I have a mild 350 chevy with stock q-jet carb, duel exhaust with ramhorn manifolds , 700r4 ****** , 3.08 gears in the rear with 26"tall tires on my '49 AD . It's not bad on gas out on the highway but I noticed alot of guys with the overdrives running gear ratios in the 3.55 ,3.73 , 4.11 range and was wondering would I get better milage going with something like the 3.55"s ? I can't check my milage now because my speedometer gearing is off and I don't want to correct that if I might change rearend gearing later on . Also I don't have a tach hooked up yet so I'm not sure where the rpm's are @ cruising speed .
wow...3.08 gears and a 700R4? with the 30% overdrive the effective gear ration is 2.156 how does the motor pull your car at cruising speed? seams to me it would be laboring hard
X2 The slower the RPM's the better the mileage,,WITHOUT luggin' the engine as 36-3 said. That'll only shorten engine life. I'd get a better carburetor so that the air fuel can be tuned for mileage too. Can a 350 get 20 mpg? lol.
A 3.42 or 3.55 gear is going to be the ideal gear for the best of both worlds. The 700R4 has 3.00:1 first gear in it so you do not want to go to a 3.73 or 4.11 because the motor will turn up too quick and then immediately shift to 2nd. The problem with the 3.08 gear is that the engine is lugging along at highway speeds in OD with the motor turning around 1500 rpm. You need to shoot for a highway cruising rpm of about 2000-2200 rpm to get the engine more into it's torque rpm range which you can do by using a 3.42 or 3.55 gear. You will have better power at hand when you mash down on the pedal and you should get a little bit better gas mileage because the motor is not lugging along and straining to pull the vehicle along. Roger
Main problem with 'economizing' larger displacement motors (yes 350 is large when thinking in those terms) the motor is designed to breathe deep, not shallow. You could tune it better, re-jet, for maximum economy (if that's your goal), with 3-2s on progressive linkage. Tall gears are an aid in that direction but when it's all said and done, with a car that heavy, 22mpg was about the best it could get originally with it's smaller stock motor with a single 2bbl. I can't see you ever getting there with a 350.
you can check your mileage if the speedo gearing is off, just figure out how far off it is and correct for that. in AZ the highways have a milepost every mile, it's pretty easy to see how far off the odometer is. or use a GPS to find out how far off it is. I had a 90s Crown Vic with 3.08 gears and overdrive, it got over 25 mpg on the highway. It's probably a bit more aerodynamic than your pickup, and that makes a big difference on mileage (and driveability) with gearing like that. But a stockish 350 with Qjet and low 3 gears with overdrive should be able to get 20 without much effort. Adding more gear will make the truck more fun to drive (it will accelerate better), what are your priorities?
I have a o/t 98 burb that gets 19.5 on the freeway with a/c full on and 80 mph with cruise. 71 Caprice average 18. Both 350's with auto trand. One carbed, one infected. You can do it.
The donor for my 49 GMC was a 91 Fleetwood. Stock, the caddy had 305 TBI, 700R4, 3.08 rear, and 28 inch tires. It was rated at 23MPG. (and that was with the longest car made in the U.S. for 1991) I'm running the 305/700R4, with 3.42 rear and 29 inch tires, I just got the correct speedo gears, so I will be checking my mileage this week. p.s. finding the right speedo gears was a PITA. Rich
My old Suburban was built with a 700R4 and a 2.73 rearend. I managed to pull 17 out of it on a 4 hour drive that I ran around 75 most of the time. It never felt like it was lugging, though. And I even pulled trailers with it on a few occasions. Have one now with a 3.73 and the milage is about the same as the other one, average about 15. Having the aerodynamics of a brick is probably the biggest issue there. Have been thinking about swapping the rear from the old one over, but the new one has an electric speedo and I don't know if I can correct for the difference. Generally to gear for maximum fuel milage you need to know your motor's torque curve. If you cruise around the peak torque RPM, you should get the best fuel milage. I'd just use an RPM calculator (many of them available online) and plug in different rearend ratios until I got close.
it's not that simple....you're running at part throttle, so you really need to look at a 3d map of BSFC vs rpm vs torque
...and if you're running a carburetor, make sure that it isn't revving so low at road speed that the carb's still drawing off the idle circut.
