otherwise identical vehicle with the same total overall gearing to the wheel in 1st, 2nd, 3rd, OD, would one be better somehow than the other? ...or is this a marketing gimmick that throws back to adding a tailshaft overdrive to 3 speeds on performance cars in the 50s and 60s and it just sounds good? In the end both vehicles spin the same rpm in any of their 4 available gears at any identical speed. Just curious.

Oh and to add to the stats: My 1991 Suburban with a 350 TBI (8.5:1 compression) and a 4L80E would get 13 MPG an flat highway with 3.91 gears and 33x12.50s and got 13.5 MPG on flat highway with 4:11 gears and 37x12.50s. In both cases it's "sweet" spot was about 75-78 MPH, anything less and I lost MPG, anything more is difficult to maintain, takes too much throttle and wants to lug with even unnoticeable changes in grade. I am convinced that that truck simply burns gallons per hour regardless of speed.

[derrickmanx1]

Well the reason I went with the overdrive was so I could reap the benefits of the lower gearing. Bottom line it helps taking off. Besides if my truck was set up with 2.61 gears and a thm-400 it wouldn't come close to my 4l80e in driveablity. It wasn't close when I changed from the 3.08's. It was such a big difference when I had my 496 it wasn't even funny, so much quicker. I will do 4.11's if I ever have to go back into the rearend now.

[skorpioskorpio]

Hmm wasn't really my question, my question was why is a greater than 1:1 ratio more desirable for your 4th gear than a direct drive 1:1 if the engine revolutions to wheel revolutions ratios are all the same first through last. I understand the desirability to the extra gear, it's more about the why make that extra gear an overdrive. When 4 speeds replaced 3 speed manuals they didn't add the extra gear as an overdrive they increased the number of gears (top gear still 1:1) and redistributed the ratios.

[derrickmanx1]

Originally Posted by skorpioskorpio

Hmm wasn't really my question, my question was why is a greater than 1:1 ratio more desirable for your 4th gear than a direct drive 1:1 if the engine revolutions to wheel revolutions ratios are all the same first through last. I understand the desirability to the extra gear, it's more about the why make that extra gear an overdrive. When 4 speeds replaced 3 speed manuals they didn't add the extra gear as an overdrive they increased the number of gears (top gear still 1:1) and redistributed the ratios.

All things considered the thm-400 and 4l80 gear ratio's are the same until you get to the overdrive. The gear ratio's are 2.48-1 in 1st. 1.48 for 2nd and 1-1 in third. The 4l80 carries a .75-1 overdrive. When you run a turbo 400 on 3.08's your final drive ratio in first gear is 7.64-1 as opposed to 9.25-1 with 3.73's(3.73's pull better). If you consider at 1.00-1 is still 3.73-1 and the 3.08-1(which is better for cruising). When you add the overdrive of the 4l80 it puts it at 3.73X.75=2.80 final drive for cruising not considering tire size. So for me to take advantage of taking off more efficiently and cruising at a lower rpm the overdrive make good sense. It does come at a custom cost. For me i'm 30 miles from anything so I put more hiway miles on my vehicles. You can increase the gear ratio of your first gear to get the same effect though. The 700r4 or 4l60 has a 3.06 first gear ratio. They just aren't as strong and reliable behind the kind of power my engines have been putting out.

[skorpioskorpio]

OK, I get that 4 gears are better than 3, not what I was asking, apparently my question is too complicated and it seems to be diverting the thread and this was not my intent. I thought it was a pretty simple question.

[ubtripn]

Actually I like the question and it's related. Everybody above knows far more than me but I got the 700R4 overdrive because I thought that on the highway it locks the torque converter for better cruising mileage. (Or something to that affect.) This tranny was one of my few good luck moments I have had with my 68. I walked into a junkyard, paid $150, installed it and it runs perfect. Shifts perfect too. I had to have a 2 drive-shaft system with a carrier bearing redone and I asked why not one drive shaft? The answer had something to do with vibrations or harmonics, something like that.
Anyway,
I was under the impression that an optimally geared, aspirated 350 could be set up in a 2 wheel drive to hit 19-20 mpg if driven mild. This is my target. I am trying to see if it is possible.

I would love injection but that is the same cost as ac is going to be for me so I know which one wins =)

[luvbowties]

Originally Posted by skorpioskorpio

OK, I get that 4 gears are better than 3, not what I was asking, apparently my question is too complicated and it seems to be diverting the thread and this was not my intent. I thought it was a pretty simple question.

Just read this post & read its preceding one: why have an od for the 4th gear (rather than re-distribute the 4 gears ALL between 1.00 and some higher number)? No, your question is not too difficult for us to answer!!!

