Ok...I have been here a while, and, instead of asking a lot of questions, I have been reading TONS and TONS of posts...enough to know that the majority of my questions have been asked about 1000 times before. So far I have gotten all my answers without even having to ask anything...BUT...now I have a question about something I read in an old post. What I read was: My question is, if that is truly the case...what would be the best rear end for my setup then? 28 Tudor, 327, 700r4, small 4bbl, fenderless, NOT hopped up a lot. I am going with that motor/tranny combo because it's what I have, and it's all fresh and free, in case anyone wants to know. I work about an hour away from my home, and would really like to drive my car to work on the weekends at least. I know that there is no "magic" formula to make me get 50+ mpg, but, I don't think 20mpg would be unattainable at 70 mph. My ORIGINAL idea was a set of 3.08's...that I ALSO have lying around. If I stick with them...what tires should I run? If I go with...say...3.73's or 3.42's, or even 3.90's....what tires would be best? Any help is greatly appreciated... I am just having trouble wrapping my head around this one....
I have a zz4 with a 700r4 in my coupe. The rear gears are 3.50's and the rear tires are 31" tall. The car will pull an honest 22 mpg on the road. I like it.
Slap a vacuum gauge on it and experiment. It's possible to get to low of a rear end gear and while you're turning lower RPM's, that might now be the ideal place your engine wants to be causing it to work harder, therefore resulting in lower MPG.
I have a 33 Coupe, with a ZZ3, Turbo 350, 3:25 Rear Gear, 235/70R15 Rear Tires, 2800 RPM at 70 MPH, 21 MPG all day long. HemiDeuce.
Ahhhh...I see...a lot like "lugging the motor." If it's below "peak" performance levels for that "speed" then it's not running efficiently trying to make power... Did that make ANY sense??
It's not the rpms so much as it is the vacuum the engine is pulling. If the car pulls a higher vacuum in 4th than it does in 5th, mpg will be improved if you run 4th. ZZFords combo above sounds close to being on the money for running in OD. I hope to do as well, but will probably just improve maybe 10% going from 3.70's in my 32 roadster to a 3.00. 30" tall tires in both cases. And a non-OD T-400. I think a 3.25 would be about right for my combo, but the 3.70 and 3.00 are all I have for a 28 spline 9". Anyway, 15-16 mpg on the highway, but running pretty fast. 75-80 most times and these little roadsters - and coupes - are about as aerodynamic as a brick. I don't expect in-town mileage to change much - 11-12 mpg right now - but if I can get the highway mileage to 18 mpg or more I'll be happy.
You got it. Manifold vacuum will teach you a lot about how "efficient" your setup is. I know I've learned a lot watching my father tune a car with solely a vacuum gauge. He also had the similar issue with too low a gear in the rear end and he noticed it on his vacuum gauge. Was over working the motor trying to haul a tear drop with the car. I believe he was around a 3.00 rear end and actually went up to a 3.50. Manifold vacuum increased, car ran cooler, performed better and got an additional 4-5 MPG to boot while hauling the trailer.
my thinking says to me , that if you get the SBC running down the H-way at 2000RPM..your "Lugging" the engine. Therefore it isnt in its working range of RPM and it is really working harder and getting low MPG, than it would be at say 2500 or 2800. Not to mention the 327's do like a bit higher rpm than the 350's probably due to the shorter stoke. Im thinking of running a 3.00 gear with a 27" rear tire 350/700r4 set up. i know the 1st gear might be short..but i think the Highway at O/D will be right on the mark..(i hope) dam i must type slow..
