What rear end do I have and how far do 35" tires throw me off

[mike70gtx]

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?

[85_305]

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.

[mike70gtx]

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.

[85_305]

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.

[STANGMAN893]

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.

[NorCalDiesel]

Originally Posted by mike70gtx

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.

[85_305]

^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?

[NorCalDiesel]

Originally Posted by 85_305

^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?

[85_305]

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

[NorCalDiesel]

Originally Posted by 85_305

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

Mickey Thompson Performance Tires & Wheels

225/75/16 = approx. 690 revs per mile