you can send the check now. My '06 Dakota gets better mileage between 2600 and 2800 then it does between 2000 and 2300. The sweet spot is definitely between 2600-800 for mpg. It's all about where the engine makes torque. To make a statement like that shows how little attention people pay to how an engine runs. C9 has it nailed with the vacuum gauge trick.
Remember, driving in a gear that "feels strong" is very different from the best fuel economy condition. If that were true, we'd be screaming our engines all day long in first gear. Also, not to doubt anyone's actual experience, but you need to repeat an experiment several times to make sure it's not just a "bounce" in the data, e.g., wind at your back vs. in your face, going down hill vs. up, filling at a station that clicks prematurely vs. one that lets you overfill -- lots of variables.
Good point on the flat windshield. I use Rain-X and it's interesting to watch the water flow on the windshield. Top on or running topless, light to moderate rain has the water running in circles on the glass until it gets heavy enough to flow off and down. If I remember aero theory correctly, doubling the speed squares the drag so forgive me if I look askance at the higher mpg at much higher speeds. A while back someone posted running about 4 grand with 4.11's and getting 22 mpg in a roadster. I'm doubtful about that. Probably a one time thing - if it happened - and that was due to the pump clicked off with less fuel in the tank and the previous fill-up had the tank topped off. Short gears many times equal an incorrect odometer and that can add to the illusion of higher mpg. Pitman's 18 mpg comment sounds very realistic in the earlier cars. If I ever get the 3.00 diff in I'm hoping to touch on 18 mpg in my roadster, but what may kill that - as it does now - is my propensity to run up the on-ramps at about 3/4 throttle. Gotta let it out sometimes, right? I'm guessing that 18-19 mpg is about what you're gonna get driving one of these aerodynamic bricks if you're running carburetion. Roadsters and coupes running EFI and computers are reporting over 22 mpg and as high as 27 mpg. Those figures sound reasonable, but the higher ones were probably accomplished at 60 mph and under. To an extent, a lot of guys don't know how to figure mileage and just think they're getting terrific mileage. My pal, a rather casual guy all things considered bought a Dodge crew cab, 1500 model with Hemi, auto and all the bells and whistles not long after I bought my Ford F-150 SuperCrew which more or less is the same truck as the Dodge considering all the nifty stuff they both came with. I was getting 14-15 mpg in town and just touching on 18 mpg running the desert highways - 75-80 per - with two passengers and a couple suitcases along with a small tool box. Add the 20' box trailer and mileage went to a steady 10 mpg loaded. With the trailer empty, it still ran 10 mpg with an occasional 11 mpg. Which shows you that aero and not weight is most of the problem. My pal starts reporting 20 mpg towing his flatbed trailer. A little better aero, granted, but 20 mpg struck me as more of a lightly loaded truck bit. And he was running fairly high speeds along with he tends to drive faster than I do unless we're traveling together. Anyhoo, I find he's figuring his mileage on tank fill-ups and not doing the math. When I figured out the mpg he was getting by noting the gallons pumped in, his truck was getting about the same mileage as mine. Funny part here is, he bought a 2500 diesel Dodge crewcab with 4wd. Since it was hitting 20 mpg - for real - he started driving it all over the place cuz it got better mileage than his Durango or Jeep. I didn't say anything about the little fact that a couple miles per gallon extra were outweighed by the little fact that diesel per gallon cost quite a bit more. Nuff said there....
Depends on where the sweet spot is. For a very short stroke motor, it might be 2800 rpm. But, as you say, it's not likely in a car. And for a monster ship of pipeline engine, it might be 150 or even 50 rpm. The funny thing is that a chainsaw at 10000+ rpm and a ship at 50 rpm might well have a similar piston speed (in feet per minute). Correct, the sweet spot is not the same as peak torque. It's a combination of friction, flame speed, enrichment, manifold pressure, pumping losses, and in-cylinder turbulence effects.
Oh....I thought we were talking about changing gearing to change RPM, and keeping the vehicle speed constant. as in the original question, which had to do with rearend gearing.
