tires gas milage

[350wagon]

My 93 e350 has 225/75 16s , I was wondering if a 245/75 16 would help the gas milage , being they would be taller. any help or suggestion would be appreciated.

[pfogle]

Probably not. The taller tires would confuse the computer. It reads RPM data from the speedometer, and the taller tires will mess with the speedo reading. You'd have to recalibrate the speedo to make it work right.

[QuadVan]

Noit sure it would help as you would also be increasing your frontage area becasue the van would sit a bit higher. What I have found is running the E rated tires with 65 psi made a big difference over running with 35 to 40. Bumped my gas mileage by nearly 8%.

[Ian F]

Originally Posted by QuadVan

Noit sure it would help as you would also be increasing your frontage area becasue the van would sit a bit higher. What I have found is running the E rated tires with 65 psi made a big difference over running with 35 to 40. Bumped my gas mileage by nearly 8%.

Agreed - I get the best mileage when I pay close attention to tire pressure. Even with my E-150 with incorrect P235's (yeah... I need to get XL's or LT tires, but not sure if I'm keeping the van...).

[86rustbucket]

Not to mention that there will probably be some increase in the overall rolling resistance and weight of the LT245's vs. the 225's. These will definetly have a small affect on the fuel economy of the vehicle.

[ACESN8S]

Your speedo will be to slow by 7.7%. That comes out to reading 60mph on the speedo and actually going 64.6mph

How did I figure this out? Well.....I cheated. Go HERE, it's from a Miata site.

Patrick

[Adventure]

I get different answers using that site. The sidewall is still 75, the radius is still 16, and the tread width goes from 225 to 245. It's about a 4% change (225 tires are slightly smaller in circumference).

A 225 tread is the tire used on dual wheel applications of the E350. Because of the reduced width you can have 2 tires with a 5" offset rim and it fits nicely in the wheel well. The 225 is also used on many of the cargo vans while the 245 seems to be used more on the wagons. I have 225's in the rear and 245's in the front (converted E350 15 Passenger DRW).

As for gas mileage, changing tire size will do nothing. Fuel consumption is based upon the mass of the vehicle, the frontal wind resistance, and the efficiency of the engine / drive train. Notice how tire size and gear ratios don't come into the equation. They do, however, affect your ability to start the vehicle rolling (which kind of consumes fuel) and the ease in which a vehicle can pull a weight (again related mostly to starting the mass rolling).

Force = Mass x Acceleration

[pfogle]

The 75 is the sidwall height ratio. In 245 are 1.2 inches taller, which makes a drastic difference in the speedo reading, and consequently the rpm signal sent to the computer.

[Adventure]

The 225's are stock on an E350. The 245's are optional. Ford will and does install either on the E350. The speed / RPM is detected by a pickup in the differential. With 245's, you have a 1.2 / 30.2 increase or approximately 4%. That is, at the same RPM, your speed is actual 4% higher. At 60 MPH, this means 62.4 MPH for real. This is not a hugh difference, it is well with tolerances. Your fuel mileage will not change because it takes 4% more fuel to now maintain that RPM.

[spnrgy]

OK here's how you figure out your speedo error:
I'll give examples for a 285/50R20 vs. the stock 255/70R15

1) figure out your sidewall height
(width x sidewall ratio) / 25.4 = sidewall height in inches
EXAMPLE: (285 x .50) / 25.4 = 5.61"
EXAMPLE: (255 x .70) / 25.4 = 7.03"

2) figure your overall diameter
(sidewall height x 2) + rim diameter = overall diameter
EXAMPLE: (5.61 X 2) + 20 = 31.22"
EXAMPLE: (7.03 x 2) + 15=29.06"

3) figure your circumference (distance all the way around the tire)
3.14159(that's pi) x diameter = circumference
EXAMPLE: 3.14159 x 31.22 = 98.08"
EXAMPLE: 3.14159 x 29.06 = 91.29"

4) figure the ratio between the two circumferences
original circumference / new circumference = ratio
EXAMPLE: 91.29 / 98.08 = 0.93

So, with bigger tires you are covering more distance per revolution so multiply your speed by the ratio to determine the adjusted speed.
EXAMPLE: 55mph x 0.93 = 51.15mph
EXAMPLE: 60mph x 0.93 = 55.8mph
EXAMPLE: 65mph x 0.93 = 60.45mph
So to stay within the law (if that's your thing ), if you want to go 55, keep your speedo at 51mph.