Stock, the '91-96 Caprice/Roadmaster/Fleetwood/Impala SS used the 700R4 (or it's electronic equivalent) behind a 265/305/350 with 2.56 or 3.08 gears and 27-28" tall tires. Mileage in the 20s and no lugging of the engine. Granted, they were either TBI or MPFI, and more aerodynamic than a '40s brick. I doubt you'll have any issues with "lugging" the engine. Mileage will be better than with a 3.42, 3.55, etc.
These trucks are NOT heavy, mine only weights 3200lbs but they're not aerodynamic. The '95 Impala the motor came from weighed 4200lbs, got mid 20's with a 3:08 rear and achieves it max torque of 330ftlbs at 2400rpm. I figure I should be able to get around 25MPG (US gallon) or 30 MPG (Imperial gallon) with my truck. I'm running a 3:42 in my truck (26.5" tall tires) but I can't tell my MPG till I get the damn 700r4 to go into 4th.
My '40 Chevy convert which weighs in at about 3700 pounds has a ZZ4 crate motor with a 600 Holley, block huggers and 2-1/2" exhaust, 700R4, 3.55 gears and 26-1/2" dia. rear tires. On my last 7000 mile round trip to Bonneville with a loaded Mullins trailer behind I saw a few tanks in the 21/22 range and one at 26 but the overall average was right on about 16-1/2. That average is about the same as the lifetime average on one of my 4.2 6 cyl trailblazers. The trailblazer would see 21/22 but, well you see where that's going. We tend to call our mileage out as being whatever the best number ever was - human nature. I'm happy with the '40 as it'll do what it does every day, all day long, in comfort. Just one example of the mileage combination. Charlie
True. And even then it's going to vary from vehicle to vehicle and even one tank of gas to another. I'd be able to get better milage if I could get gas that wasn't 10% ethanol, for instance. But that's another story.
Since I don't have any equipment for doing live calculation of Brake Specific Fuel Consumption, with today's gas, and under varying conditions, I use the next best thing. I run Oxygen sensor in the right header collector, and an air-fuel meter. Sneer if you want, but it made for a 12% increase in mileage on the freeway, and no perceptible loss of power, with just minor carburetor tuning.
LMAO, obviously you have no real experience with Quadrajets. They're really about the best street carb out there and do very very well.
Street SBCs are most efficient in the 2200-2400 cruise range, lower lugs the engine too hard. You're also going to find with a 3.4-3.7 gear the around town mileage might get better too, since the engine will accelerate with less throttle. Sometimes combos like yours get a small mileage gain from an exhaust crossover pipe. Modifying your distributor for the most rapid advance curve your fuel allows can also find a smidge of mileage. good luck!
Quadrajets are tunable. My son and I followed some advice given by a GM tech in making some small changes to a quadrajet out of a 79 Corvette. Car is a 51 Merc, 355 Chevy, performer cam, flattop pistons, 1.94 chevy heads (can't remember chamber CCs, but think they were out of a 350-350 engine). 700R4 trans with 3:31 rear gears. 1800rpm is 65mph. Before metering rod changes we got around 19mph driving 60 to 65mph on two lane roads. Trip back from Mokan drags at 75-80mph we got 17mph. Primary jets were 71 and metering rods were 43's. Recommendation was to disconnect carb pump, do a quick stab at the throttle. If it didn't hesitate, it was too rich, if it bogged terribly it was too lean. Correct would just be the slightest hesitation, but pick up immediately. Our carb did NOT hesitate. Recommendation was to only make changes to metering rods by 10% so we put long 46's in it. (It was supposed to use short rods but I didn't check and told my son to get the wrong ones ) On a rainy day trip from Indy to the Pileup the Merc got 25.5mpg. The mileage and speed is taken from a GPS and we fill tank very slowley, rocking the car until gas its to the tip of fill-tube. On the return trip back we went mostly Interstate running at 75-80mph and the Merc got 21mpg. Granted, this is only a couple of trials and I've gotta admit the car feels a little lazy on part throttle. We think the short 46's will help resolve that issue. Prior to the metering rod change the Merc ran quarter at Muncie in 15.5 with a 95mph trap speed. Not sure how it will run now, but it's a custom, not a drag car.
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