One more quick one. A long time back dad told me that if a torque curve graph was available for your engine, gear it for the peak torque rpm. And pay attention to the vacuum gauge. I don't know why more people - specially rodders - don't run them. A terrific tool to drive by and an on-board diagnostic gauge as well....
i just did a 1000 mile road trip in my wagon , it has a 305 / 200r4 , 3:42 gears . on the interstate i maintained 70-75 mph , never saw above 2400 rpm , vacuum gauge stayed around 20" and i got a best of 22 mpg. that was with 500 lbs of driver /passenger ,luggage ,a full cooler , extra parts , tools etc. now...when i'm just farting around , meeting the GF for dinner ,heading to the parts store whatever , i'm usually anywhere from 30 mph to 65 mph and the tach rarely goes above 2000 rpm , and the vac. gauge usually waivers in the 15" range i average 15-17 mpg . at times it does lug around the 1500 rpm mark if i'm in 4th (overdrive) and the converter is locked. i think if i kept the cruise range 500 rpm higher i would get much better mpg .
i did the drive by vaccuum gauge deal for a couple weeks... once you get used to tickling the peddle , you'll be amazed at what the mileage will do... i could keep mine at expressway speeds and still keep the motor at 16 to 17 inches....was idling at 19.5 as well... brandon
There is a really good way to estimate this if you have the right equipment. There is something called a "Performance Map" for every engine. It has a sweet spot for best fuel economy. Lowest BSFC or highest thermal efficience defines the sweet spot, more of a sweet zone, actually. On the "Y Axis" you have load (or torque, or MAP). On the "X Axis you have speed (rpm, or, strangely, piston speed in feet-per-minute). If you don't have the map you'll need a dyno. But you could probably get a map for a similar engine (5 liter V-8, stock, similar bore stroke and hp). If you know the approximate road load curve for your vehicle and all of your transmisison and rearend drive ratios and tire size, you can plot curves and help yourself locate the best efficience for a given road speed. We used to do this in the engine lab back when i was a teaching assistant. It's pretty cool stuff. I'm not sure if there are any recent good references on it, but i think we used Rogowski or Taylor. BUT THE SHORT ANSWER... Most likely, you'll want to lower the rpm -- that's why they make overdrives. But just don't take it too low. For rules of thumb, look at what newer cars and trucks with automatics run for rpm at highway and freeway speeds. 1500 rpm? 1800 rpm? Those may be a good target. Maybe 1200 rpm would be too low -- but i'm just guessing there. --Matt P.S., this would be a good search line in Google for further reading: "performance map" and bsfc way
As a first shot at that graph you were talking about, see "Figure 1" on this link: http://www.mautone.eng.br/apostilas/motores_combustao_interna/Golverk_SAE920683.pdf
FWIW: Charles Lindbergh knew a thing or two about getting peak miles per gallon. Or swim. If I recall reading correctly Lindbergh was hired by the US Army Air Corps as a civilian consultant to help them get the best possible fuel economy and load-carrying capability out of the F4U Corsair fighters in the Pacific theater. Lindbergh taught pilots to adjust the mixture to just on the lean side of peak cylinder head temperature, set the throttle at a lower than traditional setting, and to increase the pitch on the props(like an OD). He was basically telling pilots that to improve their chances of getting home, they had to slow down. Drove the pilots nuts to throttle back, but flying at a slower airspeed was more efficient. That's my recollection without research. I stand ready to be corrected by better informed or more ambitious readers. Now, slowing down in order to fly a wing at the most efficient airspeed doesn't translate as readily to driving a car. But maybe the engine operating tips might be of use.
Based on what I know (And I'm no expert by any means) and what you say you have, and also what you say you're looking to get out of it, it seems to me that with such a light car you will most likely be looking into the 3.20ish to 3.50ish rear gear range if you run a standard 235/75ish size tire. Once you find the tire size you like, then you will be better able to calculate exactly what ratio to run. Dont forget to play with that vaccuum gage setup too. Like everyone else said, you WILL learn alot about your particular setup. -GothY-
2200-2400 seems to be a small chevy's sweet spot. Probably the best series of articles I've ever read on the subject were done by super chevy about 25 years ago, and involved a first gen monte carlo with a 350. Surely somebody has scanned and booked that set for grey market purchase.
i remember back in my chevy II super chevy days , they used to have a shop in california that would do the jiffy tune on your car , and would chassis dyno before and after ....old school place , does the name roger jennings ring a bell...think that was his name... did a lot of distributor work and tuning... i actually ran into a local guy that lived out there. this guy had done his el camino , and said it was night and day...performance and fuel economy wise.... brandon
My 57' was getting 27mpg with a stockish L03 305, TH700R4, 3.36 rear gear, and 255/60R15 tires. On the highway, I am right at 2000rpm. I need to fix the carb now, since the stupid thing has started leaking internally into the throttle bores. Now I only get 15-17mpg. I am not impressed with Q-jets.