Man....I couldn't check my post for all day and night, but when I did you guys REALLY came through! A LOT of ground covered here...and a lot of great info! Thank you all very much. I see that reading a million posts before asking a question was well worth it! Jody
Thank you, Squirrel. That IS what I was talking about. I believe any carberated V-8 car at 3,500lbs or lighter will get better HIGHWAY mileage with a higher rear (2.70 range to 3:08 or so) than with a low rear end (3.76 and up) at the same speed. Keep in mind that "highway speed" around here is 75-80mph because the speed limit is 75 in Kansas and Oklahoma. I also believe those claiming better mileage at higher speeds are pretty much full of it, since it takes a lot more power to push a car through the air for each increase in speed. I'm not saying you're lying, I am suggesting you are overlooking other variables that made the difference. For example, I used to get much better mileage going West-to-East from Toledo to Akron than going East-to-West from Akron to Toledo because it is windy as hell along that route and the wind blows hard to the East. So you could easily go 10 or 20 mph faster AND get better mileage with the strong tail wind. I will, however, be issuing million-dollar checks immediately to anyone who can prove their old, unloaded car without a trailer consistently gets better highway mileage with a 4:10 rear than with a 3.00. (If the motor is mild or stock and it's tuned properly and in good running condition.) Pulling trailers uphill doesn't count, and neither does anything made in 1996.
Ya know, I wonder if hauling one of the nifty little teardrop trailers could improve mileage due to the draft effect? Seems like it might clean up the aero drag situation in the back. Granted, the trailer doesn't have an engine to push with, but overall the whole package tied onto a Deuce roadster would be a bit more streamlined than the roadster by itself. Drag may be lessened to the point the extra weight doesn't penalize the overall package. Fwiw, dad used to get 20-21 mpg running the highways in his new at the time 50 Ford sedan (with OD). With a teardrop on the back, mpg was 19. Loaded down with gear for a family of five, tent for the kids and all the rest including two sets of golf clubs. Looking back at the completely stock 50 and it's ability to run 65 across the country along with somewhat better aerodynamics than a Deuce, it doesn't seem like we've come all that far mpg-wise. I wonder what the ol' flatmotor would have gotten with EFI?
I think everyone's chains are being pulled just a tad tight here and the original question is getting lost. All the guy wanted to know is can your rear end gear be too tall for HIS car which is a 28 Tudor, 327, 700r4, small 4bbl, fenderless... hot rod. The answer is yes due to several factors already covered. Tire height, engine build, weight, etc... Slapping a vacuum gauge on it is a very cheap and effective live tool to dial your ride in to achieve the best with what ya got. And I would like my million dollar check please. My roadster improved gas mileage with a 3:73 rear end compared to a 2:56 with 31" tall tires, mildly hopped up 6 cylinder in an 1800 pound car. Vacuum gauge reading on the highway read ~10-15 in. and after the gearing change read more consistently between ~15-20 in. at the same road speed.
With my 260 cid Cobra, single 4 bbl, T-10 4spd., 9 inch diff./ 3.25 gears & 225/15 inch radials on my 32 steel roadster, will produce 25 mpg at 2200 rpm all day, BUT not in mountains! And only if I stay under 70 mph.-----Now lets be serious, my average crusing speed is a steady 75 mph, up & down the terrian, mountains & all. I love the old saying "There's more to life than GOOD gas milege"-------------Don
So...by this formula, if I did 70 mph with 3.08's and 30" tires, I would run approximately 2400 rpm's. Assuming that this is under IDEAL conditions (no head wind, level travelling, no load but the car/driver/fluids, and I don't mean beer...we will calculate that later...) then I could ASSUME that with the added resistance of the Model A form (a large brick) and other variables, that there would be anywhere from 200 to 600 more RPM's just to get down the road. This would put me at, or around, 2600 to 3000 rpm's at speed. Keeping in mind the small carb, less than 500 cfm, keeping the pedal off the floor, and a stock setup on the motor, no over-bore or big cam... then 20mpg should be do-able if that is in fact the prime running area per the vacuum gauge, right? Or am I still lost?
