Tire size change,speedo fix,hows everyone addressing it?

[Snakes&horses]

I apologize for the redundant topic. There are a ton of people on here that have changed they’re tires to a different size resulting in the speedo being off to some degree. What and how ( if at all ) did you address the difference ? I’ve gone in the rear from 275/40/19-729revs to 305/35/20-710revs. I know it’s not a huge difference and according to the tire calculator speedo will read 70 with actual being 71.8 . I can live with it if I have to but I’d rather it be accurate. Any help or suggestions to correct it would be appreciated

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[Zooks527]

I apologize for the redundant topic. There are a ton of people on here that have changed they’re tires to a different size resulting in the speedo being off to some degree. What and how ( if at all ) did you address the difference ? I’ve gone in the rear from 275/40/19-729revs to 305/35/20-710revs. I know it’s not a huge difference and according to the tire calculator speedo will read 70 with actual being 71.8 . I can live with it if I have to but I’d rather it be accurate. Any help or suggestions to correct it would be appreciated

You might want to try seeing what your actual speed is when timed over a known (measured mile) distance. Many (most?) manufacturers tend to set their speedometers to read a few mph high, and you may find that you're closer to accurate than you were before.

 

[frank s]

I used a hand-held Ford Racing Performance Parts appliance that installed the FRPP tune. Wheel Rp Mile is adjustable in one-mile per hour increments. It took some work, trying to get the actual measured speed, as mentioned above, to match the displayed speed, which did not match the purported Wheel Rp Mile supplied by tire manufacturer.

Second adjustment was to attach a GPS digital speedometer, (a little slow to keep up with quick acceleration) to which I became so accustomed I had no idea how fast I was going when I changed cars. Lucky for me the current car has a digital MPH display in there somewhere. I haven't done the real-speed assessment, yet, but it's out there somewhere.

 

[Hooch180]

You can specify wheel size directly using forscan.
You need to edit: 726-12-01
And value can be calculated here: https://docs.google.com/spreadsheets/d/1mWnuQZlDTUGjKjmJje1QJIR8Z2c7Kf8KBZxKnw_8EqU/edit

 

[Ronin5.0]

I went from 18 to 19s. According to radar, I'm right on or +/- 1 mph. I'm pretty good with that as I would think (as mentioned above) the margin for electronic error is probably greater and or your ability to discern 1mph at a glance is doubtful. If you have a scan tool, great but I certainly would not pay for a correction that small.

 

[Rash]

I went from 235/55/18 to 275/35/20. The various calculators out there say it's 1 mph off. Too much trouble to change for such a small difference.

 

[ctandc72]

Keep in mind, the S550 is different from previous generations.
The ABS / wheel sensor reads actual wheel speed.
The first thing I'd do is use a GPS and check your actual speed versus your indicated speed and then go from there.

 

[hiccup]

Look at it like I do..though not much but say on a 250 mile trip my gtpp will only log on about 230 something miles..I have 285/40/19 in rear and its like driving the car and getting extra miles and warranty life ..
Now if the new tires had a smaller circumference that would bother me a bit though I'd probably enjoy the feel of a bit more gearing..

 

[Blue Moon]

The Cobb tuner lets me input tire circumferences. I can even put in different front/rear sizes for staggered setups; that's supposed to help the electronic traction control keep things stable.

 

[Norm Peterson]

I apologize for the redundant topic. There are a ton of people on here that have changed they’re tires to a different size resulting in the speedo being off to some degree. What and how ( if at all ) did you address the difference ? I’ve gone in the rear from 275/40/19-729revs to 305/35/20-710revs. I know it’s not a huge difference and according to the tire calculator speedo will read 70 with actual being 71.8 . I can live with it if I have to but I’d rather it be accurate. Any help or suggestions to correct it would be appreciated

Those revs/mile numbers are not the true numbers (there's a correction related to tire compliances - it's around 3% - that none of the online tire calculators or their developers seem to know about). That said, your 71.8 vs 70.0 comparison does still hold. As does the notion that OE speedometers tend to be set to read a little high rather than low. Might even be a legal requirement in some places (Europe?).

I really don't understand this obsession with getting "perfect" speedometer accuracy. Needles have width, and digital or even GPS-based numbers don't always display the precisely correct speed either. Driving at "70-ish mph" should be good enough.


Norm

 

[Zooks527]

Might even be a legal requirement in some places (Europe?).

It's not a legal requirement in Europe, but in some countries the car makers are liable to owners if the owner gets a ticket and can show it's because his speedometer reads low. As a consequence, the manufacturers add a few percent to the speedometer to make sure that it should always read at or above the actual speed with the as-delivered tires. Many make the actual speed available via the OBD or a hidden diagnostic mode on the dash.

 

[Norm Peterson]

It's not a legal requirement in Europe, but in some countries the car makers are liable to owners if the owner gets a ticket and can show it's because his speedometer reads low. As a consequence, the manufacturers add a few percent to the speedometer to make sure that it should always read at or above the actual speed with the as-delivered tires. Many make the actual speed available via the OBD or a hidden diagnostic mode on the dash.

I'm good with that definition. Makes it a de facto regulation rather than a law that's officially on the books.


Norm

 

[Snakes&horses]

Thanks to everyone for the input and suggestions. I’m going to check it against gps but it sounds like it really isn’t much to even worry about. For me it wasn’t even about the speed as much ( or being obsessed with 100% speed accuracy)as it was about the mileage on the car being inaccurate but once again,probably such a small amount not worth worrying. Thanks again guys, input was greatly appreciated

 

[VelocityB]

I put the same size on the rear as well(305-35-20) and according to GPS my speedometer is reading 3mph slow according to the digital dash. At 72 I'm actually going 75. I've only got a 3.15 rear gear(19 GT Premium) and the 10 speed auto. My biggest thing is if the car is shifting optimally at the strip. Sucks knocking my gear down even further but I had the wheels and tires on my 18 mt82 equipped car with a 3.55 gear.

 

[BlownGP]

I'm not to worried about the speedo being off but I was wondering what I should do if I did want to correct it..

I have 285/35/20 front and 305/35/20 rear. Both bigger than stock obviously but if I use forscan it looks like I can only put in one value.
I have a Ngauge with a lund tune cane you put both front and rear tire sizes in it?

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