Good post!! A couple additions : Lets asssume you are all running a carb, the carb has different circuits but lets talk about the power valve and power circuit. I am going to say in this example it came stock with a 11" power valve . This would mean at 11" inchs of manifold vacumm and lower the valve would supply additional fuel to the engine based on load. For the sake of discussion lets say this is perfect! Now change the cam shaft add more carbs, manifolds etc and the engine now runs down the road with a lower than normal vacuum etc. NOW the power valve is open more causing fuel economy to fall way off etc. This has got to be considered on every hot rod ever built. Item 2: Rear axle ratio, if the rear gear isnt tailored to the vacuum power valve curve etc the engine will again lug , making the power valve come in sooner resulting in loass of fuel economy and driveability will be poor at best.... Item : Camshafts Camshaft all have a usefill band of operation. One i see often is a engine with a " RV" camshaft in it. The band of opereation is very short meaning you must gear to operate in that band. Many camshaft run great at higher or lower rpms based on profile etc... Again a loss of vac hurting the entire system. The good news is that all carbs are tuneable in one way or another . May need different parts (power valve) or a spring trim etc ..... Good post ! keep those brain cells going..
That might not be truly the case. When I put overdrive in my 39 chevy with a mild 350 and 3.23 gears, highway mileage went from 15 to 20. Most cars made in the last 25 years have overdrive because they need it to get decent mileage and keep up some performance. Late 70s american cars didn't have OD, but needed to get better mileage, so they usually had rearend ratios in the mid 2s. Now, if you set up an engine with a wild cam, etc. to make high rpm HP and consequently it runs like crap at low RPM, then choprods' statement could be true. For most engines, it's not. Anyways, you probably want rearend gears in the low 3s, like most newer cars have.
I will give a million dollars to anyone who can show me a car that gets better gas mileage at 2800 than at 2000 rpms! Pulling a trailer with a 5000lb SUV? Maybe. Pulling along a 1928 Ford? No way in hell. The motor could practically idle and pull that car down the road at highway speed. As long as the thing isn't bogging down going up a hill, the gears aren't too high. Period. Why do you think so many new cars turn around 1800-2000rpms at 70 mph? And I'm talking about 4 and 6 cylinder cars. EDIT: (I typed this before I read squirrel's response. I was trying to say exactly what he said, but in my less diplomatic, detailed, and thought out manner)
Not a correction, just a bit more information. I'm pretty sure Lindbergh was flying and teaching P-38 pilots since it was the Marines who flew Corsairs. He did a lot of one engine out training as well. The P-38 being the big twin engine, twin boom fighter built by Lockheed and fwiw my all-time favorite. All aircraft have a bunch of speeds. Best climb speed over an obstacle. Best climb speed over distance. Approach speed. Stall speed clean. Stall speed dirty (flaps down etc.) And more. Looking at my pilots manual for a Cessna C-150C trainer it shows cruise at 105 mph, approach speeds with flaps down at 60-70 mph and short field landings with flaps down at 59 mph approach speed, flaps down stall at 50 mph. Along with all these speeds is best glide speed (@70 mph) at gross weight. Usually used during times of engine failure, it trades the least altitude lost for most distance gained. Every plane has such a rating and this is one of the things Lindbergh was teaching. Fly best glide speed along with setting engine A/F mix and prop as Northern Skink notes. Granted, the very fast P-38 would be dealing with higher speeds, but just as it is in the little Cessna it's difficult to make yourself cruise at best glide speed, but it uses the least amount of fuel to stay level, stretch the remaining fuel out and reach your intended destination. Destinations change sometimes, lowest fuel level I ever put down with was a little under 1/4 of both tanks remaining. That due to the weather closed in at the destination, got fairly well closed in at the home field where I was attempting to return to, so I went a little further north and inland until I found a field under VFR conditions and landed there. The P-38's a highly complex airplane and managing the engine properly is something Lindbergh was able to pass along and more than few P-38 drivers got back from a long mission over water. Most interesting part about Lindbergh's time in the Pacific were the missions he flew with the pilots he was teaching. He was instructed not to fire at enemy aircraft. Strong rumors were that he shot down at least one Japanese airplane. Denied strongly by the higher-ups, but muchly bandied about during pilot bull sessions and talk in the hangars. Sometimes a guy's gotta do what a guys gotta do....
