Anyone found the most fuel efficient RPM for freeway speeds for an MCS? My original impression was that because of all the low-end torque, you could cruise at 2,000 to 2,500 RPM at freeway speeds. However, on a recent trip to San Jose & back (200 + miles) it seemed that running at about 3,000 RPM gave equal or better numbers on the instant MPG readout.

That got me thinking about the dyno charts showing a drop in torque at 3,000 RPM. IIRC, someone commented that the drop was to get the best results in European carbon emission tests. I wonder if that leads to 3K being an efficient cruising RPM. Any thoughts?

 

if your car is stick... just cruise in 6th gear.

 

Yeah- I've noticed that right about 3,000 RPM seems to be more efficient when in the same gear- 6th is almost always the best for economy at freeway speeds that I've noticed. So far 5th seems better for longer uphill climbs that need more than maintenance throttle though.

 

When I am cruising above 59 mph, I leave it in the 6th.
If I am going up and down in speed up to 59 mph, I go up as high as 5th but not into the 6th.

At 40 mph, if there is no traffic ahead I rev it up to 4K rpm and drop it into 2nd and put the pedal to the floor and the car flies.

 

Good question, but unfortunately, there's no good way to predict an answer by looking at dyno plots, gear ratios, or anything like that.

The number you're interested in is called "Brake Specific Fuel Consumption", or BSFC. The BSFC is how much fuel you have to burn per hour to produce one horsepower (or one kilowatt, depending on whether you're using metric or English units). Since the amount of horsepower required to cruise at a certain speed is the same, regardless of what gear you're in, you want to find out which gear puts you at the engine speed with the lowest BSFC.

Unfortunately, there's no way to predict BSFC - it has to be experimentally measured, and for any given engine speed, BSFC varies with throttle position, so even if you went to a dyno shop that was set up to measure BSFC, you'd have to do dozens of runs, using a different throttle position each time.

On the bright side, BSFC generally increases with engine RPM, so if you're talking about *cruising at a steady speed*, you're almost always better off in the highest gear available. The exception would be if you chose such a high gear that you "fall off the bottom of the torque curve" and are lugging the engine (i.e. you wouldn't want to cruise at a steady 20 MPH in 6th gear, even if you could keep the engine running that slowly).

 

I was finding that dropping into 5th, to get up to 3K rpm, didn't lower the instant mpg numbers and possibly raised them. Perhaps I had enough head wind to make 5th necessary, but I don't recall strong winds. If you have a chance try it out and see what results you get.

 

Well, I'll be driving from California to Texas and back (3200 miles round trip) next month, so I'll have a chance to take some measurements.

Unfortunately, I have a first-gen car, and my torque peak is nowhere near as wide as yours, so my readings won't be good for much more than general information.

 

How's about 55 mph in 6th gear, AC off, cruise control on, Syntec 0W20, tires +5 psi, downhill, windows shut, side windows pulled in, antenna off, drafting behind an 18 wheeler, with a 50 mph tailwind, first thing in the morning after a 12 hour fast. It won't get much better than that.

 

I have noticed this as well in my car. While I've not yet done a whole lot of highway driving, I take a specific route to work every morning where I average about 40-45 through a lot of twisties. I find that if I stay in second or third - right about 3000-3500 rpm - my real time mpg seems to stay 40+. If I drop to fourth, it will fall to about 25ish. Again, this isn't to say that the OBC is totally accurate, but it's not far off, considering that the computations for air/fuel mixture, timing, etc. are done by the same system.

 

I just put it in 6th and keep with traffic. Usually stay around 3500-3900 RPM when traffic is fluid. I really don't see how you can cruise on the freeway at 2000RPM and not get ran over.

 

That's running at a pretty good clip. You will find that in California when there isn't too much traffic, but when it gets heavy it tends to slow down to 60-65.

 

I live in California, and commute from Sac to SF every week. Traffic is generraly very fluid in the mornings on the 80, but you're right, as you get closer to SF it slows to a crawl where you are lucky to even see 60mph. We were driving to South Tahoe last weekend, and were lucky enough to catch some open highway. It's amazing to see how fast people drive here (in Ca) when the roads are clear

 

If you're getting 40+ MPG through twisties something is wrong!

Seriously though, the OBC is fairly primitive. It absoutely does NOT calculate air fuel (These car's don't have a wideband on them anyway) or timing and use that to determine MPG. It has a preset set of parameters telling it what MPG should be at certain positions of throttle input, that's about it.

My car has 440CC Injectors in it, my car regularally tells me I'm getting 40+ MPG cause comparatively, even though each injector pulse is a LOT more fuel than stock (100 CC's more) it's not pulsing as much. Obviously I know cruising on the freeway at 70 MPH i'm not getting 46 mpg. I wish!

 

Aw man, 60-65 is slow? That's where we start out!

 

Well, it does slow down to stop & go creaping during rush hour, but when traffic is moving, there are certain freeways in California where driving the speed limit is considerably below flow of traffic.

 

So what did you get lest weekend on the "twisties"?

 

Have to be close to the semi to stay out of the turbulent flow and a laxative the night before and no water for a week prior!!!!