Do 19" wheels throw off the speedo??

[GotAViper]

Now that I have the 19" ccw 505a's on my Gen I, will it throw the speedo off?

will it read faster or slow? what can I do to fix it?

The stock gen I wheels were 17" right? i would assume its a big enough difference going up 2"?

Thanks

 

[GotAViper]

More information:

The stock sizes for 93 vs. new ones

335/35/17 vs. 355/25?/19 pirellis

 

[plumcrazy]

its the overall height of the tire that does. not the wheel.

check both outside diameters of the tires

 

[snampro]

the rim size doesn't matter, only the overall tire diameter. for example, the stock 335/35-17 has an outter diameter of 26", I believe the pirelli you listed is very similar in overall diameter and would not cause a problem with the speedometer. However, the 345/30-19 that I am running on my rear has a 27.4" overall diameter and did cause an inaccuracy in the speedometer. I has since installed new diff gears which also changed the speedo. To correct for this I installed a Unitrax speedo cal box that I bought from Roe Racing.

If you need help calculating the difference in your speedo due to tire changes shoot me a PM.

you can get tire data from tirerack.com or the manufacturer

 

[RoadiJeff]

Use this tire size calculator that provides the speedometer error data as compared to stock.

http://www.1010tires.com/TireSizeCalculator.asp

According to the info you provided the new tires will indicate that you are driving 60 mph when you are actually driving 60.5 mph - not enough of a difference to be concerned with, IMHO.

Here's another online calculator, if you want a second opinion: http://pw1.netcom.com/~sgalaba/tiresize.htm

 

[OKViper]

With my GTS, GEN III 19" rears with a 3.45 rear gearset is showing 70mph on the speedo and my GPS shows 68.4, so 3.45 seemed to even it back out.

 

[GotAViper]

Thanks for responding guys.

I have one other peice of info that I'm not sure how much it will effect it.

I have 3.55 gears as well, will that change it any more?

Thanks

 

[snampro]

ANYTHING that effects the final gear ratio effects the speedometer calibration, that includes chnages in gear ratio, different overall diameter tires, etc.