Thinking out loud, large cars

[mcrews]

Again, I hate to beat the dead hoe.......BUT.......
to all:
1. The name of the game is rpm at 60 or 55 or what ever you 'cruise' at. In my Q45, w/ the 2.78 ratio and the one size up 255/45/18, I'm turning 1820rpm at 62mph. On a flat freeway, that will get me 30mpg all day long. At 2000rmp the mpg drops noticablly.
2. Again, why lower it. REALLY. why? I can tell you from firsat hand experience there is no noticable difference in mpg w/ lowering.
3. BUT your ride quality will suck. period. So knock your self uot. use lowering springs. what ever. But you will regret it. the spring is the major component in the suspension. the minimal change in tire size has very little impact on the quality of the ride. A 1 inch change in springs is not a little thing.
4. Besides, if you measure the current ride hieght with the factory ride hieght, I bet the car sits 3/4" lower right now!
5. a much better option (if you are convienced that you have to be lower is a front lip or airdamn. So of the truck-modders have really mastered the art of the lower front dam.
and they still have the factory ride quality.
Why suffer a worse ride, that cost you 3-400 dollars to lower, when there is 0 payback in terms of noticable mpg???

(ps. fud this was not directed at (you). it was aimed at the other posters.

[mystere485]

Originally Posted by mcrews

Again, I hate to beat the dead hoe.......BUT.......
to all:
1. The name of the game is rpm at 60 or 55 or what ever you 'cruise' at. In my Q45, w/ the 2.78 ratio and the one size up 255/45/18, I'm turning 1820rpm at 62mph. On a flat freeway, that will get me 30mpg all day long. At 2000rmp the mpg drops noticablly.

245/45/18 to 255/45/18 changes the width not the height

Originally Posted by mcrews

2. Again, why lower it. REALLY. why? I can tell you from firsat hand experience there is no noticable difference in mpg w/ lowering.

"Active ride height that lowers the vehicle at speed, which Ford employed on the Lincoln Mark VIII and which appears on Mercedes-Benz vehicles with Airmatic suspensions. According to Mercedes, "Lowering the ride height at speed results in a 3-percent improvement in drag." Car Aerodynamics 101

To put Cd changes in perspective, Frasher put some numbers to a hypothetical sedan. Our imaginary car has a curb weight of 3,527 pounds, a Cd of 0.30, a frontal area of 23.7 square feet and 9 pounds of rolling resistance for every 1,000 pounds of weight. According to Frasher, "If we put a gas-burning engine in this car, expect reasonable performance and drive it on a combined driving cycle, we can expect to get 23.8 mpg…. Add 10 percent to the drag coefficient, we'll now get 23.3 mpg…. Take 10 percent from the drag coefficient, we'll now get 24.3 mpg."
Improving Aerodynamics to Boost Fuel Economy — Edmunds.com


65+ Vehicle modifications for better fuel economy - EcoModder.com

According to the book i have, a 1992 crown victoria has a drag of 0.34 and a grand marquis of same year has 0.36. Those are not bad numbers considering the size of vehicle.

Originally Posted by mcrews

3. BUT your ride quality will suck. period. So knock your self uot. use lowering springs. what ever. But you will regret it. the spring is the major component in the suspension. the minimal change in tire size has very little impact on the quality of the ride. A 1 inch change in springs is not a little thing.

I understand that; anti-roll bars, shock rates, and camber adjustments should be taken into account when modifying springs as well.

Originally Posted by mcrews

4. Besides, if you measure the current ride hieght with the factory ride hieght, I bet the car sits 3/4" lower right now!

Because of the saggy springs no doubt!

Originally Posted by mcrews

5. a much better option (if you are convienced that you have to be lower is a front lip or airdamn. So of the truck-modders have really mastered the art of the lower front dam.

A front air dam and belly pan is in the plans.

Fud2468, sorry for hijacking your thread.

