I've set the cruise at different speeds while on the highway and checked the mileage using the instantaneous mpg display. I live in a very flat area. If it weren't for overpasses I'd never go up or down a hill.

I find that at 60 mph the needle would swing between 35 and 45 mpg. Lowering the speed to 55 caused the needle to stay between 40 and 55 mpg. However, further lowering the speed to 50 or 45 mph did not improve the mileage. I'm guessing that there may be a couple of factors. Since it's an ECVT the engine is staying at it's most efficient speed regardless of the car's speed. However, you would think that the increased aerodynamic drag at 55 would cause it to use more gas. Perhaps that doesn't come into play until you reach, and go above, 55 mph.

What do you think? Just trying to figure out why economy does not improve (for me) below 55 mph.

Keep in mind I realize that if I dropped the speed a bit more and did pulse and gllide, or simply let the electric motors do the work the economy would improve. I'm just referring to this case where the ICE is providing the power.

 

I've always heard that aerodynamic drag doesn't start to become significant until ~55mph, which would mean that FE wouldn't start to drop significantly until you pass the 55mph mark. I could be wrong...thats just what I've heard...

 

Greengirl-the 55 mph is an important number,but I think the 55-60 mph number is the speed at which aero drag = rolling resistance(tire resistance, gear friction,lube drag etc-all the "friction" and bending of the tire lugs). There is plenty of aero drag under 55 mph, but it is less than roling resistance. It dropsand increases dramatically(relative to itself) as the speed goes up and down.
Funny,I have also noticed that on various vehicles-hybrid, and non hybrid- that I get better mpg readings at higher speeds-up to a point. This is true for our Prius. At kinda low speeds-30 mph or so-I'll have readings of mid 30 mpg, but I can get 50+mpg at 55 mph or so. I rarely show a reading(non EV, or except when rolling off throttle) much above 40 mpg at 25-40 mph. I can get reading well above 50 mpg at 50+ mph.
I taking these numbers off the top of my head, but they are ballpark.
I have noticed the same thing for the Honda Pilot,and Chevy Prizm-better mpg at 50-55 mph than at 35 -40 mph. I have even noticed it with the Pilot locked up in 5th gear.
Orcrone, I have figured-like you-that it indicated a engine sweet spot, but with the CVT's and CVT equivalent transmissions, that doesn't make sense anymore.Besides, it never was a very satisfactory explanation since no sweet spot should be worth more than the aero drag penality.Thanks,Charlie

 

I think you'd find that at 55mph the engine rpm is quite low (not far off of idle), producing not much power and using very little fuel for a decent vehicle speed. Even my Sienna will manage to get into the low 30s mpg with cruise set at 55mph over long distances on nice, smooth, level roads. Slowing down any further might save just a tiny bit of fuel usage but you are getting less distance over a given time period. On the other hand, speeding up a bit the engine still operates quite efficiently up to a point, but I believe aerodynamic drag increases as a square to the vehicle's speed so as you move past 55mph drag increases rapidly (ie 70mph has 4X the drag then 35mph). That means more and more fuel for each mph increment on the speedometer.

 

Theoretically a car should get its best mileage at the slowest speed possible in its highest gear. So it's perfectly reasonable with a normal transmission that a car will do better at 55 than 35. I guess I'm surprised by a car with an ECVT doing that, regardless of whether it's hybrid.

 

One of the features that Toyota has utilized well is the Atkinson cycle for the ICE.

IMO most ( all?) vehicles are 'over designed' for power, especially for highway driving, due to the need to generate significant torque for start up and acceleration, thus the Otto cycle. When these over-powerful vehicles finally do get up to cruising speed they are generating significant amounts of power that are not needed. In 95+% of vehicles it's just dissapated.

Most midsized 4c autos like the ICE Prius and the ICE Camry cruise at about 2000 rpms while at 55 mph. The V6's such as the Sienna turn at about 1800 rpms, but burn more fuel due to the fact that 50% more cylinders are involved. Using the Atkinson cycle for the hybrids allows these vehicles to cruise at about 1600 rpms - but even this amount is often too much power for what is needed. When the power is not needed the hybrids versions of these vehicles either turn down the rpm's to under 1000 or use the extra power to charge the battery.

By using the Atkinson cycle which is weak at take off and lacks torque for passing situations Toyota has been able to keep the speed/fuel usage graph mostly linear; i.e. the faster you go the more fuel you burn and the worse is your fuel economy. This is contrary to the 'normal situation' we have known til now that getting up to cruising speed in an Otto cycle engine by burning a lot of fuel at the outset is the best situation. Until drag/friction kick in above 60 mph in an Otto cycle engine the faster you go the less fuel you burn and the better your fuel economy!!

 

While driving around nags head this week (it's flat and lots of low speed limits) I was able to maintain 45 mpg average (raising it on the 35 mph roads (long straights, few stops), easily staying in the EV mode a lot. In traffic it dropped off some as I started and stopped 0-45 mph several times. I found though that at staedy 52 mph I was able to maintain the 45 mpg rating, but with some effort (attention) reguired. When I went to a wildlife reserve and did a lot of 20 mph cruising, without making stops, I found that my milage suffered greatly.

It seems like the TCH rolls quite easily and once moving takes little power to keep it moving (on the flats). However when moving along slowly it's still a lot of mass to move if you're accelerating or simply trying to slowly accelerate it uses the ICE.

Interesting, if you don't pay attention to the driving style (I had to concentrate on a long trip- it seems easy around town ) I'll find myself driving along getting 20 to 25 mpg on the instant readout and then I let off the gas and then reapply the throttle lightly and the gage drops to 40 mpg. It's a never ending process but doing so can put you from the 33 mpg average to the 38 mpg average with a little effort. It's a "touch" you have to learn to feel as you're driving but with a little practice it's pretty easy to do.

 

Accelerating quickly really doesn't hurt that much, but accelerating slowly for a long time most certainly does. You want to get to your speed and keep it there, or get into EV mode if going slowly enough -- you can accelerate slowly in EV mode without a problem, of course, but when the engine starts running, the car gets 20 mpg or worse when accelerating even a tiny bit, at low to moderate speeds.

 

Following is a graph for Prius, but the graph for Camry Hybrid should be the same pattern since they use the same HSD technology.
Above 42mph, higher speed means worse mileage.

Ken@Japan

 

I've only had my TCH a week, but it seems to get better mileage around 65 mph than 55 mph. I can keep the instant mpg display between 55 and 60 mpg rather consistantly on level sections of freeway at 65, but not at 55.

I filled up for the first time yesterday and drove around anywhere between 60 and 70 mph depending on if I was going up or down hill. After 69 miles (round trip to work) I parked today with the computer showing 46.0 mpg for the tank average! This was mostly highway driving with the AC on going home (with about a 5 mile stretch of < 20 rush hour traffic.) The only effort I made to help the gas mileage was to slowly drop speed when going up hills until I was close to 60 and to let the car speed up to about 70 when going down hill. On the flat I kept my speed between 65 and 70, backing off to 65 if the instant mpg showed < 55 mpg.