What is involved in speedo calibration?

[elwesso]

I am looking at some new tires (even though I dont need them)

The optimal size for me to run on my Q is 255/45-17 and 255-40-18.. however, these tires are expensive for something good..

255-45-17

Sumitomo HTRZ II... 98W, $112

Kumho ECSTA MX. 98Y, $125

255-40-18

BFGoodrich g-Force T/A KDW 2.. 95Y, $180

Bridgestone Potenza S-02.. 95Y, $229

So Im not too pleased with these load ratings, as Tech just said in another post a lot of handling can be had by just having a better load rating.. So I looked at some 255-50-17 and 255-45-18.. The problem is there is a 4% difference in speedo..... THe tires are a perfect match, but Im not sure I like the difference in the speedo.. The load ratings make it seem more worth the price...

255-50-17

Kumho ECSTA Supra 712, 101W, $107

255-45-18

Sumitomo HTR Z II 99W, $161

Now looking at the prices, the difference is minimal... However the load rating makes me want to do it, because it the same width, but a taller sidewall for better load handling.. Theyre all considered Z rated (150mph+) so the sidewalls should be really stiff...

Thus, I ask, how might one calibrate the speedo in the Q for this.....?

EDIT.. Let me add there is a 4% variation between the larger and the smaller size... the 255-45-17s match up perfectly to OEM.. the 255-50-17 are 4% different.. The corresponding 18in tire i listed matches perfectly to the 17 i listed.....

[Q45tech]

http://www.blackrobotics.com/yb_home.htm

$100 cheap solution

[elwesso]

THANK you tech!!!!!!!

[JT_MONEY_2004]

Funny you started this post--looking at the same thing today. i plan to replace my wheels and tires prob in jan or feb. replacing w/ 245-45/18,, was looking at this site, shows my change to be 6.1% too slow change. http://www.miata.net/garage/tirecalc.html

I thought there was an onboard system computer to recalibrate--didn't think i needed to buy an add on system.

[texasoil]

It's important to note--you need the loaded radius of the tires, not the external diameter. I'm not sure which Tere Rack uses in their Revs/mile information. For my sl-600--I measure the distance from the concrete floor to the exact center of the wheel to ensure front to rear balance--otherwise the damn anti-slip computer (can't be defeated) will think a wheel is slipping and apply the brake or back off on electronic throttles

[Q45tech]

People don't realize that the same size tires [say 245/50/16 or what ever] will actually be quite different between brands and manufacturers. The 15" have more variation than the 18" usually! The less you pay the more from perfect.

On older Q have been able to mix and match 215, 225, and 235/15 front and rear but not on same axle. The ABS [3 channel] is more forgiving than the traction control [4 channel] updated unit.

Modern cars use the front ABS sensor/computer as a speed sensor for engine/transmission ecu.........so front tires need to be precise in size and inflation.

Measuring loaded radius accurately is a good way to approximate the tire sidewall stiffness. If you know the weight on tire accurately and have an accurate pressure gauge. Notice same tires same inflation on Q front have more deformation [less loaded radius] due to the extra ~~ 100-170 pounds of weight on each front tire [up to 17%] more than rear [empty gas tank ].

This extra front weight is what creates the understeer as the tires with the most weight will have higher slip angles.

Obviously a 17% greater weight cannot be compensated by just a 8% stronger front tire [say by 235/60/15 on front] but half of it can be!When the gas tank is full the weight balance gets better and with 235 front 215/225 rear the Q is tire LOAD Capacity balanced depending on drivers weight and how that changes left /right side to side balance.

Using 6 psi higher inflation in front [35>41] can accomplish part of what a stronger front tire does at the expense of tire patch shape........not an optimal solution but better than nothing.

I split the difference 235 on front at plus 3 psi higher inflation and maybe 225/60/15 rears.

Understand I experiment with lots of rear tires as the rears are less demanding [sustain less abuse] than the front on a Q [high caster camber]. 99% of time you will find my Q with the Michelin Pilot H4 in 235/60/15 on the front.

The cheaper newer experiments go on the rear........brand new tires less likely to fail and it lets me try even Kumho and other import brands to play in summer/even winter..........if a new tire has problems surviving the Q rear you can imagine what would happen on the front.Learned this the hard way in multiple experiments over the years.

I know this is contrary to wisdom about best tires on rear [but I've never had a rear failure [the tires are all too new].

A rear $60 may not last like a front $110+ tire but the effective cost is the same if you get 13-15,000 miles from it and you might have better traction [softer compound] during the life cycle. Unfortunately many of my experiments have ended with giving tires away at 8,000 miles.......they seem to heal when placed on lighter less demanding [camber] cars but we know the internal damage has been done!

[PoorManQ45]

I split the difference 235 on front at plus 3 psi higher inflation and maybe 225/60/15 rears.

Are there any negative aspects to putting smaller tires in the rear, and larger tires in the front?

Also, do you know what the tread wear rating of the OEM tires that the Q45 came with is/was? How close should one try to stay to that OEM tread wear rating for the best possible braking?