Gas Mileage???

I have a 2002 Diamante with 34,000 miles on it. I am very disappointed with the gas mileage. Recently, I discovered I was only getting about 21 miles per gallon with at least 80% highway use. The rest was regular city driving. My wife has a Grand Marquis with a v8 that gets around 24 under the same conditions. The only thing different about the car from stock is a set of 18 inch rims and tires that were on the car when I bought it. What kind of gas mileage should I expect? Do the oversized rims and tires have anything to do with my dismal gas mileage? Is there anything I can do to significantly improve gas mileage? By the way, I do not have a heavy foot. Thanks

Papasmith, a second generation Diamante will attain as much as 38-40 MPG. This requires the engine speed to not exceed 2100 RPM. Using this RPM with fourth gear, the vehicle speed will be about 55 MPH. But even slight and infrequent RPM increases will significantly reduce fuel efficiency.

I have consistently completed about 185 miles on a 1/4 tank capacity of gasoline in a 2001 Diamante. Using the fuel capacity specification of about 19 gallons listed in the user's manual, this places approximately 4.75 gallons in every 1/4 of a tank holding. 4.75 Gallons x 38 MPG = 180 miles for every 1/4 of fuel tank capacity. Perhaps a 700 mile range from a full tank.

Maximum fuel efficiency in any machine is based upon how much work or frequency the device performs. Lower engine RPM's support less fuel consumption. Considering the manufacturer's statement of a 5000 RPM limit in the Diamante 3.5 Liter engine, this engine operating at 2000 RPM uses 40% of Total Available RPM Range. This is a good fuel efficiency for vehicles that have a gear ratio to keep engine speed at 2100-2200 RPM and vehicle speed at about 55-62 MPH. The Diamante's transaxle has the advantage of a high final drive gear ratio making this possible.

Some drivers might look at an RPM dashboard guage with a listing of 9000 RPM and instinctively think they have up to 8000-9000 RPM available. They might think operating the engine at 4000 RPM is less than half of the listed range and a desirable fuel efficiency is possible.
With the Diamante's engine RPM limit of 5000 RPM, operating the engine at 4000 RPM is using 80% of the Total Available RPM Range. Fuel efficiency is almost at the lowest at this condition.

Fuel efficiency is a direct result of engine speed. Lower RPM's mean less times the intake valve will open and less times fuel will be sprayed into the combustion chamber.

Improved fuel efficiency during non-cruising operation, city driving, can be reached by not exceeding 2000 RPM. It is a slow process, especially when accelerating from a standstill, but it is possible. Releasing the throttle pedal when the RPM reaches 1900 will cause the transaxle to shift into the next higher gear. Repeat until 4th/final gear is held. Acceleration will be that of a tractor-trailer vehicle and perhaps not practical, but always an option.

Finally, Papasmith, larger tires do place an increased load and resistance on the vehicle and powerplant but a lower profile tire may have less rolling resistance than a higher profile one and may not affect overall operation in a significantly negative way.