Tire size

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condorman

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My 1986 Ford Econoline 150 van currently has 235/75R15 tires that need replacing. I am thinking about going up a few sizes to get better gas mileage. Is there any downside to this besides less torque and the speedometer being off?
 

blueblood

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condorman said:
My 1986 Ford Econoline 150 van currently has 235/75R15 tires that need replacing. I am thinking about going up a few sizes to get better gas mileage. Is there any downside to this besides less torque and the speedometer being off?

Probably poorer handling and change in turning radius.
 

ronnyd8719

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I did this on my Pickup and on the highway it helps, but in the city it will lost mpg. The bigger tires take more power to start. I only added a half inch taller tire. I am told it is hard to tell if you will get better or worth. It has allot to do with gearing in the motorhome.
Ron
 

King

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Since gas milage is usually calculated using the speedometer odometer. a larger tire size will indicate lower gas milage even though the actual highway milage should increase.
Art
 

Karl

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Art,
True, but you must consider the relative power required to turn the larger tires. The larger the tire, the more power it takes to turn it at a given load.

For a given tire size; say 15x70, there can be considerable variation in their circumference. A place like Tire Rack will give you a comparison of revolutions per mile for various tires of the same size. Also, you need to consider the cost of replacement wheels and possibly hubs too. You can't just put 16" tires on 15" wheels, or 16" wheels on 15" hubs. You must also consider the clearance between the taller tire and the wheel wells, shock absorbers, and steering linkage - may be more trouble than it's worth. 
 

King

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"The larger the tire, the more power it takes to turn it at a given load. "    Horsepower is proportional to RPM times Torque.  A larger tire will require more torque to turn it, but at a lower RPM. The amount of power required to move the vehicle over the road wont change.  Generally speaking, internal combustion engines are more efficient at lower RPMs.  But the change may not be measurable.
Art
 

Karl

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Generally speaking, internal combustion engines are more efficient at lower RPMs.
They're most efficient where the horsepower and torque curves intersect - regardless of rpm. The point I was trying to make was that a particular engine/gear/wheel combination may not have enough oomph to climb a hill in, let's say third gear, whereas a smaller diameter tire may. With a significantly larger tire than normal, you may find yourself not being able to use top gear in many normal hill climb situations; in extreme situations you may not have enough even in the lowest gear to climb a mountain pass.
 

King

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No..  They are most efficient at or below the maximum torque point.  My 69 Vette 350 torque and horsepower intersect at 5250 RPM, and the redline is 5800.  No one has ever suggested driving around at 50 MPH in first gear is the way to save gas.  (The Vette gets 16 MPG around town and 24 MPG on the highway.)
The rest of your statements are absolutely correct.
Art
 

King

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No...  I did...  You said "They're most efficient where the horsepower and torque curves intersect - regardless of rpm."    I disagree.
Art
 

Whiskyecho

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Is a "possible"/"not possible" mpg gain/loss on a 1986 Ford Econoline 150 van really worthy of this banter?

Dump the van and get something more fuel efficient - the price of vehicles from this vintage is probably less than the price of a set of new tires      ::).....bill
 

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