speedo gear when going to 17's

[jammer84_03]

been searching but to no real help. i did the spaw to five lug and 17's to replace the 4 lug 15's i had. i have the stock 2.73 gears (do to the gearing in the tremec i have i dont need to up the rear end gear). now my question is, under highway speeds roughly 70-80 mph, my speedo is anywhere between 5-10 mph off. is there any speedo gear i can put in to correct this?

 

[Mustang5L5]

You shouldn't need a speedo gear if you use the correct size tires. On the Mustang, the correct tire size with 17's is either a 245/45/17 or a 275/40/17. If you have either one of these tire sizes on the rear then there is no change in speedo gearing due to the overall diameter of the tire being the same.

Now if you use a different tire size, then it's a different story.

What size are you running??

 

[jammer84_03]

the 275 40.....just a bf g-force kd

 

[jammer84_03]

actually i think it is a 45

 

[Mustang5L5]

Well then that's your problem. A 275/45/17 is an inch taller than the 275/40/17 you should be running. So your speedo is slow by 5-8MPH around 65MPH.

Only way to fix it is to run the correct tire next time.

 

[jammer84_03]

got ya.... sounds right though. going to a 275 with a nitto though

when i replace these i mean with the new tires i will have the right sidewall