do I need speedo calibration for going 20 inch ?

[UT-Engineer]

I have a similar issue. I had the 19" wheels and now have 20" wheels. TBH i am not sure if the overall tire diameter increased but my GPS speed is .5-1mph faster than my cars DIC speed. I also wondered if a recalibration is needed.

[nancygrl]

Just check the two "sticky" threads at the top of this "Wheels and Tire Talk" listing. One is by nineball and the other is by dragoneye. Very useful info. I used both when making a decision on my wheels and tires to make sure that I picked a package that didn't measure/weigh more than the stock ones.

[DraggnFrameGMC]

Originally Posted by Sir Nuke

YES IT WILL....it will effect them all..........If you put on TALLER tires (1.5" larger diameter) you speedo will be registering 60 MPH.....when you are ACTUALLY running 63 mph......and for every hour you drive at 60....you would have actually gone 63 miles.

so you heve two different effects going on. Although you will be thinking your going slower than you really are......your also showing fewer miles on the odometer than you have actually put on your car.

Yeah, I always knew the speedo was off because the speed sensors read off the wheel hubs but I was curious about the odometer on the Camaro, because in my Duramax it reads off the tranny so it never effected the mileage. I guess the lower mileage does kinda work in my advantage though

[Jay_LHD3]

Even if the 20s w/tires weighed the same, you would feel some difference from the 18s. No different than people going from 20s to 22s. Because even if they weighed the same, the mass (of the hoop/barrel and the tire sidewall) has been moved out farther from the center (even when the tire height is the same).

That is why you hear everyone on here always talking about 'show' or 'go', when it comes to wheel size!

[melvinakshay]

can you guys explain what a rolling diameter means ?
I mean I went from 18 to 20, doesn't that mean the diameter increased by 2 inches ?

[DraggnFrameGMC]

not necessarily. As Nancygrl explained, when you measure overall diameter youre actually measuring the tire. If youre 18" was say a 245/55/18 which equals 26.61 inchaes overall and now youre running a 20" at say a 275/40/20 which equals 26.66 overall. The overall diamter is almost exactly the same. So if you took a marker and marked on the "B" of BFG on the 18" and put a mark on the "P" of the 20" Pirelli it and took off with those letters pointing at "12 o'clock" on the tire, it would take the same lenght of road for those letters to make a full rotation back around to the "12 o'clock" position again. Now if you had a 18" at 26.61 and a 20" at 28.1. It would take the 18" less length of road to make a full rotation where as the 20" would be a further distance for a full rotation. Hope this simplified it and wasnt too confusing.

[Sir Nuke]

Originally Posted by melvinakshay

can you guys explain what a rolling diameter means ?
I mean I went from 18 to 20, doesn't that mean the diameter increased by 2 inches ?

Draggn explained it below.....but what it is, is the CIRCUMFERENCE if the tire....or how far the tire travels with ONE rotating of the tire, not a rolling DIAMETER.

Originally Posted by DraggnFrameGMC

not necessarily. As Nancygrl explained, when you measure overall diameter youre actually measuring the tire. If youre 18" was say a 245/55/18 which equals 26.61 inchaes overall and now youre running a 20" at say a 275/40/20 which equals 26.66 overall. The overall diamter is almost exactly the same. So if you took a marker and marked on the "B" of BFG on the 18" and put a mark on the "P" of the 20" Pirelli it and took off with those letters pointing at "12 o'clock" on the tire, it would take the same lenght of road for those letters to make a full rotation back around to the "12 o'clock" position again. Now if you had a 18" at 26.61 and a 20" at 28.1. It would take the 18" less length of road to make a full rotation where as the 20" would be a further distance for a full rotation. Hope this simplified it and wasnt too confusing.

[melvinakshay]

Originally Posted by Sir Nuke

Draggn explained it below.....but what it is, is the CIRCUMFERENCE if the tire....or how far the tire travels with ONE rotating of the tire, not a rolling DIAMETER.

Yes, I think that makes sense. One rotation of the wheel covers the same distance regardless of the diameter.

Originally Posted by DraggnFrameGMC

not necessarily. As Nancygrl explained, when you measure overall diameter youre actually measuring the tire. If youre 18" was say a 245/55/18 which equals 26.61 inchaes overall and now youre running a 20" at say a 275/40/20 which equals 26.66 overall. The overall diamter is almost exactly the same. So if you took a marker and marked on the "B" of BFG on the 18" and put a mark on the "P" of the 20" Pirelli it and took off with those letters pointing at "12 o'clock" on the tire, it would take the same lenght of road for those letters to make a full rotation back around to the "12 o'clock" position again. Now if you had a 18" at 26.61 and a 20" at 28.1. It would take the 18" less length of road to make a full rotation where as the 20" would be a further distance for a full rotation. Hope this simplified it and wasnt too confusing.

Just learnt something new today !!. btw, how did you get that 26.61/26.66 ? is there any mathematical formula ?

[DraggnFrameGMC]

I use a tire calculator on a website when i'm comparing customers factory wheels compared to aftermarket to give an estimated guess as to what size we can fit on their car.
http://www.1010tires.com/tiresizecalculator.asp

[Sir Nuke]

Originally Posted by DraggnFrameGMC

I use a tire calculator on a website when i'm comparing customers factory wheels compared to aftermarket to give an estimated guess as to what size we can fit on their car.
http://www.1010tires.com/tiresizecalculator.asp

yep.....I do the same thing.....there are quite a few different ones.