speedo off by how much?

[tnk_2]

Hi guys,
I am going to get a 2020 premium gt that come with the stock 18" rim with p235/50R18 tire. I am going to upgrade it to sve R350 19X10 with P275/40R19 square , don't know what I want in color yet. I think liquid graphite is my first choice.

here is the question: how much off is the speedometer going to read? is the new setup going to fit without any issues or do I have to add spacers. car will be stock height for the time being. if I would lower the car how much can I lower without rubbing?

thanks

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[Mach VII]

Speedo will be off approx. 1.5% (low) as that is almost a 1/2" taller tire. Fitment will be determine by offset which I believe the consensus is +35 to +40 ET is the ticket.

https://tiresize.com/calculator/
https://www.1010tires.com/Tools/Wheel-Offset-Calculator#

Best resources for calculating wheel/tire changes...

 

[m3incorp]

Is the speedo picking up from rear wheels are fronts...I'm guessing it picks up the fronts. I ask because of a staggered tire setups like 275/35R20 fronts and 305/35R20 rear. The fronts are only .3 taller than the stock 18" tires but the rears are 1.1 taller than the stock.

 

[SAY WHAT]

From what i read, its from the rear wheels. But im not sure if that has changed for the 18+ years.

 

[slowrey]

I'd like to know too because the sizes of the front PP1 wheels from front to back have different diameters.

 

[TopJimmyCooks]

Is the speedo picking up from rear wheels are fronts...I'm guessing it picks up the fronts. I ask because of a staggered tire setups like 275/35R20 fronts and 305/35R20 rear. The fronts are only .3 taller than the stock 18" tires but the rears are 1.1 taller than the stock.[/

The speedo gets its data from the transmission output speed sensor. Same location where I remember replacing a broken speedo cable on my ‘89 IROC, but now they are digital and also feed the cruise control data.

 

[tnk_2]

Speedo will be off approx. 1.5% (low) as that is almost a 1/2" taller tire. Fitment will be determine by offset which I believe the consensus is +35 to +40 ET is the ticket.

https://tiresize.com/calculator/
https://www.1010tires.com/Tools/Wheel-Offset-Calculator#

Best resources for calculating wheel/tire changes...

is that 1.5% across the 0-160 miles?
thanks

 

[Jonyxz]

From what i read, its from the rear wheels. But im not sure if that has changed for the 18+ years.

Always rear for rwd cars

 

[Mach VII]

is that 1.5% across the 0-160 miles?
thanks

That is correct. At 100 mph indicated you will be running about 98.5 mph.... not a huge difference at all.

 

[JohnVallo]

The speedo gets its data from the transmission output speed sensor. Same location where I remember replacing a broken speedo cable on my ‘89 IROC, but now they are digital and also feed the cruise control data.

https://www.mustang6g.com/forums/threads/speedometer-calibration.110607/page-2#post-2875272

 

[TopJimmyCooks]

https://www.mustang6g.com/forums/threads/speedometer-calibration.110607/page-2#post-2875272

I’m confused (and admittedly not a mechanic, so take it easy on me here) ...I thought that the ABS speed sensors were intended to monitor rotation disparity between wheels and the vehicle’s speed, and the vehicle speed is determined by the transmission output speed sensor (e.g.
BR3Z-7H103-A). If I’m wrong I’m fine with that. It happens.

 

[JohnVallo]

I’m confused (and admittedly not a mechanic, so take it easy on me here) ...I thought that the ABS speed sensors were intended to monitor rotation disparity between wheels and the vehicle’s speed, and the vehicle speed is determined by the transmission output speed sensor (e.g.
BR3Z-7H103-A). If I’m wrong I’m fine with that. It happens.

Wasn't meant to be hard on anyone,
This forum is to help people, right?
The ABS speed sensors ALSO "monitor rotation disparity between wheels"
The transmission output sensor is primarily used by the computer (along with final rear-gear ratio), to control shift points.
This of course is for Automatic transmission equipped cars. Manual transmissions, (Not so much)
Having changed rear-gear ratios 6 or 7 times in the S550's, as well as changed tire sizes, I had to get up to speed on what controlled what.

Hope this helps...

 

[tnk_2]

https://www.mustang6g.com/forums/threads/speedometer-calibration.110607/page-2#post-2875272

this really help me understand much better now. thanks

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