Background:
2004 F150 SCAB 5.4 with 3.73 Limited slip.
Equipped originally with 255/65/17 tires, currently rolling on 275/70/18 Nitto Tera Grapplers. New tires are 3" taller than the ones from the factory.

I have to take my rig in to the dealership due to the air bag wire harness replacement recall. And I'm trying to decide if I should have the computer flashed to accommodate the new tire size while it is there. Typically it would be a no brainer but I've checked the speedo against the GPS and it is pretty much dead on which was a surprise to me. I just don't want to waste $40-$50 on a flash if it's not needed, plus I could cause other problems with flash that I don't currently have.....if it ain't broken, don't fix it.

What do you guys think? Any benefit to flashing the computer other than recalibrating the speedo?

 

How does your odometer compare to the GPS or to highway mileposts?

Stock, the speedo is usually 2 to 3 mph higher than actual (GPS) speed.

I wouldn't bother with a reflash - but if you ever decide to get some tunes, you can use the programmer to correct it yourself.

 

"Typically it would be a no brainer but I've checked the speedo against the GPS and it is pretty much dead on which was a surprise to me."

 

Yes mine did too, But did 1 mile=1mile on your GPS?

My truck was rollin 1.6 miles to 1 Mile Gps before I reprogrammed it.

 

So the speedo could be correct but the odometer could still be off? I assumed if the speedo was accurate then the odometer would be too.

You would think that I was old enough to know better than to assume anything.....I'll have to verify the odometer to the GPS and see how to compares.

 

I don't think the dealership will be able to help you with the correct tire size. They are limited to certain tire sizes, IE, those that come as OE on the truck. Probably, the largest will be 275/65r18 (32"). The 275/70R18 is 1" larger than that.

 

Often .... YES ..... but some vehicles have odometers that are dead on with stock tires but whose speedometers are reading faster than true speed by 2-3 mph. Kind of like a built in "fudge factor" to minimize your chances of tickets, suspensions, higher insurance, and other unpleasant experiences while driving the product.

Maybe they do it so the speedo is correct with brand new tires at max inflation pressure? If they set the speedometer to be dead on with worn out tires at 15-20 psi below recommended pressures like many people run them, they'ld be getting speeding tickets the day the leave a tire shop with new meats and full air.

When I carry my GPS, I find that my two T-Birds and the Merc GM are each about 1 mph fast (speedo reads faster) while my '07 FX4 is more like 2 faster and wife's Forester is between 2 & 3 faster .... all at 55-60 mph. My '77 F-150 with 3.50 gears and C-6 is just about perfect with it's near new 31/10.50-15 tires at 45 psi .... but once the tires wear some the speedo will stay same as truck slows down some.

 

Yep, my 07's tires are 275/55-20 and nominal outside diameter is 31.9" (when new and mounted and inflated). Seems all the tires Ford sets them up for are near 32" OD .... except Raptors.

That 33" versus stock 32" is a increase of 3.125%. That's the difference between ...
... 60.000 mph with the 32" tires ...
... and ...
... 61.875 mph with the 33" tires both at same driveline RPMs
(I just did the math, 33 / 32 = 1.03125 ..... 1.03125 X 60 = 61.875)

Good chance that speedo may wind up just about dead on then?

Here's some links I find helpful.

http://www.1010tires.com/TireSizeCal...?action=submit

http://www.tirerack.com/tires/tirete...conversion.htm

 

FYI, dealer charges at least $100 for a flash. Add $150 (or less) and get an Edge and do it yourself and you'll get some gauges too. Add $200 (or less) and get a Gryphon w/ some tunes.