Cruising 80 to 100 MPH across the desert and about 30 minutes of
street driving
with two dozen street lights gave me 18.6 MPG. 216.7 miles.
11.645 gallons.

Paul Lamar

Ageless Wings wrote:

Paul, and other smiling RX-8 owners ...

I've had about 15 fillups now, and have been keeping track of the mileage
for each one. Been filling up after the low fuel light comes on...usually
takes between 12.5 and 13 gallons then. Nice to know I've got a good

buffer

of around 60 miles to find another gas station if I ever get too far out

in

the country!

The worst mileage was when I tried that one tank of 92 octane ... 17.76

mpg

(229.7 miles - 12.931 gals.).  The best was one tank of 87 octane when I
traveled the entire trip to Pittsburgh around 70-90 mph ... cruise

control

was set at 78 ... 25.59 mpg (334.8 miles - 13.083 gallons).

All of the rest have been between 19.4 and 20.85 mpg, averaging 19.84
mpg...all on 87 octane gas...one tank on 83.

The low fuel light came on last night as I pulled into the garage.  I had
spent the better part of the day in the southern tier, down near
Pennsylvania...mostly high speed country roads, two dirt roads that

amounted

to about 5-6 miles total.  No expressway driving. Have 275.5 miles on the
"A" odometer (143.9 of them yesterday, recorded on the "B" odometer).

If it

takes the usual, about 13 gallons, this tank was 21.19 mpg. I also have a
trip back from my parents on this tank, which is about 55 miles, also

mostly

country roads. The rest of the tank was very short, local drives

(groceries,

library, home improvement stores, friends homes, etc.).

The other extreme is 14 gallons (haven't seen that yet in any of my

fillups,

though) and with that number, it still gets 19.68 mpg on this tank.

My driving is always a good mix of suburban and country roads ... about

half

and half. Almost no city driving at all.  Most places around here I can

get

to with only a few traffic lights...no more than 5 or 6 at worst.

All my fuel purchases have been Mobil or Citgo, if that makes a
difference...doesn't appear to in the numbers above... About equal

amounts

of both (a local Citgo station discounts 10 cents a gallon on Sundays!).
The highest mileage was on Mobil, as was the lowest one! The current

tank is

Citgo.

Harley

Fuel used is of course proportional to HP needed.
Drag on a car is roughly proportional to the square of velocity.
(That is aero drag alone while there is rolling drag as well.)
So aero drag at 100 MPH is 10000X while aero drag at 70 MPH is 4900X
or over twice as much.

HP is drag times velocity or 10000X drag times 100 mph = 1,000,000X HP.
HP is drag times velocity or 4900X drag times 70 mph = 343,000X HP.

Almost three times as much HP and three times as much fuel as it
takes to go 70 MPH.  18.6 MPG is pretty good for a car going those
kinds of speeds.

BTW the freeways in So Cal move at 80 MPH to 85 MPH on average.
Some times you look down and everybody is going 90.

Now I was not going 100 MPH all of the time of course. One has to
slow for curves if for no other reason you need to find out what is around
the corner. The other thing you need to watch out for in the desert
is dips. Dips can launch you through the air for a couple of hundred
feet at high speed and they are hard to see.

Getting back to MPG there are other secondary effects going on such
as engines tend to get more HP out of a gallon of fuel the more wide
open the throttle is. The torque peak on the RX8 high HP engine is
around 5000 RPM so that is the cruise speed that will give the best
HP per pound of fuel or BSFC assuming the throttle is wide open at
that RPM. (Variable pitch prop.) Stop and go driving also effects
the fuel used of course and I was also doing some of that.

Paul Lamar

Back when I was young and single, I bought a new 1983 RX-7 in October 1982.
In December 1982 I drove it from Phoenix to Kansas. I normally got around 20
mpg in the car, and did on that trip except for one tank. I drove nearly all
the way across New Mexico behind a speeding Cadillac doing around 90 mph. On
that particular tank I got nearly 30 mpg. I couldn't believe it or explain
it. It never happened again. It must have been reduced drag following a
bigger car, is all I can figure.

Perry

Your still young Perry :)

Herb Sanders in my ex 1983 RX7 automatic reported 23 mpg at 70 MPH on his 
way from Phoenix AZ to Austin TX just last Sunday night.

Paul Lamar
 
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