17" wheels, 45, or 40 profile, what will keep the speedo accurate?

[Henceforward]

If I did a 5 lug conversion, and used 17"x8" 1999 Cobra wheels, what profile tire would allow the speedo to remain accurate.

40, or 45?

Thanks for the help guys

 

[10HoleLX]

A 275 40 17 and a 245 45 17 are very close in height (275 being a bit taller).
45 series would keep the speedo more accurate.

 

[mike93lx]

the "40 or 45" number isn't a measurement of size is mm. it is a percentage of the width of the tire that gives the sidewall height. a 205/40 17 tire is significantly shorter than say, a 325/40 17 (23.5" and 27.2" respectively).
so to properly answer a question like that, you have to know what width you want to use.

just trying to make sure you understand what you're looking at.

 

[89MustangGX]

A 275 40 17 and a 245 45 17 are very close in height (275 being a bit taller).
45 series would keep the speedo more accurate.

They're actually the same height -- if you want to get into real small measurements the 275/40-17 is shorter by hundreths of an inch.

 

[Michael Yount]

Mike and GX are both correct; and either the 275/40 or the 245/45 17's are very close to the height of the stock tire on the Stang. Either will result in your speedo being very close to reading the same way it did with the stock size tires on the car. Which doesn't necessarily mean it's accurate by the way.

 

[trbkrb]

Here is a pretty good tire size comparison calculator:

http://www.miata.net/garage/tirecalc.html

But as always, tires the same "size" may not be exactly the same size from tire maker to tire maker.

the 245/45/17 is almost the exact same diameter as the stock tires, per the above link.

 

[jrichker]

Check to see if there is a speedometer shop in your area. If there is, ask about the Stewart-Warner speedometer gearbox. I believe it is either a 777 Series Drive Joint Kits and Parts or 666 Series Drive Joint Kits and Parts. It is a small gear box that fits between the speedo pickup gear on the transmission and the speedometer. It has quick change gears that allow you to choose almost any tire and rear end gear ratio you want. This will allow you to get the accuracy with within 2%-3% or less. The drawback is that it isn't cheap