Hi,

I have 16" steel wheels (from the Work Special). These are 16", but not sure of the width.

The manual says the stock sizes are 225/57-16 and 245/75-16.

I am looking to go to 17" wheels and wanted to make sure the speedo doesn't end up too far off. That should be a function of overall tire height.
So...

OD = 2 x H + WD
= 2 x (W x AR) + WD

where: OD = Overall Diameter
H = Height of the sidewall
W = Width of the tread
AR = Aspect Ratio
WD = Wheel Diameter

note: use 25.4 mm per inch for the metric conversion.

e.g.-
for the stock tire at 225:

OD = 2 x (225 x .75) + (16 x 25.4)
= 743.9 mm
= 29.28 inches

for the stock tire at 245:

OD = 2 x (245 x .75) + (16 x 25.4)
= 773.9 mm
= 30.47 inches

So the speedo will vary by 3.88% depending on which of the stock sizes that were installed by the factory. At an indicated 65 MPH, you could actually be doing 67.5 MPH. The nice police officer with the radar gun would not likely pull you over for 67.5 MPH, but if you were thinking you could get away with doing 69 in a 65 zone, and were actually going 72 you might not be so lucky.

So if I go to 17x8" wheels, here is a close tire size:

OD = 2 x (285 x .60) + (17 x 25.4)
= 773.8 mm
= 30.47 inches

That is a 285/60-17 tire.

Another one that would work would be:

OD = 2 x (265 x .60) + (17 x 25.4)
= 749.8 mm
= 29.51 inches

That one is a 265/60-17.

This is a 2WD truck, so I am not looking to go to 33" tires and go off-roading. It is just going to the 'Depot or Lowes, car camping, etc. But even so, not sure I really wanted to go to 60 series tires.

On older cars, I have swapped the gear in the transmission that drives the speedo cable, but am guessing this is all electronic these days. Does anyone know if there is a simple way to adjust this for different tires sizes?

Thoughts or suggestions?

 

Forgot to mention: to give credit where it is due... the +1 tire size concept was pioneered (IIRC) by the Tire Rack as a way to go to a larger wheel/tire but keep the speedo reading correct. This is esstentially what the equations above show.

 

My build sheet says the steel wheels on the Work Special were 16 x 6.5".

The tires were 225/75-16 per the build sheet.

So I will probably shoot for the 265/60-17 tires as that will keep the speedo closer to correct.

 

You can get a 17 inch wheel and stay on or around the same diameter of the factory set up and there is no need to re-calibrate the speedo...

 

Thanks Dakota Raven. That is what the calculations above were meant to determine (OD = overall diameter).

But the common tire sizes I have been finding in the brands I like are closer to the 30.5" or 31" mark than the 29" OD my truck came with.

So if I do go to a larger OD tire, then I may need to do something to make the speedo read accurately.

 

Now I might be wrong on this but IIRC to what RMindy said that your year(99) reads from the tone ring in the diff and that tire size changes will not affect speedo, Im not sure if it was gears or tires but one does not affect it, pm RMindy and he should know

 

Tires would still affect it, but gears would not. Having the sensor on the tone ring has it reading the revolutions of the axleshafts, effectively past the gears.
Gord.

 

a superchips tuner will adjust the speedo, and get rid of any death flash issues. hypertech is peculiar about its speedo adjustment and often times does not work, so don't get one of those. i have one that doesn't work...

the dealer can also adjust the speedo by setting the pinion factor. they have canned settings for 225/75-16, 245/75-16, 265/75-16. they can also set others by going to "other" and setting the "revolutions per mile". cost of this should be no more than one hour of labor, maybe less.

it might be cheaper to buy common tire sizes and recalibrate the speedo as opposed to buying odd sized tires.

 

Might be a little late to the party, but this will help determine the sizes you want:

http://www.1010tires.com/TireSizeCal...?action=submit

or this

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

 

On mine I didn't go to bigger diameter wheels. Went from 16x7 to 16x10 wheels with 265 70 tires. Not what they call a +1, I guess it's a +1/2. The tires were .2 inches larger diameter than the 245 78's.