jskennedy

drove down to the coast this friday & decided to utilize the long stretches of flat road to get some mpg avg's

ran various speeds for 10mile stretches to & from the beach & averaged the results

surprisingly & unfortunately the best mpg was @ a speed slower than i want to travel

also surprised mpg was a high as it was @ 85mph

LexBob2

Interesting. For example, there is a 18.8% decrease in mpg's between 65 and 75.

sleepysurf

I have found the MPG computer to vary quite a bit from min to min, so I doubt it's a reliable measure of current gas consumption. The cumulative "AVG MPG" number seems to correlate better with actual (calculated) consumption.

MEP7557

That would explain why when I'm on the freeway my avg mpg does not go up radically. I drive about 75 mph. That will teach me. Thanks for the chart, it may not be dead on but I think it gives people something to go by.

wa3cuj

jskennedy,

My wife would say that you need to get a life!!! You and I may have similar backgrounds (engineering) which leads me to do things just like your chart.

This is a great depiction of why Richard Nixon lowered the speed limits to 55 mph back in the 70's when oil supplies were a problem.

Thanks for the time and effort.

jskennedy

the min-to-min graph does vary a bit

mine had less variance @ higher speeds

the #'s i posted were all collected from the avg-mpg

i've compared the avg-mpg reading to the actual miles-driven divided my gallons-pumped-in-tank & it's always been amazingly accurate

antonio98

Thanks for posting your findings!

jskennedy

i'm a information junkie who enjoys tuning, tweaking, & maximizing output

my background is computer science

RXSF

The car should get the best mpg at speeds near 55 and it does decrease rapidly after that. However, I think you normally do get more than 20 at 75. On flat stretches at 80, my RX330 which is less fuel efficient than the 3.5 engine can get 23-24mpg. THere are many factors such as wind and even a minor gradient change to affect those numbers drastically.

jskennedy

i'm with you on the wind & gradient changes

what are you basing your 55mph claim on ?

RXSF

if you just search on google for most efficient speed, everyone claims that the magic number is 55.

jskennedy

i remember reading long ago 55-60 was ideal

i believe the 55mph # comes from the nixon era

3 speed trannys may have been the norm back then

the best mpg should come from the lowest speed you can drive in the highest gear

never really watched the shifts that much, but i'm betting the RX shifts to 6th around 40mph

i'm pretty confident in my 40mph # for best mpg

my test is far from exact science, but i bet the #'s are good within a couple of mpg

RXSF

There has got to be a bit more than just lowest RPM on the 6th gear. It is about the energy needed to hold a speed, so I would think you need to take into account the momentum and force that pushes the car forward balanced with the energy needed to maintain the speed.

widebandph

Fantastic post. Don't get too caught up on the variables since this is a ballpark estimate. It will definitely give people a good idea on what to expect when they drive at certain speeds. Of course YMMV.

ChitChat

Wow, those are impressive numbers! Unfortunately, I haven't see anything close to that in the past four months I've had the car. I don't know if I'm doing something wrong or if the computer in my car isn't working properly. I have only put the highest octane gas in my car since I got it - 93.

For example, the calculations on my dash usually averages 13 mpg for local driving and when I combine it with highway driving at about 65 mpg it goes up to 17 mpg the most. Also, my mph (miles per hour) calculation on the dash is always around 18. How come it always stays at 18 when I never drive that slow. When I first got my car, I brought up this subject and was told that idle time is calculated into the mph calculations, but I am not idle that much to bring my 40 mph local driving and 65 mph highway driving down that far. Also, I was told that my mpg averages would go up after a few gas fill-ups, but it's always the same.

This is what I do - when I fill up a tank of gas with 93 octane, I reset the odometer to 0. Is this the proper way to get the correct mpg calculations? And, I don't know what else could be wrong with the mph calculations.

Suggestions please . . .