[fud2468]

I highjacked my own thread by starting it on large cars and then switching to minivans more recently. Then I couldn't resist getting back into the large car discussion.
IIRC, the Cd difference between the '92 CV and the '92 Grand Marquis may have been because the CV had no grill.
The public didn't like the looks of it so they added a grill for '93.

[Joenavy85]

Originally Posted by mystere485

245/45/18 to 255/45/18 changes the width not the height

you really need to read up on the specs of tires. from above: 245 and 255 are the width of the tire(in mm). the 45 is the sidewall/width ratio (there is another word for it that is currently slipping my mind) that shows that the sidewall measurement is 45% of the width. if you narrow the width and keep the same ratio you change the sidewall measurement as well.

245mm x .45 = 110.25mm

255mm x .45 = 114.75mm

very slight difference, but still not the same height

[mystere485]

That could be possible, a few places cite 0.38 for 2002 and up crown vics with the p71 police package. Do you think an outer grille block would do the trick? or would it be the smoothness of the entire panel doing the trick?

[mystere485]

Originally Posted by Joenavy85

you really need to read up on the specs of tires. from above: 245 and 255 are the width of the tire(in mm). the 45 is the sidewall/width ratio (there is another word for it that is currently slipping my mind) that shows that the sidewall measurement is 45% of the width. if you narrow the width and keep the same ratio you change the sidewall measurement as well.

245mm x .45 = 110.25mm

255mm x .45 = 114.75mm

very slight difference, but still not the same height

Ok, that makes sense, however at 10.1 revs per mile and .35 of an inch difference that there will be a noticable change in ride height and mileage?

245/45R18
Rim Width Range: 7.5 - 9 in
Overall Diameter: 26.68 in 677.67 mm
Sidewall Height: 4.34 in 110.23 mm
Radius: 13.34 in 338.83 mm
Circumference: 83.81 in 2128.7 mm
Revs per Mile: 779.6

255/45R18
Section Width: 10.03 in 255 mm
Rim Diameter: 18 in 457.2 mm
Rim Width Range: 8 - 9.5 in
Overall Diameter: 27.03 in 686.56 mm
Sidewall Height: 4.51 in 114.55 mm
Radius: 13.51 in 343.15 mm
Circumference: 84.91 in 2156.7 mm
Revs per Mile: 769.5
Speedometer Difference: 1.312% too slow
Diameter Difference: 1.3%

[Joenavy85]

read the entire post

185mm x .75 = 138.75mm

215mm x .45 = 96.75mm

add the 15" rims and you can figure out the outside diameter

[mystere485]

Originally Posted by Joenavy85

you really need to read up on the specs of tires. from above: 245 and 255 are the width of the tire(in mm). the 45 is the sidewall/width ratio (there is another word for it that is currently slipping my mind) that shows that the sidewall measurement is 45% of the width. if you narrow the width and keep the same ratio you change the sidewall measurement as well.

245mm x .45 = 110.25mm

255mm x .45 = 114.75mm

very slight difference, but still not the same height

Thank-you for the info!

[mystere485]

185/75R15
Overall Diameter: 25.92 in 658.36 mm
Sidewall Height: 5.46 in 138.68 mm
Radius: 12.96 in 329.18 mm
Circumference: 81.43 in 2068.3 mm
Revs per Mile: 802.4

215/45R15
Overall Diameter: 22.61 in 574.29 mm
Sidewall Height: 3.80 in 96.52 mm
Radius: 11.30 in 287.02 mm
Circumference: 71.03 in 1804.1 mm
Revs per Mile: 919.9

[zonker]

Originally Posted by mystere485

The door jam says its a y, and the car for sure is a 1994.

8.8" carrier, 3.08:1 ratio, non limited-slip.

OEM tires sizes for the
'94 Crown Vic is 215/70-15 (26.9" OD, 751 rev/mi) or 225/60-16 (26.6" OD, 757 rev/mi), depending on the options.

The next taller diff gear available is 2.73's, or the equivalent of changing your tires to a 30" diameter.