Larger Diameter tires

[Langan]

I think i read something on here about the 5sp trans having troubles with changing diameter on tires? Plan is to go from 245/70×17 to 285/70×17. What do you think?

[WhitH]

I did that on my E350 Quigley when I had it and there were zero issues. Having said that, I did have an SCT tuner with custom tunes and tire size was part of the discussion when they did the tunes but I think maybe that was just to get the speedo corrected?

[Langan]

SCT tuner? From were?

[WhitH]

My local diesel shop but they outsourced the custom tuning. If you don't have a diesel (can't remember if yours is?) then you can buy a unit and just adjust tire size or maybe even just take it to a shop to have the adjustment done. There are a number of much more knowledgeable folks on here than I who can likely chime in. I will say this though, there are a lot of folks running oversized tires on their rigs with the 5R100 trans with great success.

[boywonder]

FWIW I have been unsuccessful at adjusting my speedo for 285/70-17's...

It reads 7 MPH slow at 65 mph. I have played with 1der's auto-enginuity programmer and called them. Apparently on 2008+ vans, the speedo calibration is in the ABS module and that's not something that Ford exposes to the tuner crowd. My van is a 5.4L/4Rsomething trans....this is likely not an issue on 2007 and older vans.

[JoeH]

No problems running bigger tires with the 5R100. Main issue is the gearing. Not a hard and fast rule but generally:

3.73 33's
4.10 35's
4.56 37's

As for tuning if you have an SCT Tuner you can adjust the shift points, tire size (REV per mile) and gear ratio. But it won't change the speedometer/odometer. It kinda depends on what you want. More legs for the freeway or more torque and power for off road or pulling a trailer.

[86Scotty]

Originally Posted by Langan

SCT tuner? From were?

Why not add some free power while you're at it?

Custom Dynomometer Chassis Tuning Products and Services - 5 Star Tuning

[JoeH]

Originally Posted by boywonder

FWIW I have been unsuccessful at adjusting my speedo for 285/70-17's...

It reads 7 MPH slow at 65 mph. I have played with 1der's auto-enginuity programmer and called them. Apparently on 2008+ vans, the speedo calibration is in the ABS module and that's not something that Ford exposes to the tuner crowd. My van is a 5.4L/4Rsomething trans....this is likely not an issue on 2007 and older vans.

Yep, that's correct. Even with a custom PCM tune you can't adjust the speedo. If one has a Scanguage II or something similar you can adjust it to read the correct speed if you have a GPS.

With 4.10s and 35" tires I'm at or near stock and less than 1 mph off on the speedo.

[WhitH]

See I told you there would be some answers forthcoming! Langan, you should update your signature to include some of your van specs so that folks will be able to give you more direct answers to your question.