Jeff Strickland

I agree. Sorta.

If the factory fitment of speedo gear was to accomodate the largest factory
tire, then with the small tires on the stock models, the speedo should be
fast, they should read considerably higher than the actual speed.

If the speedo gear for the stock base-models is selected properly for the
tires, then changing the tires should result in an error where the speedo
reads slow with the new tires.

Your experience with the XJ was such that you swapped a tire that was nearly
30 with one that was 30, and the error that caused dialed the speedometer's
accuracy in almost perfectly. (I had a similar experience with my BMW, by
the way.) But, the OP is making a drastic change from a 26.5" tire to a 30.
This is 3" different in diameter, and 9" in circumference. That's a large
difference, and the speedo should logically not be able to display the
actual speed anymore.




"Mike Romain" <romainm@sympatico.ca> wrote in message
news:45ba8063$0$6662$9a6e19ea@unlimited.newshostin g.com...
> You need to find a measured mile or use a GPS to see what it is now. I
> have found that Jeep speedometers are calibrated for the largest stock
> tire size so yours might be right on now.
>
> Our XJ's became right on with 235's which are almost 30" tall.
>
> Mike
> 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
> 88 Cherokee 235 BFG AT's
> Canadian Off Road Trips Photos: Non members can still view!
> Jan/06 http://www.imagestation.com/album/pi...?id=2115147590
> (More Off Road album links at bottom of the view page)
>
> tk wrote:
>> I upgraded to alum canyon wheels and 30" tires from 195/75R15 steel
>> wheels and tires. Does my speedometer need to be adjusted?
>>
>> Has anyone done this?
>>
>> Tom
>>

Mike Romain

The funniest one was my CJ7's speedometer. It became proper with 33"
tires for some reason. It had 31's on it when I got it and it was slow
to the real speed. I would be doing a real 60 when the thing read 65.

I think someone swapped a gear to go to 31's from the 215's when 30's
would have been right for the speedometer...

Mike

Jeff Strickland wrote:
> I agree. Sorta.
>
> If the factory fitment of speedo gear was to accomodate the largest
> factory tire, then with the small tires on the stock models, the speedo
> should be fast, they should read considerably higher than the actual speed.
>
> If the speedo gear for the stock base-models is selected properly for
> the tires, then changing the tires should result in an error where the
> speedo reads slow with the new tires.
>
> Your experience with the XJ was such that you swapped a tire that was
> nearly 30 with one that was 30, and the error that caused dialed the
> speedometer's accuracy in almost perfectly. (I had a similar experience
> with my BMW, by the way.) But, the OP is making a drastic change from a
> 26.5" tire to a 30. This is 3" different in diameter, and 9" in
> circumference. That's a large difference, and the speedo should
> logically not be able to display the actual speed anymore.
>
>
>
>
> "Mike Romain" <romainm@sympatico.ca> wrote in message
> news:45ba8063$0$6662$9a6e19ea@unlimited.newshostin g.com...
>> You need to find a measured mile or use a GPS to see what it is now.
>> I have found that Jeep speedometers are calibrated for the largest
>> stock tire size so yours might be right on now.
>>
>> Our XJ's became right on with 235's which are almost 30" tall.
>>
>> Mike
>> 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
>> 88 Cherokee 235 BFG AT's
>> Canadian Off Road Trips Photos: Non members can still view!
>> Jan/06 http://www.imagestation.com/album/pi...?id=2115147590
>> (More Off Road album links at bottom of the view page)
>>
>> tk wrote:
>>> I upgraded to alum canyon wheels and 30" tires from 195/75R15 steel
>>> wheels and tires. Does my speedometer need to be adjusted?
>>>
>>> Has anyone done this?
>>>
>>> Tom
>>>
>

Mike Romain

The funniest one was my CJ7's speedometer. It became proper with 33"
tires for some reason. It had 31's on it when I got it and it was slow
to the real speed. I would be doing a real 60 when the thing read 65.

I think someone swapped a gear to go to 31's from the 215's when 30's
would have been right for the speedometer...

