Gear change speedo correction

Trevor008 · Apr 2, 2015

Hey guys. I have a 14 250 that came with 3.55 gears. In a month or so I'll be installing 4.30 gears. I know my Minimaxx has a function to adjust for tire size changes but what have you guys done to correct the speedo when doing gear swaps? Is it all electronic now or can I get the proper speedo gear? I've had tons of Fords over the years and each one has been different in this aspect it seems.

Thanks in advance guys

CurtisF · Apr 2, 2015

No change is needed on a gear swap. Going from 3.55 to 4.30 has no effect on the speedo.

CUZICAN · Apr 2, 2015

Yes it will change the speedo.

Easiest way would be to do your gear swap, then use a gps to see how far off it is. Then it will take some playing around with tire sizes in your mini maxx, but you should already have everything you need to fix it.

Trevor008 · Apr 2, 2015

Well that's an interesting start to this thread lol. 1 person says you don't have to do anything and the next describes how to fix it

I figured I could fudge it with the tuner via tire size changes but I was looking for a more proper way to do it.

Thanks for the replies so far though

CurtisF · Apr 2, 2015

CUZICAN said:
Yes it will change the speedo.

Easiest way would be to do your gear swap, then use a gps to see how far off it is. Then it will take some playing around with tire sizes in your mini maxx, but you should already have everything you need to fix it.

The vehicle speed sensor (VSS) is located at the top of the rear differential and reads the excitor ring, which spins with the axle shaft. The axle shaft speed doesn't change with a gear swap.

What will change is the output shaft speed (OSS) compared to the VSS. That can alter transmission shifting.

CUZICAN · Apr 2, 2015

If that is correct, then I was told wrong when I asked about it.

Trevor008 · Apr 2, 2015

That's quality info CurtisF. Thank you

The Butcher · Apr 2, 2015

For what it's worth, when I put 38s on my '15 and swapped in 4.30 gears the speedo read pretty much dead on. At 30mph it is absolutely correct. At 70MPH per my speedo, a gps app on an iPhone had me at 73. Very minor change if any.

CUZICAN · Apr 3, 2015

Out of curiosity, what gears did your truck have before the swap? My truck has 3:55's is getting 38's and I was thinking 4:10's, but if 4:30's will get the speedo back real close, then that'll be the way I go.

Trevor008 · Apr 3, 2015

That would leave me to believe that something else is at play then because a tire change would affect the speedo and need correction. If the speedo reads off of vss then a gear change and tire size change shouldn't offset

psduser1 · Apr 3, 2015

Trevor008 said:
That would leave me to believe that something else is at play then because a tire change would affect the speedo and need correction. If the speedo reads off of vss then a gear change and tire size change shouldn't offset

Tire size change is the only thing that will change the speedo reading, besides a reprogram.

Mark Kovalsky · Apr 3, 2015

CurtisF said:
No change is needed on a gear swap. Going from 3.55 to 4.30 has no effect on the speedo.

Correct!

CUZICAN said:
Yes it will change the speedo.

Incorrect!

CUZICAN said:
Easiest way would be to do your gear swap, then use a gps to see how far off it is. Then it will take some playing around with tire sizes in your mini maxx, but you should already have everything you need to fix it.

Everything needed to make the speedo right is already there. If you understood where the speed is measured you'd understand that nothing needs to change.

The vehicle speed sensor is on the rear axle. It measures the rotational speed of the axles. If th axles are spinning at 500 RPM at a given speed, when you change the axle ratio, what speed will the axle shafts be spinning at the same speed as before? The answer is 500 RPM.

The driveshaft speed will change, but that doesn't matter. That isn't used for vehicle speed.

Trevor008 said:
If the speedo reads off of vss then a gear change and tire size change shouldn't offset

Tire size DOES affect the speedo. That WILL change the RPM of the axle shafts at any given speed. That's why there is a correction in the software for tire size, but not axle ratio.

Trevor008 said:
Well that's an interesting start to this thread lol. 1 person says you don't have to do anything and the next describes how to fix it

It's now 2-1 for nothing needs to be done.

Why don't you verify how accurate your speedo is before the change, and then check it after the change. That will prove that nothing needs to be done.

Trevor008 · Apr 3, 2015

Mark Kovalsky said:
Why don't you verify how accurate your speedo is before the change, and then check it after the change. That will prove that nothing needs to be done.

I want to make sure of the answer before I do the swap so I get it done in one shot. I'll be happy to report back with my confirmation though. Thanks for the great input

CurtisF · Apr 3, 2015

Trevor008 said:
I want to make sure of the answer before I do the swap so I get it done in one shot. I'll be happy to report back with my confirmation though. Thanks for the great input

He happens to be a former Ford engineer.... so I'm sure you've got the correct answer now.

Mikeyfitz · Apr 3, 2015

^ I'd say that's about as accurate as it'll get lol

Trevor008 · Apr 4, 2015

Thanks for all the posts guys

The Butcher · Apr 4, 2015

CUZICAN said:
Out of curiosity, what gears did your truck have before the swap? My truck has 3:55's is getting 38's and I was thinking 4:10's, but if 4:30's will get the speedo back real close, then that'll be the way I go.

As has been posted on here already, a gear swap won't change your speedo reading. My post was mostly to point out that sometimes an increase in tire size doesn't wildly skew your speedo reading. Either that, or my speedo was pretty far off from the factory.

I had 3.55 gears to start with. As someone else mentioned, get a GPS read on your truck after you do the swap so you'll know how fast you are really going compared to your speedo.

Mark Kovalsky · Apr 4, 2015

The Butcher said:
My post was mostly to point out that sometimes an increase in tire size doesn't wildly skew your speedo reading.

If the new tires have a 1% lower rev/mile then the old tires, it will affect the speedo by 1%. If the new tires have a 10% lower rev/mile then the old tires, it will affect the speedo by 10%. It's just simple math.

The Butcher · Apr 5, 2015

Mark Kovalsky said:
If the new tires have a 1% lower rev/mile then the old tires, it will affect the speedo by 1%. If the new tires have a 10% lower rev/mile then the old tires, it will affect the speedo by 10%. It's just simple math.

Indeed it is simple math. What I was trying to point out is that from the factory the speedo might not be all that accurate to begin with. Since I went from the factory 34" tire height to a 38" (actual diameter 37.7"), my speedo should be off by 10% at any given speed. That isn't the case though per GPS and traffic radar signs. When my speedo reads 30MPH now, I am going 30MPH per GPS/radar, not 32-33MPH as the math would dictate.