Getting back to 61Apache's original question: "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. " and also "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? " And his stated intention to run 70 mph (minimum, if Texas freeways are like most of 'em......) My answer would be to run what you've got, cause its free, and try a tire about 28 or 29 inches tall, minimum. Here's two pges to play with: http://www.700r4.com/speedoCalc/rpmcalc.php http://riverviewtire.com/Tireheightchart.html Add a few hundred rpm for aerodynamic drag, pushing that brick down the road.... I think 2000 rpm is a reasonable goal in your light car. I also think that the money spent buying and installing different gears without testing yours first could buy a lot of gas. Use a small carb with vacuum secondaries, jetted carefully, and drive for best vacuum on the guage. Mileage should be good. Others opinions may differ, and mine is worth exactly what you paid for it. With the right car, it's measured in "SMILES per gallon" anyway. Ralph
I'm guilty of continuing the O/T talk, but as long as the trailer isn't a big ol' honker, I can answer this with an absolute YES... I work in R&D for U-Haul and our "Sport Trailer" has been a well documented, consistent mileage improver. It's a small, aerodynamic, fiberglass trailer with an overall height equivalent to most mid-size/compact car's trunk. Even loaded to it's GVW of 1,500 lbs. our tests have shown anywhere from 2-10% improvements in fuel economy. Aero is definitely the most direct means of improving your economy given a fixed RPM. I wonder if that flat-bed trailer of your buddy's isn't acting much like the extended decklids seen on Pro-Stock drag cars? His mileage may well improve with a flat enough trailer deck, coupled closely to the vehicle, along with farily low rolling resistance tires.
Aside from my off topic thinking out loud about a trailer we've not gotten too far off topic. Discussing things here on the HAMB is a lot like the BS sessions in the garage. If you're gonna ask about gearing and mileage you're going to get a lot of other advice along the way . . . most it pretty good. Anyway, interesting stuff about the trailer and mileage. My pal's problem is he doesn't pay attention nor does he care to an extent. As long as he can be first and best . . . ya know?
The '69 Mach I had for a million years has a 428SCJ in it. The cam is a Crower 297HDP. Its not stock, it was the biggest hydraulic stick they made in 1988. (.588/.586lift-236/242 degrees duration @.050) The motor had headers but was otherwise pretty much a balanced and properly assembled stocker...sort of. Yes, it was over cammed. When I swapped the noisy 3.91's out of the nine-inch for 3.50's I went from a consistent (years of data) 14mpg to 11.5 with no other changes. Its the combination that matters, not just the gears you have out back. Humming down the road at 2800RPM made the motor much happier than 2400RPM. We build plenty of motors and this problem is more common that you'd think. Both performance AND mileage are effected by the combinations efficiency at normal driving speeds. Just tossing in taller gears willy-nilly may not be the best way to improve your mileage. So let me know when that check is coming an I'll let ya know where to send that million dolllars........ -Bigchief.
I find this thread very interesting... My 36 coupe has a 350 TPI /700R4 and 2.79 gears. I get 24 MPG at 70 MPH at 2000 rpm. It loafs down the freeway. I have people tell me from behind my exhaust smells a bit rich since my rpm is not high enough and I am not pulling an adequate vacuum for the computer to adjust the timing and gas flow properly. The car is a dog off the line..... of course. I bought some 3.50 gears, which I need to put in. I have been told by several people my gas mileage should go up at 70 MPH with running the motor more in it's sweet spot of 2500 RPM. Makes sense. My chevy truck runs about 2500 @ 70 MPH. Manufacturers must optimize for fuel ..... right ?? Doug
That formula is pretty much to get you close so you can figure your rear end and what tires you want to use, but i dont think that 20mpgs is out of line in pretty ideal conditions. as far as i can tell in my own driving experience the formula comes in real damn close on rpms, guess you'll have to get the car on the road and see where it runs best at.
Done... I get better mileage in 5th at 2600-2800 than I do in 6th at 2000-2300. I have every gas receipt and a mileage log for the 46k miles. The math and the computer agree. I get better mileage in 5th at 2600-2800 than I do in 6th at 2000-2300. I have every gas receipt and a mileage log for the 46k miles. The math and the computer agree. Two trips to Joplin and back from NC. NC to NJ. and a bunch more. I love how the BS changes now. Face it some combinations do things you can't comprehend.
