What happens when you change rear end gear and you have a chip. Is it just the fact that the speedo is off calibration or does it affect shift points. I plan on going from a 3.08 to a 3.73 rear gear.

 

You hit the nail on the head. With the 373 gears it does change your shift pattern. You will need your chip reburned. A good idea is to get the 373 from a junk yard and while you are there get the peice off the transmission so your speedo is correct. That is if they have 373 gears for your year. check your owner manual to see if they offered 373 for your year.

 

You hit the nail on the head. With the 373 gears it does change your shift pattern. You will need your chip reburned. A good idea is to get the 373 from a junk yard and while you are there get the peice off the transmission so your speedo is correct. That is if they have 373 gears for your year. check your owner manual to see if they offered 373 for your year. I believe that any change that you do to your engine. I.e cam, pulleys, rear end ect. yopu need to tell them about that so they can program the chip to fit your mods. I even think larger tires will effet the chip as well.

 

Hi MhzMan,

If you do a gear ratio change or a tire size change in the 1999 & newer F-150's, that needs to be corrected in the Superchip, or your speedometer, odometer and shift points will be off, and the "feel" of the shifts won't be quite right, either.

"TomatoTruck" offered some good points in his helpful post.
However, because you have a 1999 F-150, you do *not* have the traditional reduction gear (commonly called a speedometer gear) in the tailshaft of the transmission that he was referring to when he was talking about going to a junkyard, so you can't do it that way at all. Nor is that a good way to obtain a speedo gear, the way to do that is just buy a new one for under $20 at any Ford parts counter, as it's plastic & you don't want to install a used plastic gear coming from a vehicle that's been junked because it was wrecked.

1998 is the last model year to have a speedo gear in the F-150, all 1999 & up models need to have this done in the powertrain programming, and we can handle that.

The ECU looks at not only the VSS (vehicle speed sensor) data stream, it also looks at things like engine rpms, output shaft speed, etc. to *confirm* VSS data. If the speedometer is off, then it's all off (odometer & shift points too) as it's basically a VSS-dependent system. It will allow the vehicle to operate and be driveable if the speedometer is off, but it won't be optimal by any means, and me personally, I wouldn't drive it hard or do any towing with a significant speedometer error due to the shifts being off.

Give us a call if you are going to do the 3.73 gear change, and we'll be happy to make the arrangements to correct the programming in your Superchip for the change to 3.73 gears. I see you have a centrifugal blower on there so you obviously must have a custom program in that Superchip. As long as there is a sticker on the back of it that properly identifies the program inside that Superchip, we can do the changes needed to that same file for you so you don't have to get a complete new custom program done, we can just change your existing program so your tuning stays intact.

 

Thanks Mike, you are always very helpful.

 

I just went from a 3:08 rear end to a 3:73 and my speedo was off about 9 mph, I checked it through one of those neighborhood watch speed checks, at 45 on my speedometer the cop box said 36mph. I have a 2001 SCab and after I had my flip chip reburned I noticed the shifting takes place at a lower speed, more low end torque now with the new gears, so the truck can handle a lower rpm shift point. I let my wife drive my truck last night after I had the new gears and flip chip installed.......hope she comes back soon, I miss my truck. LOL
J.J.

 

Good to hear, can't wait to get mine done.