What rear gear would give best mpg for a 351c 2v with p235/60r15 tires?
Generally your peak efficiency is at peak torque. If you know where peak torque falls you can calculate the rear gear from RPM, tire circumference and transmission high gear ratio.
If that were true then cars wouldn't have overdrive. Looking at dyno torque curves, peak torque is usually around 6000 rpm. That's certainally not the best rpm for mpgs. The ideal rpm for MPGs is at idle speed, but there is no power at idle, so that's not practical.
As for rear gear, it's a compromise. You want the tallest gear (lowest number) yielding the lowest rpms for mpgs. The problem is that at lowest rpms the car is not drivable at speed because it has little power at low rpm. So as a compromise, most cars do well at about 2000 rpm at hiway speed. You can calculate your rpm at different speeds at ring-pinion.com,
using different gears, tires and tranny gears.
HMMMMMM
Fuel Economy, Engine Efficiency & Power
by Rad Davis, Virtual Vairs
Philip Stevens wrote: I'm trying to understand fuel economy and engine efficiency. Does an engine run most efficiently at maximum torque, or maximum horsepower, or where? Also, is the point at which an engine runs most efficiently also the point of maximum fuel economy? I've also been told that manifold vacuum is related in some way to fuel economy. Anyone care to elaborate?
Rad Davis answers: Ah, a technical question! Just what I like.
An engine (no accessories), running at full throttle (we're ignoring power valves and other enrichment gadgets) has its lowest fuel consumption/unit power produced at the torque peak. The horsepower peak is a mathematical artifact of increasing RPM coupled with decreasing efficiency, and is not usually a good place to run for max. economy. Unfortunately, unless you're running a generator, or pump, or something like that, it's not a particularly relevant value, because automobile drivers *do* have engine accessories and power valves (late corvairs, anyway) and *don't* spend much time at full throttle at the torque peak.
OK, now a little quick theory: The reason that economy (efficiency) maximizes at the torque peak is because this is the place where the engine is inhaling the greatest amount of fuel and air into the cylinder which, when burned, makes the peak amount of cylinder pressure and thus the maximum torque.
So the instant you close the throttle some, you're losing efficiency and your fuel economy (expressed per unit power output) goes down. Again, if you're generating electricity or pumping water, this is a bad thing.
But cars are in a variable load environment. You may be going downhill at 60 mph, and only need 1/2 hp to maintain speed. So the engine is horribly inefficient in producing this 1/2 hp, but you're demanding so little compared to what the engine can make that your economy (expressed per unit distance traveled (like MPG)) is extremely high.
If you then complicate things by including parasitic power losses to the cooling fan, vacuum advance, power enrichment, and aerodynamic drag, things get pretty murky, at least from a theoretical standpoint. The good news is that we can make some emperical generalizations:
Assuming that the engine is operating in its powerband (roughly 1200 RPM to 4000 RPM on two-carb corvairs), and throttle is between zero and 80% of travel, the slower the engine is turning, the better fuel economy will be. So upshift early--fuel economy is always best in top gear. This is especially true with a Corvair, because the power consumption of the cooling fan is a cubic function of RPM.
The slower you go (assuming the 1200 rpm/80%/top gear rule) the better fuel economy will be, because aerodynamic drag is the largest single power sink above about 35 mph. I can't find my 110 HP Stock Engine Test Report (lots of stuff still in boxes), but best fuel economy for the late sedan with that engine was something like 32 MPH, according to the dyno numbers.
The less you start and stop, the better fuel economy will be. Every time you start the car, you waste all the kinetic energy as heat by using the brakes. Starting from the stoplight sucks up power (and fuel) replacing that lost energy of motion. Accordingly, a lighter car will get better fuel economy irrespective of any aerodynamic changes. It takes 3200 lb-ft to raise my Greenbrier one foot. It only takes 2300 lb-ft to raise a late coupe that far. Every time you climb a hill this comes into play.
About vacuum gauges: These make more sense on cars with huge engines. If you've got something like a caprice with a 454, the fuel economy of the car is related almost exclusively to the amount of fuel the engine is demanding at that instant, regardless of speed. Our standard joke about our '73 400 CI Pontiac was that it got 13 mpg uphill, downhill, a/c on or off, towing a trailer or not. The reason for this was that the engine could make something like 350 HP. And in normal operation, you used maybe 60 HP. Highway cruise was something like 20 HP. So in typical driving, you could get around with 20% of the engine's total power output. The engine is in a hideously inefficient regime all the time, so all you can do is try to minimize the gross fuel consumption.
Engine vacuum is a good approximation of just how much fuel and air is going into the engine. Vacuum is near zero at full throttle. Higher vacuum=less fuel. So if you use the vacuum gauge as a guide, less is better.
Where this falls down is when it runs into situations when the engine *can* be steady-state loaded near full throttle and the torque peak. In our Pontiac, this would be pulling a trailer climbing Mount Everest at 60 mph in top gear. Fortunately, Corvairs have smaller engines relative to their weight, and weigh less than that Tin Indian did, too.
So if you take something like my Greenbrier (Corvair minivan, if you don't know), and drive down the road with a low rearend ratio (3.27:1) and tall van tires at 65 MPH, you're either at about 60% throttle or you're slowing down. Climbing even a 6% hill on an interstate you get up to 75% throttle or so. So you're in a situation where the engine (as a black box) is starting to be very efficient. Sure enough, the Greenbrier never gets less than 20 MPG on interstate trips, and set a record of 28 MPG driving through the mountains of Tennessee in heavy traffic. By the vacuum gauge, the fuel economy should suck, but in practice, it's better than most turbo corvairs can manage, even though the van is heavier and less aerodynamic.
This is the secret of cars like the Geo (now Chevy) Metro--it doesn't weigh anything, has no frontal area, and has a tiny engine (1 litre). So the little engine still has to work quite hard to drag even such a light car around, and is working very efficiently most of the time. Result: 54 MPG.
