hi, I have larger tires (275/55R20) on my '12 SC L322 than stock and have been trying to adjust the speedo to correct for that using the IIDTool (bluetooth version). The speedometer reads about 3MPH less than the actual speed so i am driving at 3MPH over what i it says i am. .

I tried changing the Tyre Rolling Radius to 383mm as measured, but that made no difference. So i cranked the setting up to about 400mm just to see if that did anything, but no.

I did some more searching and found that the Tire Diameter might be the right setting to change, so adjusted that to 32". That seemed to make the problem worse and now it is reading 4MPH off.

Any recommendation on how to adjust this? TIA!

It’s the IPC number - drop it by 2 values if you are that far over. 2012 TDV8 Vogue (L322)

Thanks JayGee, will give it a go.

That makes sense now that i think about it - iirc i bumped it up an inch to 32" in that setting and it made it worse, so i apparently went in the wrong direction since it added 1MPH to the wrong indicated speed.

Now i'm wondering about the actual tire diameter since the it should theoretically be around 31.9" (810 mm) by calculating it.

I measured the radius from center of the wheel down to the ground and got 383mm, which is less than the 405mm you'd expect if the tires were actually 810mm diameter. However, the bottom of the tire is flattened, and the top may be bulging up? Also, the tires are worn down quite a bit even if not yet to the wear bars. I wouldn't have expected them to lose more than an inch though. Also, if i started at 31, and it was already off 3MPH, that would seem to mean they lost several inches due to wear.

(The GPS speedo i am using seems accurate from comparing to the reading when driving by speed cameras.)

Looked the tires up on the Discount Tire site to see what the diameter for my actual tires is and they show: OVERALL DIAMETER 31.89" - 32.13"

The information pop-up says: "Measures the tire height from top to bottom when mounted on a wheel and inflated."

Which is what i was expecting.

Changing the IPC number from 32 to 30 did the trick. thx again!