[dagnuten]

While searching thru the archives, (looking for some cheapo mods to someday do when it aint so stinkin hot out...lol) I read that changing tire size did not affect your odometer reading, something about the sensor location. I always thought it did, so 1st, does it? And if so, how could you calculate your mpg used if it is off?

I went from stock to 315's. I've searched for some kind of converter to tell me what I'm getting. I've found some that give the mph differences but thats about all. I've been calculating by hand my mpg just to tell if there was any problems. There arnt any, just curious what my mpg is, am I really getting @ 16 with my 315's? Does the tire size throw off the odometer reading?

Dan /ubbthreads/images/graemlins/cool.gif

 

[TheHammer69]

[ QUOTE ]
While searching thru the archives, (looking for some cheapo mods to someday do when it aint so stinkin hot out...lol) I read that changing tire size did not affect your odometer reading, something about the sensor location. I always thought it did, so 1st, does it? And if so, how could you calculate your mpg used if it is off?

I went from stock to 315's. I've searched for some kind of converter to tell me what I'm getting. I've found some that give the mph differences but thats about all. I've been calculating by hand my mpg just to tell if there was any problems. There arnt any, just curious what my mpg is, am I really getting @ 16 with my 315's? Does the tire size throw off the odometer reading?

Dan /ubbthreads/images/graemlins/cool.gif

[/ QUOTE ]

Tire size DOES effect the speedometer/odometer. You can have a Ford dealer change the Revs/mile in the ABS computer to fix or you can get something like a Superlift Truspeed to adjust it.

Hammer

 

[SmokeyWren]

[ QUOTE ]
I read that changing tire size did not affect your odometer reading, something about the sensor location. I always thought it did, so 1st, does it? And if so, how could you calculate your mpg used if it is off?

[/ QUOTE ]

Either you read it wrong, or you were reading a thread written by an ignorant boob. /ubbthreads/images/graemlins/frown.gif

Changing the rear axle ratio does NOT affect the speedo or odo or tripmeter. But changing tire diameter does. /ubbthreads/images/graemlins/cool.gif

[ QUOTE ]
I went from stock to 315's. ... Does the tire size throw off the odometer reading?

[/ QUOTE ]

Yes. The math is fairly simple. There is a direct relationship between tire revs/mile and the speedo/odo/tripmeter. And the effective rear end ratio.

Stock and factory option tires on your 2002 were 649 to 657 tire revs/mile. Stock A/S tires had 655 revs/mile.

Most 315/75R16s have around 590 to 605 revs/mile. Since the Dealer's equipment won't fix less than 601 revs/mile, let's assume your new tires have 601 revs/mile, and that your stock tires had 655 revs/mile.

655 minus 601 = 54 divided by 655 = 0.0824 = 8.24 percent

So assuming your speedo/odo/tripmeter were accurate to begin with, then they are now 8.24 percent slow. When your speedo shows 70 MPH, Officer Bob is seeing 75.77 MPH on his Lidar. When you fill up with fuel after 400 miles on the tripmeter, you really traveled 433 miles. And your 3.73 rear end ratio "feels like" it's only a 3.42.

Three options for the speedo/odo/tripmeter error:

1. Live with it. /ubbthreads/images/graemlins/frown.gif

2. Find a Ford dealer that knows how to calibrate the speedo to 601 tire revs/mile. Most dealers than can and will do it will charge you about one hour labor rate. Some dealers claim they can't do it. /ubbthreads/images/graemlins/crazy.gif

3. Buy a SuperLift TruSpeed calibrater for a coupla hundred bucks and do it yourself.
http://www.superlift.com/accessories/truspeed.asp

To get your performance back to stock, the only reasonable fix is to change the rear-end ratio. 3.73 plus 8.24 percent = 4.04. But the big tires are going to eat up some power too, so the commonly-available 4.10 gearset should get you really close to the stock oomph.

Of course, if you have a 4x4, then you'll have to change out both the front and rear gear sets.

 

[DNDARNETT]

I went from 16's to 18's, and the speedo is about right on now. My speedo was about 3 mi/hr faster than i was actually going. Another words, when my speedo was reading 70mph, i was really doing about 67mph. Now when my speedo reads 70 mph, i'm doing 70mph. Thats all if my officer next door has a calibrated radar detector.

Hilbilly

 

[dagnuten]

Will my Dp chip have, or capable of, fixing it?

 

[tpace]

I went from stock tires to 315's. I had a Ford dealer do the calibration. I spent somewhere around $70-$80. I have a GPS in my truck so I am able to compare what the GPS is saying to what the speedometer readout is and they match.

 

[TheHammer69]

[ QUOTE ]
Will my Dp chip have, or capable of, fixing it?

[/ QUOTE ]

No

Hammer

 

[itzlen]

You can also get a Hypertech Programmer. It will let you program for tire size, and also boost for 3 different stages of power.

 

[Bella PSD]

What SmokeyWren said. My Odometer is off by 9% with a GPS. Add that back when you fill up. Or get a Superchip true speed. I like the true speed because as the tires wear down or you change size you do not have to go back to the dealer.

I only dropped 100 rpm at 70 MPH from 285's to 315's. 2000 rpm with 285's at 70 mph to 1900 rpm at 70 mph with 315's.

Louie