FINAL FORMULA: old tire over new tire
[((((width x sidewallratio)/25.4) x 2) + rimdia.) x 3.14159]
divided by
[((((width x sidewallratio)/25.4) x 2) + rimdia.) x 3.14159]
equals your speed ratio!


wow, i don't think I can even follow the damn brackets...


1) width x sidewall ratio(put a decimal point in front of the number) = sidewall height in mm
sidewallMM / 25.4 = sidewallIN

2) (sidewallIN x 2) + rim = diameter

3) dia. x 3.14159 = circumference

4) old tire circumference / new tire circumference = speed ratio


Then multiply your desired speed by the ratio and you get what your speedo should read!!

[spnrgy]

Responding to the previous post... fuel mileage is not a linear equation; that is, it varies depending on what gear you are in and the RPM of the engine at a given speed. So you can't just apply 4% everywhere. Bigger tires WILL affect your fuel mileage because you will require more torque and more RPMs at takeoff to turn the bigger tires. Once you are up to speed there is virtually no difference in the mileage based on tire circumference. HOWEVER, air resistance over the larger frontal area (as mentioned above) of the tires adds in (but it's so small as to be virtually negligible). The BIG difference is the height of the chassis and body off the road. More air flowing under and across the bottom of the vehicle where the surface is extremely irregular increases your air resistance significantly (significantly in scientific terms only means a repeatable percentage over 1%). Not knowing the air resistance coefficients for the stock and altered heights, I can't say for sure what the difference would be, but if I had to take a stab at it, I'd say a 2" lift would increase your air resistance 3-5%.
For good mileage, choosing your speed is important (as are other tips above i won't repeat). If you can cruise at a speed where the rpm are relatively low within a given gear, but not too close to the point where it downshifts, that will give you your best mileage. Keep in mind though, that you don't want the rpm at so low a level that the engine is bogging or chugging. Keep a healthy margin from the shift point. This applies to cruising though; for more hilly and variable driving, higher rpm will give your best mileage. Driving style in and of itself can make a HUGE difference in mileage. Much more so than swapping tires. Pay attention to reasonable acceleration, and limit your speeding up and slowing down as much as possible. In bad traffic, I like to keep 100 or so feet in between me and the car in front of me so that when he stops, I can keep idling ahead, and often not have to brake before he starts moving again. Basically, I'm letting slack out and taking it up to remain at a more constant speed. Of course you do always have the lane dodging *********s to contend with...

[86rustbucket]

Originally Posted by Adventure

As for gas mileage, changing tire size will do nothing. Fuel consumption is based upon the mass of the vehicle, the frontal wind resistance, and the efficiency of the engine / drive train. Notice how tire size and gear ratios don't come into the equation. They do, however, affect your ability to start the vehicle rolling (which kind of consumes fuel) and the ease in which a vehicle can pull a weight (again related mostly to starting the mass rolling).

Force = Mass x Acceleration

Sorry, you're not correct on this one. Changing tire sizes will affect fuel economy. In fact, sometimes changing tire brands can affect fuel economy. Its known as rolling resistance. It is affected by the tire's size, weight, tread compound, footprint shape and size, inflation pressure, etc.

Automakers specify a rolling resistance requirement for every new tire that is designed for a particular vehicle. The rolling resistance is specified for one reason... fuel economy. As a result, the automakers are willing to accept certain tradeoffs for better fuel economy (lower RR), including traction, wear, ride, etc...

Rolling resistance does affect the vehicle's ability to get rolling, it also affects a vehicle's ability to keep the tire rolling.

If you would consider a force balance on the vehicle, the force required to move the vehicle down the road is counteracted by the 1) wind resistance (related to the coefficient of drag and surface area) and 2) rolling resistance of the tires on the ground.

[Ian F]

Originally Posted by 86rustbucket

The rolling resistance is specified for one reason... fuel economy. As a result, the automakers are willing to accept certain tradeoffs for better fuel economy (lower RR), including traction, wear, ride, etc...

Yep! It's actually one of the tricks all the eco-cars use: narrow, high pressure tires. ...and why they rarely score well in handling tests... engineering will only do so much for tire-grip when your contact patch is the size of a quarter...

[350wagon]

second question then ,would a spoiler on the front to deflect the air from under the van help?

[ACESN8S]

It's supposed to. But how much? I'm not sure.

Patrick