I just ordered new wheels and tires for my 09 FX4

I went with BFG 285/65/18 All Terrains and now I have to recalibrate the speedo.

I want to use my xcal3 but I think I need a number to put in there if I'm not mistaken.

Is there a calculator to determine that? or has anyone already done this that can help me?

Thanks

 

Here ya go - don't fergit yer 3% squish

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

MGD

 

man thanks, i knew i read that somewhere but could not find it

 

how do i account for the 3% squish? on the final number?

 

Sure - close enough fer Gubmint work, lol.

Just remember that a reduction in effective diameter (and hence circumference) results in an increase in effective revs/mile - in other words, it's inversely related.

Can the x3 do single-digit rev/mile increments? My X2 cannot - to nail it would require the tuner to change the value in the tune.

Just get it close enough. Check with a GPS if you can.

MGD

 

Take the number from the website calculator and multiply it by 1.03. This will give you the revs/mile with a 3% squish. In other words:

No squish: 619
3%: 637
4%: 644
5%: 650

MGD (as usual) is correct. Verify with a GPS or (even better) the odometer over a known distance (preferably 10 or more miles).

GPS odometer/Truck odometer x Old tire diameter or revs/mile = New/corrected tire diameter or revs/mile

Make sense?

 

Yes! thanks!

I honestly knew you would sniff this one out and add real value

MGD

 

Based on the tire calculator here is the difference i will see with my ne tires:

Tire Size Comparison

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
275/65-18 7.0in 16.0in 32.1in 100.8in 629 + 3% 0.0% -stock
285/65-18 7.3in 16.3in 32.6in 102.4in 619 +3% 1.6% -new BFG's

the difference is minimal

 

Tire rack has a "specs" feature in the tire sizes that gives you revs per mile also, but you would still have to figure your squish. Your tire for example is 634 revs per mile according to the specs.

 

I found a new iPhone app that calculates revs per mile.

And best of all, it's free!

http://itunes.apple.com/ca/app/auto-...446649009?mt=8

Cheers!