i just put a set of 235/40/17's on my stang today, and it seems like it threw the speedo off some?...would it do that, and if so, how much is it off?

 

235/40/17...are you sure that is the size? Thats like 2.5 inches smaller than what you had on there?

If that size is correct, you are about 10% slower than what the speedo reads but they would look like **** on the car so i think you have your size mixed up

 

Wrap a tape around them and divide 63360 by the diameter in inches for the revs/mile.
OEM 17s are about 740 revs/mile.
You need a $300 tuner to adjust the PCM to display the correct speed with
smaller diameter tires.

 

Your sidewall height is way low for 235 width... how big is your wheel gap now?

 

yea, they are 235/40/17....and yea, there is some massive wheel gap in there, but im gonna lower it a couple of inches in a few weeks, which will cure that problem....and i think im gonna buy a tuner aswell, cause ive been wanting one anyways, so i guess ill just go ahead and get it when i order my springs.....any sugestions on where to buy springs?

 

springs aint going to cure that much gap! I think the most you can get is 2" from sprtlines, if you do that plus the combo you have on there now, you will be bottoming out on everything you find and will probably leave the exhaust laying on the road somewhere if not worse

 

Yeah dude.. if you lower the car 2 inches, your car will be lowered 4 1/2 inch in total due to the low sidewall height of your tires..

I don't think that would bid well on the roads.