when I am idling in gear, my 2010 RX 350 tachometer reads 750.

I am wondering -

is this about right in comparison with other owners?

In general is there any relation between rpm and mpg? e.g similar vehicle with lower rpm yields better mpg.

 

~750rpm seems about the right range to be idling in gear. This also depends on how warm your engine is at the time of idle; cold starts idle will be at a bit higher RPM than a warmed up car.

As far as MPG goes, if you're idling, you're getting 0 mpg regardless of which car you're driving since you're just sitting there with your engine running. This excludes hybrids and electric cars of course =)

But if you're moving, then generally speaking if all other factors are equal and your rpm is lower while traveling at the same speed as the other similarly equipped vehicle, then yes you'd get higher MPG.

 

I haven't driven a vehicle like yours but 750 seems right if idling in gear, I may be wrong but it seems a bit low if your a/c is on. Also factory equipped tachometers are not the most accurate and can be compared with the readings from an obd II scanner.

Are you having any unusual symptoms? If not, I wouldn't be too concerned. BTW, a dirty MAF sensor/iac valve/pvc valve, vacuum leak will likely affect your idle system.

Duplicate engines at higher rpm's will consume more gas under equal engine load/drive trains/tires, Newer transmissions usually incorporate over drive which lowers the engines rpm to save fuel ( probably the main reason for the newer larger Lexus engines to maintain their mpg vice the older smaller cid ones).

Dissimilar engines will vary in their mpg depending on how efficient their combustion cycle is as well as cid of their engines if other factors are equal.

Grades/brand of gas or special additives? That'll open up a big can of worms.

In my experience, driving technique can help out the most for better mpg as well as proper tire inflation and not hauling about unnecessary cargo if your car is running properly. Turning off the a/c helps too if you're so inclined.

 

My 2013 is below 750 ( 710-730) when idling.

 

Seems normal to me.My 2010 Is around there too. Maybe 780. Im not sure. but its under 1000 for sure. What kinda gas mileage do you average?

 

Mine is actually 625 (it is right in the middle of 500 and 750). I observed around 17mpg in stop-go traffic on Premium gas (93 Octane).

Yesterday for the first time, I pumped mid-grade (89 octane). It is 87-89-93 only. I could have used 91 but there is no 91 available in the station i used. Anyway, with 89 i did not see any loss of power or for that matter noticed any difference. I am now watching mpg.

 

I guess you have a 2011 or older, hence the premium " recommendation".
Don't worry, even with regular 87 octane, you' ll be fine.
You will lose some horsepower, but unless you're in a race track, i don't think you'll even notice the difference.

As for mpg, i didn't see any significant difference.
I did an extensive comparison a few years ago with this, making sure i go the same exact route and same "season" driving.

But if you're not comfy about the idea, stick with the recommended fuel type.