The answer is key to why we want overdrive in the 1st place: fuel mileage. There was a different "why" in the why we wanted a 1:1 output 4-speed in muscle cars, like back in the 60's: then, we wanted to get to the end of a 1/4 mile distance quicker. Solution: keeping the rpm's up in the power band by losing the 3-speed & installing a 4-speed. Then space the 4 gears, at better steps than the 3-speed, all between 1:1 and some higher numbered ratio. TWO SEPARATE GOALS!

A simple answer about overdrives that I think of is an illustration that all of us can easily relate to: Overdrive enables one's vehicle to "operate at 60 mph while his engine is only operating at 50mph". All else being equal, that ought to give better fuel economy.

With a final tranny ratio of less than 1.00 (like .70) it means your output speed (of any pair of driveline components, like: output shaft speed vs. input shaft speed of tranny, rpm of driveshaft vs. rpm of engine, input speed of driveshaft to differential vs. output speed of crankshaft), is faster than your input speed. This means that while vehicle is running 60mph, the engine's rpm is only outputting 38.5 mph--0.70X55mph=38.5mph. Sounds like money from home, huh?

Life experiences teach us that any time you gain something, you've gotta give up something. To gain this (seemingly free ) vehicle speed, you gotta lose somewhere else. In this case you lose torque which is twisting force, or pulling/towing ability, or ability to accelerate. First gear takes off faster from a standstill than 3rd gear. In order to "gain" that take-off ability, you've gotta give up something.....you give up how fast it will ultimately go in that gear.

Overdrive ultimately offers the best of two worlds: (1) It allows us good gearing in 1st 3 gears to take off fast, or to let engine take off easier [["with enough gears, a washing machine motor may be able to move a freight train...albeit verrrry slowly"]] and(2) It allows a really long-legged final drive ratio to give us good mpg. Gearing of 4.11 has stood out for years as being a good drag-car differential ratio. Its drawback was it was not good for interstate driving because it spun the engine too fast. Add an overdrive gear, like a .70:1, to use only on the highway, and it's like having a 2.88:1 differential: 4.11X.70=2.88.

There is a caveat in all the above (that may sound like bull****, yet TRUE). The "law of diminishing marginal returns" affects this scenario like it does in any input-output situation: too low a final drive ratio and efficiency/mpg actually falls, just like too much ice cream actually makes us sick.

I'm sure the op can one day determine his differential ratio which will help him determine which way he may need to go. Based on the limited info he's given so far, sounds like he's got something close to a 4.11. Using ratio-and-proportion, and comparing to a poster who has given all needed parameters to 'work with', and comparing op's info to it, we come up with 4.00:1 ratio for op, one I'm not familiar with. But 4.11 is closest I am familiar with.

If 4.11 be the case, he'd be wise to leave it alone and see what his new engine will actually do as he breaks it in. May find he has a sweet spot already with that rear end. As he drives and tests, he can be on the lookout for a 3.73 on-the-cheap and for even a numerically lower one also. Only takes a reasonable time to swap diff-to-diff, and then see which way he's progressing--or regressing--as he drops ratios the 1st time. Only then will he know which way to move next. He can always sell such rear ends--so no actual expenditure, only a temporary investment.

Good luck, op and all others who came up with related questions. I learned a lot from these discussions, too!
sam

[blackdawn]

i see what you are saying in your question and im shooting in the dark so take it for what it is but since all previous transmissions where mostly 3 gears with 3rd being a 1:1 and all of the current rear ends setup with that gearing in mind they probably just chose to go to a .7:1 overdrive as the next logical gearing place as opposed to just rearranging the gear ratios to all fit under the 1:1. yes you still need to play with the rearend gears when you do this swap, but you can go up tword the 4x4 and other 3.73 and 4.11 gearings rather than down to gears that where not used at the time.

[blackdawn]

as to the ops hope on his setup hitting 19-20, even with all of the nice gearing your problem will be in the carbed crate engine, even a perfectly built for mpg carbed 350 just wont hit those numbers. the engine is too big to get that kind of fuel economy in the non-aero brick. if you think about it in new trucks with small blocks with fuel injection which has the computer constantly adjusting the fuel to put in the smallest amount it can in in all circumstances is only getting 20-22 freeway. not to mention they have come leaps and bounds farther in aerodynamics. i have found that unfortunately you have 2 options in these trucks, put in a cheaper setup and live with 12-16mpg, or drop a large amount of money up front and get 18 on the freeway if you are lucky by getting an engine built for just that. then there are always the diesel projects, which are more fun than savings most of the time.

sorry to rain on your parade, its something i have asked many times and found the hard truth on. its still going to be worth your while to do those upgrades, but dont get your hopes too high.

