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Just got an 03 Excursion with the 7.3. Right now it has a lift and 35" tires. I'm thinking of dropping it back down so my wife is a little happier and get some regular size tires to help with fuel economy. First question, how do I tell what kind of rear end I have? Is there a code some where on vehicle. Also how far off do 315/75/16 tires throw my speedometer off?
 

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It should say right on the door panel vin tag. use the online decoder to figure it out. also, the diff should say itself.

My obs has 4.10's stock, and w/ 35's I'm off 5mph for every 50mph. If you have 373s stock, I couldn't tell you how far off, but I'm willing to bet very closely.

Those 35's aren't hurting your mileage one bit. Keep them.
 

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Thanks for the response. The code on door was c1 which looks like 3.73
My speedo app on my phone showed me about 3 mph over at 60, but not sure if it is accurate. It seems like most of my reading says that the big tire could cost me a mph or 2 with fuel, but your saying it shouldn't hurt too bad.
 

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If anything the bigger tires are giving you more like a ... 3.23 or 3.00 gear, for instance. Not exact, just giving an example. Although, I never like the ideas of 3.73's in a diesel built for towing. They seem too high.

But the speed that your speedo will be off is linear; ie a 7% variation.. 10% variation. My 35's are something like 10% bigger than stock, therefore my speeds will be 10% off. So by 50mph, I'm 5mph off, and by 100mph I'm 10mph off. Usually I multiply my final fillup by .10 and that gives me a more accurate number that I've experienced between stock, 33's, and 35's.
 

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Up to 55 it is just about right but gets further off faster.70 will be about 3-4.I had that setup when I first got my truck.Now running 20" rim and 35" tire and its off 5mph.
 

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Just got an 03 Excursion with the 7.3. Right now it has a lift and 35" tires. I'm thinking of dropping it back down so my wife is a little happier and get some regular size tires to help with fuel economy. First question, how do I tell what kind of rear end I have? Is there a code some where on vehicle. Also how far off do 315/75/16 tires throw my speedometer off?
From the factory your Ex would have a Sterling 10.5 with 3.73's in the rear, the 4x4's have a D50 up front as well.

IIRC, You would have had 265x16x75's on factory 16 x 8 inch wheels. That puts you at 1850 RPM at 65 MPH. That is "supposed" to be the sweet spot for fuel econ.. 1850 rpm.

So with that info you can figure the rest out for tire size. On factory wheels without an offset (Spacers) I was able to run 305x16x70's which are about 33" and that is mathematically about 3% difference from the factory tire diameter. You might want something between the factory and 305's

Here is a little math with explanation that you can use to figure out speedometer and rear gearing. It will give you an idea of how far off factory specs you are with different sized tires.

Stock tires for '99 -'04 models have 655 revs/mile. 295s have 626.
655 minus 626 = 29 divided by 655 = 4.4%.

So your 3.73 rear axle will "feel like" 3.73 minus 4.4 percent = 3.57. Your speedo will be 4.4 percent slow, so an indicated 70 on the speedo will be 73 MPH. Your odometer will be 4.4 percent understated, so 400 miles on a tank of fuel per the tripmeter will be an actual 417.6 miles.
 

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^interesting math there. How would you figure the same out for an OBS truck? How many revolutions per mile would a stock OBS tire be, compared to 35" tires?
 

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^interesting math there. How would you figure the same out for an OBS truck? How many revolutions per mile would a stock OBS tire be, compared to 35" tires?
The math works regardless of the truck or car. It is based on the distance a tire moves in one revolution. So if you chalk marked your tire where it meets the road and roll it forward 1 revolution and measure how far it traveled forward in inches there you have it. Or take a string and wrap it around the diameter of the tire and measure it, should come out around 600 inches

Whats your stock tire size?
 

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dont remember.. either 225/75/16 or 245/75/16. I went from stock, to 33's, to 35's.
 

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Those 35's aren't hurting your mileage one bit. Keep them.
I would have to disagree, even though the overall gear ratio may be taller for lower RPM's at cruise. The increased rolling resistance and additional rotational mass can definitely hurt mileage. I saw this in my own truck, removing the level kit and 35's and going back to stock height and 285's on my 2002 truck made almost a 2mpg improvement...
 

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Mickey Thompson Performance Tires & Wheels

225/75/16 = approx. 690 revs per mile
Which would make my 35's about 5% larger than stock... correct?

I would have to disagree, even though the overall gear ratio may be taller for lower RPM's at cruise. The increased rolling resistance and additional rotational mass can definitely hurt mileage. I saw this in my own truck, removing the level kit and 35's and going back to stock height and 285's on my 2002 truck made almost a 2mpg improvement...

My truck w/ 6"/35s gets 18mpg in the city every time. Now, if I were to go back to stock 30's and stock ride height(or whatever the heck they are..) do I think my mpg would go up? I'm sure it would.. but 18mpg with what I've got right now is plenty good enough for me :D
 

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Which would make my 35's about 5% larger than stock... correct?
Yes



My truck w/ 6"/35s gets 18mpg in the city every time. Now, if I were to go back to stock 30's and stock ride height(or whatever the heck they are..) do I think my mpg would go up? I'm sure it would.. but 18mpg with what I've got right now is plenty good enough for me :D
That is good for 4 tons of truck
 

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^Yes indeed it is.. I sure the hell am not complaining :D
 

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^Yes indeed it is.. I sure the hell am not complaining :D
Well, I'm thinking the OBS's have it over the 99.5 - 03 7.3's with just the AC singles. They are more efficient than the AD Splits we have from the factory. When you run AC Stage I's in your SD 7.3's your fuel econ increases in my experience.

I have never gotten any better than about 15MPG no matter what I do when running AD's, plus the Singles require less HPOP oil psi.

I love the OBS's, they never get old!
 

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Our stock ac's are MUCH more efficient than the ad's. The ONLY reason International went to split shots is to reduce noise emissions and to reduce n0x emissions. Split shots do require a ton of oil though... why is why the SD is nice when upgrading sticks; you already have the 17*.
Although, my 6spd super duty used to get 21mpg consistently in the city.
 

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Our stock ac's are MUCH more efficient than the ad's. The ONLY reason International went to split shots is to reduce noise emissions and to reduce n0x emissions. Split shots do require a ton of oil though... why is why the SD is nice when upgrading sticks; you already have the 17*.
Although, my 6spd super duty used to get 21mpg consistently in the city.
If I got 21 mpg I would have it advertised on the side of the truck:laugh:
 

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hahaha. I know I loved the hell out of it. If I ragged on it and raced my buddies d-max, I could dip to 18mpg or on the lowside, 16mpg in the city.
 

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Not to hijack the thread but since you guys are already on the topic of injectors.

I have a 95 f350 with rosewood stage2 sticks and am very seriously contemplating moving to an excursion. Would the rosewood stage2 single shots work in an excursion?

I'm sure this is a slightly noobish question but I'm still learning.

Thanks
Eric


Sent from my iPhone using Autoguide
 

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Yes as long as the excursion got a chip that was programmed for them.
 
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