The "form drag" of most A-models is tough. That windshield resembles pretty much a barn door when full height and straight up. 20+ years ago a 350 SBC mildly warmed, w/3.08 in a coupe netted me about 18mpg. The vac guage is a help, to learn how the engine is responding to power needs, under given conditions. Certainly there can be evaluated, a sweet spot, that yields best mileage for a given set of weather, speed and factors that influence the yield. Part of the question sounds like, "at what rpm is the engine most efficient?"
It seems to me that how much power you have compared to what the car weighs is also a big factor in deciding what gear ratio gets you the best mileage. A big truck hauling a trailer needs a different rear end ratio than a lightweight Model T powered by a big block Chevy. Also the efficient RPM range of the engine depends a lot on what camshaft/heads/intake you have. If you have a radical cam and a tunnel ram intake, it'll run like crap and lug at low RPMs, so you need a higher ratio gear to keep it from lugging. But if you have a stock truck engine with a short duration cam in a lightweight car, a lower rear end gear would give you better mileage, since a stock truck engine might be very efficient at fairly low rpms. The aerodynamics of the car makes a big difference too and what speed you're cruising at makes a big difference. If you pick a ratio that's perfect to cruise at 55 mph, it might be all wrong if you really cruise at 75 most of the time. A full size van with a flat face plowing through the wind uses up a lot more horsepower to cruise at 75 than a Corvette or something.
"I will give a million dollars to anyone who can show me a car that gets better gas mileage at 2800 than at 2000 rpms!" I have one now, and have owned several (1970 Coronet 318 with 2.76 gears, better mileage at 80 than 50) - I prefer cash. What's missing from the "see this chart/graph/map"? Those all show full throttle. Not relevant. The best mileage is obtained at the lowest engine speed at which you have (almost) zero reversion (the intake inertia ABDC is high enough that flow does not go backwards), and overlap flow is all positive (intake to exhaust). Anything below that is bad, anything above that is wasteful. This is not the torque peak (which may be several thousand RPM higher), or best thermal efficiency, its the lowest point of the power band where running any faster takes more internal drag. If the cam is too hot (or ports too big) mileage will go up with RPM... until you reach that point.
I put too tall of gears in my 37 chevy p/u and the mileage is worse,I have a 235 and a T-5 with 2.79 gears and the gas mileage sucks now. It runs great and is quick off the line but I think the 6 is struggling a little and drinking more fuel,I was told the 235 gets thei highest vacumn at 2200 to 2400 RPMs and with 3.43 gears I will reach it. Its more then putting in tall gears and getting better mileage which I found out the hard way. Jeff
I will PM my address to send the check out to me. Two examples.. I drove my 55 Ford to Bonneville this year. I had not driven it that far before, so I was being nice to it. 2200-2500 was about 65 MPH max. 15 MPG. I drove faster coming home, 2800-3200RPM the whole way. About 75-80MPH+. Made 18MPG I could feel the engine come alive at 3000 rpm's. It just wanted to keep pulling. With my one bias ply spare on there, I did not want to see the best it could do. I also have a 77 Chevy crew cab truck with a 454, 400 trans. Its built for towing. It does way better with a load, and pulling a car up the Cajon Pass at 95 @ 4500 RPM's gets way better mileage than empty on the freeway. I know people say 4500 RPM is bad for 454's, I've never had an issue yet. I like blowing past the new turbo diesel trucks with 6000 pounds or more behind me. I could use the million bucks ASAP. I'll even take payments!
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