[skorpioskorpio]

It would seem to me that the efficiency would be more optimum in a 1:1 final drive it's the gear that the transmission has the least number of spinning parts. It also means the driveshaft is spinning at a lesser RPM rate. So efficiency wise a 4 speed with a lower first should be more efficient in it's highest gear ratio than the 3 speed +OD that a 700R4 represents. The straight 4 speed in it's high gear with a lock up converter is running as a completely locked shaft from crankshaft to pinion, or optimal efficiency. So I don't understand the argument that OD is more efficient. Remember we are talking about 2 identical cars whose engines and axles spin at identical rates at all road speeds differing only in transmission gear ratios and differential ratios.

My understanding is that Indy and NASCAR do not run OD transmissions where their top gear needs to be their most efficient one, especially with fuel rules that basically mean that a 10% drop in efficiency means you don't finish the race. What it's worth.

[blackdawn]

i simply guessed at why they would do it the way they did, but that would be a great question for one of the big car company engineers.

[luvbowties]

Originally Posted by blackdawn

as to the ops hope on his setup hitting 19-20, even with all of the nice gearing your problem will be in the carbed crate engine, even a perfectly built for mpg carbed 350 just wont hit those numbers. the engine is too big to get that kind of fuel economy in the non-aero brick. if you think about it in new trucks with small blocks with fuel injection which has the computer constantly adjusting the fuel to put in the smallest amount it can in in all circumstances is only getting 20-22 freeway. not to mention they have come leaps and bounds farther in aerodynamics. i have found that unfortunately you have 2 options in these trucks, put in a cheaper setup and live with 12-16mpg, or drop a large amount of money up front and get 18 on the freeway if you are lucky by getting an engine built for just that. then there are always the diesel projects, which are more fun than savings most of the time.

sorry to rain on your parade, its something i have asked many times and found the hard truth on. its still going to be worth your while to do those upgrades, but dont get your hopes too high.

Personally, I second your opinion about the actual mpg the op should expect. I also agree with a reason why: our lack of aero's we face in our 60-66 trucks is a factor that greatly hinders our mpg's. I also agree that a smaller cubic inch engine than a 350 CAN get better mpg.

In 1955, buyers could get a 2 door sport coupe, 8 cylinder with 4-bbl carb, 3 speed with overdrive, with a 4.11 rear end--overdrives normally came with the 4.11 either with V-8 or 6-cyl. I bought such a car, used. It would deliver 23+ mpg. when driven 65 mph on a certain 230 mile trip, mixed yet more hwy driving. It did so quite a number of times, as I'd go from our service station to a Texaco station 230 miles away, where I'd fill back up--on my way to school. Repeated the exact trip over a period of 5 years!

I eventually demolished the 3-speed overdrive; the cheapest temporary fix was to install a standard 3-speed trans that had identical 1st, 2nd, & 3rd gears as the od had. This combination, when driven on that same 230 mile trip at 65 mph, would get only 14 mpg--the rpm's stayed too high!?

I later swapped differentials for a 3.70:1; the mileage went up to 16+/- mpg.(but NEVER 17); it no longer felt like the engine was fixing to blow from too-high rpm. Still later on, when I found a 3.36:1 differential, the car would get maybe just a tad better on that same trip at the same speed, but felt anemic, esp. from zero to 35--the car 'lugged' in some speed ranges, when compared to the 4.11-2.88-combo the overdrive afforded!?

I feel like the ease of accelerating from a standstill with the 4.11 to about 30 mph was a plus for mpg. Then when it went into od at about 30, it had enough momentum to be able to effectively use the 2.88 it was then "seeing". Seems like this range of gears is what gave the overall good mpg--the engine NEVER saw a lot of rpm's.!?

I firmly believe that our 60-66 trucks, even using the mfg's latest technology, are INCAPABLE of achieving the mpg that the newer trucks are getting. I also feel there is a maximum mpg we can hope to achieve, say when driving half the distance in town(20-30-40 mph) and half the distance on the hwy(55-60-65 mph). On such trips & running a proper carb, our trucks' best will likely max out at around 15 mpg. On the same trips with proper fuel injection, I feel they may do 2-4 mpg better. All this is based on what I'm seeing plus "part of what I'm hearing". I still like and believe in the advantages of (1)overdrive trannies AND (2)electronic engine management, to include managing fuel injection and gear shifting. Regardless of the mpg, I like the way our vehicles behave with these new technologies. And when they do yield better mpg, I like it even more.

[ubtripn]

Man I have have learned a ton here everyone. Thank you!

Well, I can handle 12-15 since I don't have a car payment but I am going to still maximize the efficiency as much as possible. It's my daily, around town driver. I use other cars for road trips. I was just hoping there was a set-up that could hit close to 20. Anyway, first step is to identify my rearend when I can find the time. Thanks again all.

On a related point we took two Celebrities on a road trip once. One had a six banger and one had a 4. The six cylinder got the better mileage since it wasn't working as hard so I know there is a point of diminishing returns somewhere.