Speedo correction for 33's

[dieselbigot]

All this talk of tires and my purchase today of a spanky new set of 305/70 Nitto's has me thinking about a speedo correction gear. When I had my 80 I bought a speedo gear from Slee..is there such a beast for the 100 to correct for 33's?

 

[98LXand99LC]

Christo has the Superlift Tru-Speed Calibrator for $195.


http://www.sleeoffroad.com/products/products_interior_main.htm

 

[80and100cruisers]

I forget what the stock tire size was for my 99 cruiser. I just assume it was 30 inches, and that my 305s are 33 inches, meaning there was a 10% increase. So, I assume that the speedo is 10% off, meaning 30mph is really 33mph, 70mph is really 77mph, etc. My reasoning is that the 33 inch tire covers 10% more ground each rotation than the 30 inch stock tire. That seems like a huge difference, can anyone correct me if my logic is flawed.

Anyways, if Im' right the 10% rule is a cheap/easy method to judge your true speed.

-Matt

 

[Landpimp]

good excuse to buy that GPS

 

[Kaderabek]

I have a Tru-Speed in my 80. Works great.

TJK

 

[mtycruiser]

I forget what the stock tire size was for my 99 cruiser. I just assume it was 30 inches, and that my 305s are 33 inches, meaning there was a 10% increase. So, I assume that the speedo is 10% off, meaning 30mph is really 33mph, 70mph is really 77mph, etc. My reasoning is that the 33 inch tire covers 10% more ground each rotation than the 30 inch stock tire. That seems like a huge difference, can anyone correct me if my logic is flawed.

Anyways, if Im' right the 10% rule is a cheap/easy method to judge your true speed.

-Matt

OE tires are 31.2" and the difference to 33" is 5.4%, your logic is good but the difference not as big as you stated.

 

[dclee]

I forget what the stock tire size was for my 99 cruiser. I just assume it was 30 inches, and that my 305s are 33 inches, meaning there was a 10% increase. So, I assume that the speedo is 10% off, meaning 30mph is really 33mph, 70mph is really 77mph, etc. My reasoning is that the 33 inch tire covers 10% more ground each rotation than the 30 inch stock tire. That seems like a huge difference, can anyone correct me if my logic is flawed.

Close. Actually, your stock tire size is 31 inches, so it's really more of a 6% rule. Which, of course, is more of a pain to do in your head.

 

[80and100cruisers]

Thanks, that was just the info I was looking for.

-Matt

 

[hoser]

I find that most OEM speedometers read low anyway. Yeah, get a GPS and see what your actual speed is.

 

[spressomon]

Circumference difference

Actually if you compare the circumference of both sizes (275-75/16 & 305/70-16) you arrive at a net difference of 1.7%

275-75/16
D= (.75 x 275/25.4) x 2 + 16" = 32.24"
C= Py x D
C= 101.29"

305-70/16
D= (.70 x 305/25.4) x 2 + 16" = 32.81"
C=Py x D
C= 103.08"

The difference in percentage between the above two circumferences is only 1.7%. We all know the actual measurements of each brand of tire will probably be different than the theoretical diameter.

 

[NMuzj100]

That is a 5.4% difference for 285/75R16 vs the 275/70R16 stock tires.

 

[ryanjl]

Actually if you compare the circumference of both sizes (275-75/16 & 305/70-16) you arrive at a net difference of 1.7%

275-75/16
D= (.75 x 275/25.4) x 2 + 16" = 32.24"
C= Py x D
C= 101.29"

305-70/16
D= (.70 x 305/25.4) x 2 + 16" = 32.81"
C=Py x D
C= 103.08"

The difference in percentage between the above two circumferences is only 1.7%. We all know the actual measurements of each brand of tire will probably be different than the theoretical diameter.

Correct me if I'm wrong, but I think the stock tires are 275-65/16's, which are 31.65" in diameter, which equals a 3.54% difference.

Comparing the %delta in diameter is the same as comparing the %delta circumfrence, but is just easier since you are leaving out the pi calculations that you would do on both sides of the equation.

/nerd

 

[tabraha]

Bahh... percentage this; percentage that.

I checked my speedo on 285/75R16's and at an indicated 65 I was doing 68 via GPS monitoring. That was without changing speedo calibration at all. To me that didn't warrant speedo correction.

If you can just get a buddy to ride with you and check the speedo with a handheld GPS unit.

Tad

 

[e9999]

who cares about 6%, sheesh....

 

[spressomon]

Stockers

1999 100 series stock tire size: 275/R7516.

 

[hoser]

You sure it's not 275/70R16?

 

[jp213a]

who cares about 6%, sheesh....

STATE TROOPERS! 6% of 80 MPH is almost 5 MPH. That's the difference between a speeding ticket and no ticket.

 

[spressomon]

Corrected

You sure it's not 275/70R16?

Yes you are correct...the stockers are 275/70R16....