I keep hearing that for best mileage one should not drive more than 65 mph.
I assume it’s best to drive in 5th gear with manual transmission. With my 2000 SAAB 95, unlike previous cars I’ve driven, at 65mph, I can barely drive in 5th, because it’s only running at about 2,000 rpms; so it seems to me that my engine would run more efficiently at higher rpms.
Am I right? And if so, is engine efficiency more important or the wind resistance? I’m not just looking for an excuse to drive faster, but my car feels smoother at about 72 mph than at 65.
A manifold vacuum gauge is really a better indicator of shift points than a tachometer is. The more closed the throttle is, the lower the rpm of the engine’s sweet spot, so, while this rpm probably would lug the engine if the throttle was wide open, that is not the situation at 65 mph. In fact, at that speed, the throttle will probably be so closed that 2000 rpm is actually too high, that is past the engine’s torque peak rpm.
Take a look at what closing the throttle does to a typical engine’s horsepower curves.
One reason they’ve added additional gears to transmission (3 speeds used to be normal) it to let the top gear be set to maximize mileage. You use a wider throttle opening in 5th than you would in 4th to maintain 65 mph. This reduces the pressure drop across the throttle, increasing engine efficiency. So no, you won’t get better mileage at a given speed at higher rpms (as long as you’re not lugging the engine).
And you’ll probably get better mileage at 65 than 72, but there are rare exceptions.
Yep, in a lot of cars, the top speed in fourth is higher than the top speed in fifth gear because the lower gear lets the engine rev enough to make the horsepower necessary to achieve the car’s top speed.
There is no simple answer to that one. For example I seldom drive my car at 2,000 RPM.
Even the term engine efficiency is not easy to define. Do you mean it can give the most power, the most torque or the most efficient use of fuel as measured by speed or the most efficient use of fuel as measured by mpg.
I doubt if there are many cars out there that will get better mileage at highway speeds (over 45 mph) at any gear less that the highest. There will be very very few cars that get better mileage at 72 than at 65. How about trying it yourself.
Keep below 65 on one tank of fuel and then keep at 72 on the next, keeping track of the mileage for each tank full. Tell us what you get. I get my best mileage at about 40 mph in high gear.
Determine your “power band.” Start at low rpm while just rolling in 2nd gear. Twist it’s tail. At a certain rpm you will feel the power surge. You are entering the power band. Keep your rpm’s above that point to produce power. Look up the horsepower and torque curve charts. The rpm’s where they cross is generally the most efficient engine operation. These points form your power band for daily driving.
The racers rule of thumb for aerodynamic drag:
Aerodynamic drag at 40 mph is called 1 drag unit. For every 10 mph increase, the drag unit doubles.
With these facts and a modicum of experimentation you can drive much more comfortably and efficiently. The power band in my Saabs didn’t “come on” until about 2500rpm, so it is very likely that 2000 rpm is below your power band. You may have to shift down at 65 and let the engine “hum” or let that puppy roll and produce power! Fill up immediately before and after roading the car to check your mileage. A couple trips using both methods of driving should give you a more definitive answer.
“Look up the horsepower and torque curve charts. The rpm’s where they cross is generally the most efficient engine operation.”
All horsepower and torque charts cross at 5252 rpm. If the engine can’t rev 5252 rpm, the graphs will never cross. Most all dyno charts were measured with the engine at wide open throttle and are irrelevant at part throttle. Do a dyno pull at 10% throttle and you will see a much different torque graph with the peak occuring at a much lower rpm. That’s why automatic transmissions are programed to shift at lower rpms when the throttle is not wide open.
RPM is only ONE factor in determining best gas mileage. Wind resistance is one of the MAJOR factors that effect gas mileage. If your engine rust optimally at 2k rpms which in high gear means you’re doing 70mph…you’ll probably get WORSE gas mileage then driving 55 even though the engine isn’t running at peak efficiency…The wind resistance at 70 is far far greater then at 55.
Agree with Mike that wind resistance is the most important factor over 40 mph. Engine efficiency is not the deternmining factor as to what gear you should be in. Depending on the horsepower of your engine, you stay in the highest gear possible for steady cruising. If you need to pass someone , you gear down!
Drive 72 MPH then. I seriously doubt there is a significant amount of difference with that engine which is incredibly efficient.
These curves are interesting but useless in the absence of the figures for horsepower reuired to overcome the air, rolling and acessory load at various speeds.
This was first observed by Road & Track magazine when testing the dual carb 1955 Chevy V8. The car went faster in SECOND gear that in THIRD! This was one of the first US V8s that could really be wound up.
Needless to say at 95 mph the gas mileage in second gear was dismal and merely bad in third gear. As stated before, matching the rolling and wind resistance of a vehicle to the engine efficiency curves has to be done by experimentation; you can’t sit behind a desk and do that. On racing and rally cars the good manufacturer experiments a lot to get the optimum gear ratios.
We keep getting posts on this and each OP asks the same question and gets the same answer.