Speedometer calibration through tire size adjustment, will it clear?

[Ducatislave]

Vehicle: 2013 3.8L Sedan base model. 41,500 mi.

I'm trying to find out if anyone has tried this yet, and if so did the tires fit without rubbing?
My speedometer if off by 6% by my math and confirmed to two different GPS units and a cell phone running simultaneously.
If I am going 100mph on the speedo I am only actually going 94mph.

Speedo reading vs. Actual speed
50 47
60 56
80 75
100 94
120 113
140 131
160 150
180 169
200 188

According to my math as well as a tire size calculator I believe that changing from a 225/55R17(which is essentially a 27" tall tire) to a 225/65R17 (a 29" tall tire) will correct the speedo to within 0.3% accuracy. It is only about a 1.5lb weight increase per tire and a height increase of 1" so handling and braking should not be affected much if at all. Plus I will get the added benefit of clearing most curbs too.
I have spoken to the dealership about the issue and asked if this can be corrected in the computer by changing the number of and ring pings mer mile.
I have done this using EFI live on my old 2004 GMC and the math isn't terribly hard to calculate.
According to the book and labels the tires on it now are the correct size. They claim that they are not capable of reprogramming the ECM to alter the speedometer.
My biggest concerns are, #1 it is not accurately recording the real miles on the odometer. If my 6% caltulation is correct, then it would mean that the real mileage on my car is not 42.5k but more like 39.9k. And #2 it is not correctly calculating my mpg, or more to the point I could get better mpg with large tires.
Has anyone else had this issue of the speedo being this far off? If so did you have it corrected, and how?

 

[Ducatislave]

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[Ducatislave]

BUMPo

 

[pmckechnie]

I an going to respond . However I can't shed any ideas on this subject. I have no idea if the
tires will rub but I do agree with your calculations and would agree it should work.
I'm sorry I can't be of more help.

 

[Smash Boogie]

I've also noticed this on my 4.6 2011. I first became aware of it while driving in certain neighborhoods that have the flashing radar speed detectors. If I'm going about 43-45 mph the sign reads 40-41 mph. Thought it was a poorly calibrated detector until it happened again in another area, aaaand again elsewhere lol. Idk..never checked into it though.

 

[EdP]

I've also noticed this on my 4.6 2011. I first became aware of it while driving in certain neighborhoods that have the flashing radar speed detectors. If I'm going about 43-45 mph the sign reads 40-41 mph. Thought it was a poorly calibrated detector until it happened again in another area, aaaand again elsewhere lol. Idk..never checked into it though.

Happens that way with many cars. Speedometer, by law, cannot read more than actual speed so there is a safety margin built in. May save you a speeding ticket or two.

 

[Ducatislave]

Just an update on this, IT WORKS. I put Perrelli Cinturato P7 225/65R17 on and they just fit. It actually looks great since they fill the wheel well outline perfectly and clear everything with no rubbing. I have only heard it rub the liner once and that was during a very hard brake and right turn, no damage visible. It has corrected the speedo nearly perfectly, it now reads just .5-1.5 mph low at 85 mph and barely 3mph low at 100. So my calculations were spot on, it got me to 3% accuracy so no worries about a ticket due to the speedo variation. I also GAINED about 1.5 miles per gallon back.
The pictures are of a test fit I had done at Discount Tire while having a flat repaired about 2 weeks before putting the new Perelli's on.