I'm not sur the vx will react well at 35mph in 5th, already at 50mph it struggle. I think there is no point of going slower then 53mph (85km/h), for the difference in mpg. Already at 55mph (well 56mph! 90km/h) I'm doing great on the highway/backroad, and going EVEN slower will give me what, 2-3mpg MAX.
The last 5 days here temps have been in the mid to high 90'S. I disconnected the WAI becuase the idle speed on my VX was so low when the cooling fan came on the engine almost stalled before the load sensor could kick up the idle speed. I have actually run the AC some in the last week when outside temps were extreeme (hit 100 once).
MY VX has only 33,000 actual miles on it. It was totalled and stored inside in AC for 13 years, used to train insurance adjusters, still has original tires.
I have no real problem with 35 MPH in 5th gear. Of course acceleration is very slow, but maintaining 35 is no problem.
Aero drag is a square function of vehicle speed, so the faster you go the exponentially greater the drag.
Assuming your car has no issues that would affect its driveability, it shoud be fine at 35MPH. In fact my shift indicator light comes on in 4th at just over 1000 RPM. Below about 1000RPM in mine it gets a little rough because the engine is producing only 32 power pulses per second, and the flywheel is probably a little too light (stock) to control the vibration.
To get an idea of the difference in drag just square your speed. 30 MPH would be 900 and 50 MPH would be 2500 units of drag or almost 3 times the amount.
Remember the hypermilers who got over 100 MPG in the Prius averaged 36 MPH.
I reduced my maximum speed (less than 52 MPH) and limited pulse and glide to 45 or less, with DFCO but no engine off until the last 3 tenths of a mile to my house.
68.5 MPG for that tank.
I run absolutely no accessories normally (car has no radio from factory), and use gliding on any available downslopes. My normal idle speed is very low the bottom of the tach needle is even with the small slash mark just above zero.
My guess is it probably idles below 500 RPM.
As far as the WAI, it's important to understand the fuel injection imputs automatically compensate for temperature. That means in Phoenix Arizona where the record temp is close to 137 degrees the system will compensate for that temp. My WAI is very simple. I pulled the intake snorkel off the resonator pipe just below the snorkel, took some snips and shortened the horn of the snorkel just enough to allow me to reverse the snorkel from it's normal position so it draws air from right next to the top radiator hose. This is where the coolant is 180 degrees, but I would bet the intake temp is probably about 130 to 140, somewhat cooler in winter. My car also has about 2/3rds of the grille blocked from the factory.
No other aero mods and both rear view mirrors are still there.
My daily route is about 40 miles with almost 50 traffic lights to deal with and fairly heavy traffic at times. My current average is 59.31 MPG for the last 3 k miles. This car has been on the road for less than 2.5 months after being repaired from the original 1995 accident.