I have a 2007 F150 with larger aftermarket wheels and tires on the truck, however I want to revert back to my stock wheel/tire combo when I tow my car home. I know I can buy one of those 500 dollar power programmers to correct my speedo, but there has to be a cheaper way of doing this, anyone have any good info?

 

yes... you can get a superlift "tru-speed" unit. i believe it only retails for about 175-200.00. it has a switch for 2 different settings that you program it for (one for stock/one for the larger setup). this way you have the ability to change very easily between tires and still be accurate. it alters the signal going to the computer so that it regains the correct reading for mph, shiftpoints, etc.

 

they dont make it for my truck/model year

 

thats because the speedo is calibrated off the rear gears, not the tire size.

 

umm, if you change tire height, your speedo wont be correct..

 

no...

If you change the gears and don't change the tone ring, your speedo won't be correct.

 

ok, well first, my truck has an electronically controlled speedo

and if you change your rear gears, yes, your speedo accuracy will change....but that doesnt meant if you change your overall tire height the accuracy of your speedo wont change.

taller tires equal more circumference which equal more distance traveled per axle revolution. your vehicle doesnt know this unless you tell it

 

Okay... try to follow this, your rear gear spins as you are driving and every time it spins, X number of teeth are counted, it doesn't matter if the tire is 31" tall or 37" the number of teeth corresponding to the rear gear ratio are still counted. Now, if you had a chevy or dodge with wheel end mounted speed sensors that use magnets, you would need the calibration.

Fords have had electronic speedos since the mid 90s.

 

omg, seriously... think this out, your thought is mostly correct, but incomplete... the speedo is based off the revolutons of the output shaft on my transmission which is connected to the drive shaft which is connect to the pinion. I have 3.55 gears, meaning for ever for every 3.55 turns of my driveshaft, the ring gear, and by direct connection, the wheel, will turn 1 time. if my tire height is 20 inches, 1 revolution of the ring gear will move my vehicle approx 62.8 inches. If my tire height is 37 inches, 1 revolution of the ring gear will move my vehicle approx 116.1 inches...thats close to twice the distance. Tire height has EVERYTHING to do with it. The truck doesnt know how far it has moved, it just makes a calculation based on rear axle ratio combined with overall tire height multiplied by output shaft revolutions per minute to give you the speed youre traveling.

Don't believe me? Take a stock vehicle and put on tires that are 10 inches bigger in diameter and go exactly the speed limit, according to your speedometer, by a cop and see if he pulls you over.

look at this, it'll help you understand http://www.miata.net/garage/tirecalc.html

 

That's why every programmer has the function of asking you if you've changed your tire size, do you want to correct your speedo. My Hypertech walks you through how to measure your tire.

 

I was friends with a guy in high school who understood that tire size made the speedo wrong, but couldn't come to believe in why. He knew his speedo was off 5 mph at 55. (it would read 55, but he was going 60.) I told him his speed was 10 percent off, that it affected his overall GEAR RATIO, but he wouldn't believe me.

He was convinced that his speed was 5 mph off all the time, not that it was 10% off. He would not, could not sit still to listen to anyone, by golly he was right. I asked him how it could be 5 mph off at, say, 2 mph, but he just got angry...

Matts72, you may be right about how the truck counts its speed, but you are missing why it MIS counts its speed. The tires are essentially the last "gear" before the ground. If you put on a tire that's 10 percent bigger, your truck will go 10% faster at the same engine speed. If you go smaller, your truck will go that much slower at the same engine speed. The truck doesn't know this, because it's counting revolutions WITHIN the drivetrain, but you have changed the ratio of gear reduction at the tire.

It doesn't matter where in the drivetrain the rpms for the speedo are counted, it will be an inacurate indication of speed if you have a non-stock tire size, because the computer thinks your tires (the last gear before the ground) are of the stock size.

 

I was wondering why we where going down memory lane. I'm not sure but I think I detect a little bit of sarcasm. LOL. The wheel/tire being the last gear is well put. I love FTE.

 

Did not mean to sound overly sarcastic or rude, I was just trying to explain the whys of the issue after I had a couple of drinks.... no offense intended.

 

I wasn't offended. Maybe I read more into it than you meant. I though you did a good job explaining.

 

Your Ford dealer can correct the calibration with the OBDII diagnostic bay equipment,if you do not want to pay out the money for a tuner.