It takes 1/2 pound of fuel (gasoline) to make 1 hp for 1 hour. Period! The amount of work being done determines the HP required and therefore the fuel used. As long as Rpm is in the window or sweet spot of the engines set up it is not really that important. The vaccum gauge suggestion was excellant. Greater the load the lower the vacuum. Lower the vaccum the more the power valve or power piston is allowed to raise or open increasing the fuel flow to the engine. In the case of a MAP sensor and efi the lower the vaccum the more the portion of fuel controlled by the map sensor signal is allowed to increase. Gear the car so it is happy, the engne is normally operating in its power range , not above or below it. Vaccum is as high as possible. This will give good mileage. Outside the engines operating range or sweet spot will give poor mileage. Although it may not be real sceintific most of us can tell when the engine is happy and when it is not. This also why big engines sometimes give better mileage than little ones. For instance my V 10 2500 ram gives 20.5 to 20.6 MPG (imp 160 oz gal) The same truck in a 318 gives about 17. In a 360 about 18mpg. The 488 cubed V10 is loafing with the truck at all speeds while the 318 is straining constantly. Therefore the map sensor is telling the 318 efi system to deliver all the fuel it can that is under the control of the MAP sensor . meanwhile the v10 is rolling along with high vacuum and wondering what all the fuss is about. It is barely awake moving the same amount of load. This is why overdrive has come and gone several times in the last 100 years. Even though it sounds on the suface like a wonderful solution in practice it doesnt always work out. Don
I am issuing checks immediately. They are from a bank in Nigeria. I hope that's okay? My own experiences have always been just the opposite (lower-numbered gears = better mileage in the same car), but I've also always had bone stock or 99% stock motors and maybe that's made the difference.
This is a good post. I was afraid it would be one of those I get 45 mpg with a 628 CID Olds pulling a trailer. I am building a fenderless 30 A coupe. Chopped 4" with a drilled visor and a 292 CID SBC. It has a TH-350 and an 8 inch. I have 2 sets of gears now, thanks to CAL. I have a single track 3.00 and a limited slip (I think) 3.50. The cam is pretty small, it is a 260H comp cams and I have some long tube headers with a 500 CFM Edelbrock 4 barrel for it. I will be running electronic ignition and I have a set of 29.5" tall tires for it. I am hoping for 20mpg. We'll see what reality holds.
my dad had a 79 chevy pu with a 250 and got only 12mpg. he threw in a 305 with a high rise intake, 3/4 race cam, headers, straight pipe with glasspacks and a 650-700cfm carb. he still got 12mpg in the city or 70 on the highway. when he was goin 80-90mph, he got 18. it's all about the rpm's the engine was built for and the vacuum. just what i've learned.
Hey Roothawg...that's a REALLY close set up to what I am shooting for... I am in the middle of still putting the pile of parts together...keep me posted on what she does, and I will do the same if I get mine going first!
It seems that when running roadsters it's not so much the engine brand or size and maybe even not the rpm, but just the little fact that it takes X amount of horsepower to cruise the highways at a steady 70 in these un-aerodynamic wonders. One reason that a lot of combo's in similar cars return about the same mileage. There are times when I run the highway and try to take it easy, but the car still returns 16 mpg almost 17 at times. I have a smog Holley 600 cfm I bought new and it returned the same mileage the 750 cfm Carter did when I ran it. Now I'm running a 750 cfm Edelbrock, jetted lean for the altitude and still getting the same highway mileage, but in-town mileage improved a bit after it was jetted down. (About 1 mpg.) Been doing some reading on it and thinking maybe I ought to go to a set of metering rod springs one step down (weaker) from what I have. The springs are the same ones I ran when I lived at a lower altitude. If I don't end up with a mid-range bog, mileage may improve. Probably need to try this before I do the diff swap from 3.70 to 3.00. At 462" the engine could be considered large, but the cam is pretty mild with 260 - 266 degrees advertised duration. If I keep my foot out of the secondaries it should get the same mpg that a similarly equipped 327 SBC does.
Well I have a car that runs 4000 rpm @ 75 MPH and gets 37 MPG, I wonder what a 6 speed instead of a 5 spd would do for my ol Civic. My other car that is a brick can't get 12 MPG at 70 too many cylinders I guess. But damn sure is more fun to drive!
The Jalopy Journal
