: 05 DTS Northstar fuel economy

08-08-07, 11:41 AM
Just bought an 05 DTS with the 291 HP Northstar (only had 8800 miles). On a 500 mile drive home, I averaged 22.6 mpg (with cruise control set at 70 mph), which is less than friends of mine who drive older Devilles with the Northstar engine.

Is this typical mpg for a DTS?

Can anyone suggest anything to have checked on the engine to boost fuel economy? Is there a control module setting that will help highway mpg?

I will be driving interstate miles for the most part, so would like to squeeze a few more MPG out of this beautiful car if possible.

Many thanks!

08-08-07, 11:53 AM
Your model has a 3.71 rear; that rear ratio eats up more gas than earlier models which gave 28 or 30 on highway.

No economical way to increase mileage, although you could buy 22" wheels and tires, like in the hood. $2k to $4k.

08-08-07, 12:57 PM
What size are your Tires
PM me your VIN

08-08-07, 05:46 PM
Your final drive ratio is 3.71 as opposed to the Deville or DHS which is 3.11. The Deville or DHS will run 70 MPH at 1950 RPM. I suspect at 70 MPH you will be running 2300 - 2400 RPM. There is your difference. Nothing you can do. It's the nature of the beast.

08-08-07, 05:55 PM
My STS runs 24 mpg, long highway driving, at 70. Same rear ratio. Same 2300 or so rpm at 70. Smaller frontal area and maybe a few pounds lighter than a DTS. You're right on normal. For grins and kicks, drop the cruise to 67 or 68 and see what happens. Eye opener.........Then sneak it up to 72 or 73.....Ouch !! Fooling with tires and wheels will cost a bundle, and, if you go a couple of percent above or below calibrated tire diameter, it will cause false readings from a slew of sensors that measure such things as wheel rotation vs. speed = distance = fuel mileage, and you may trip a stability sensor alarm in the bargain. .......Suppose you spend....$350-750 on some "fuel mod" and gain 0.2 mpg. How many gallons will you have to burn in order to get your money back and then begin "saving" ???

08-08-07, 06:11 PM
What did the dealer do to the car?

We do tire swaps all the time between cars.
We install Vogue tyres. Instead of 225 they are 235 or the other way around.
We swap 16" and 17" between cars. We swap 17" and 18" between cars
The sticker does not reflect what we do.
How are you checking your mileage?

08-08-07, 08:35 PM
:thepan: AJ.......Read my post again..."if you change diameter by blah, blah percent". Heck, if the OEM tire diameter was 27.2" and you fitted a 27.2" aluminum wheel with NO tire (or a new Michelin Twheel), then sensor-wise nothing changes. ANY combination of wheel/tire, as long as it doesn't eat out the fenderwell, and stays very close to the same outer diameter, will work. What generally suffers is handling, ride, and wear.

08-08-07, 09:31 PM
Tire All P235/60R17............28"..........3.71 gears
Tire All P225/60R16-97S.....26.5"........3.11 gears
Tire All P225/60R16-97H.....26.8.........3.11 gears
This is 05 Deville specs

Tire All P235/55R17 fits on an 04 and prior STS.. 27.1"

But the question is how is it calculated? from the DIC?

08-08-07, 10:09 PM

My tires are 235/55/17. Any thoughts on best tire pressure for best mpg?


08-08-07, 10:10 PM

Forgot to mention....ironically, I bought this car in Brookfield, WI! From Safro Toyota.

Too bad I don't live there, and you could be my mechanic!

08-08-07, 10:38 PM
How are you checking the MPG?
The 60 series tires are almost 1" bigger.