Mike

Jeff Strickland wrote:
> I agree. Sorta.
>
> If the factory fitment of speedo gear was to accomodate the largest
> factory tire, then with the small tires on the stock models, the speedo
> should be fast, they should read considerably higher than the actual speed.
>
> If the speedo gear for the stock base-models is selected properly for
> the tires, then changing the tires should result in an error where the
> speedo reads slow with the new tires.
>
> Your experience with the XJ was such that you swapped a tire that was
> nearly 30 with one that was 30, and the error that caused dialed the
> speedometer's accuracy in almost perfectly. (I had a similar experience
> with my BMW, by the way.) But, the OP is making a drastic change from a
> 26.5" tire to a 30. This is 3" different in diameter, and 9" in
> circumference. That's a large difference, and the speedo should
> logically not be able to display the actual speed anymore.
>
>
>
>
> "Mike Romain" <romainm@sympatico.ca> wrote in message
> news:45ba8063$0$6662$9a6e19ea@unlimited.newshostin g.com...
>> You need to find a measured mile or use a GPS to see what it is now.
>> I have found that Jeep speedometers are calibrated for the largest
>> stock tire size so yours might be right on now.
>>
>> Our XJ's became right on with 235's which are almost 30" tall.
>>
>> Mike
>> 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
>> 88 Cherokee 235 BFG AT's
>> Canadian Off Road Trips Photos: Non members can still view!
>> Jan/06 http://www.imagestation.com/album/pi...?id=2115147590
>> (More Off Road album links at bottom of the view page)
>>
>> tk wrote:
>>> I upgraded to alum canyon wheels and 30" tires from 195/75R15 steel
>>> wheels and tires. Does my speedometer need to be adjusted?
>>>
>>> Has anyone done this?
>>>
>>> Tom
>>>
>

Mike Romain

The funniest one was my CJ7's speedometer. It became proper with 33"
tires for some reason. It had 31's on it when I got it and it was slow
to the real speed. I would be doing a real 60 when the thing read 65.

I think someone swapped a gear to go to 31's from the 215's when 30's
would have been right for the speedometer...

Mike

Jeff Strickland wrote:
> I agree. Sorta.
>
> If the factory fitment of speedo gear was to accomodate the largest
> factory tire, then with the small tires on the stock models, the speedo
> should be fast, they should read considerably higher than the actual speed.
>
> If the speedo gear for the stock base-models is selected properly for
> the tires, then changing the tires should result in an error where the
> speedo reads slow with the new tires.
>
> Your experience with the XJ was such that you swapped a tire that was
> nearly 30 with one that was 30, and the error that caused dialed the
> speedometer's accuracy in almost perfectly. (I had a similar experience
> with my BMW, by the way.) But, the OP is making a drastic change from a
> 26.5" tire to a 30. This is 3" different in diameter, and 9" in
> circumference. That's a large difference, and the speedo should
> logically not be able to display the actual speed anymore.
>
>
>
>
> "Mike Romain" <romainm@sympatico.ca> wrote in message
> news:45ba8063$0$6662$9a6e19ea@unlimited.newshostin g.com...
>> You need to find a measured mile or use a GPS to see what it is now.
>> I have found that Jeep speedometers are calibrated for the largest
>> stock tire size so yours might be right on now.
>>
>> Our XJ's became right on with 235's which are almost 30" tall.
>>
>> Mike
>> 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
>> 88 Cherokee 235 BFG AT's
>> Canadian Off Road Trips Photos: Non members can still view!
>> Jan/06 http://www.imagestation.com/album/pi...?id=2115147590
>> (More Off Road album links at bottom of the view page)
>>
>> tk wrote:
>>> I upgraded to alum canyon wheels and 30" tires from 195/75R15 steel
>>> wheels and tires. Does my speedometer need to be adjusted?
>>>
>>> Has anyone done this?
>>>
>>> Tom
>>>
>

Mike Romain

The funniest one was my CJ7's speedometer. It became proper with 33"
tires for some reason. It had 31's on it when I got it and it was slow
to the real speed. I would be doing a real 60 when the thing read 65.

I think someone swapped a gear to go to 31's from the 215's when 30's
would have been right for the speedometer...

Mike

Jeff Strickland wrote:
> I agree. Sorta.
>
> If the factory fitment of speedo gear was to accomodate the largest
> factory tire, then with the small tires on the stock models, the speedo
> should be fast, they should read considerably higher than the actual speed.
>
> If the speedo gear for the stock base-models is selected properly for
> the tires, then changing the tires should result in an error where the
> speedo reads slow with the new tires.
>
> Your experience with the XJ was such that you swapped a tire that was
> nearly 30 with one that was 30, and the error that caused dialed the
> speedometer's accuracy in almost perfectly. (I had a similar experience
> with my BMW, by the way.) But, the OP is making a drastic change from a
> 26.5" tire to a 30. This is 3" different in diameter, and 9" in
> circumference. That's a large difference, and the speedo should
> logically not be able to display the actual speed anymore.
>
>
>
>
> "Mike Romain" <romainm@sympatico.ca> wrote in message
> news:45ba8063$0$6662$9a6e19ea@unlimited.newshostin g.com...
>> You need to find a measured mile or use a GPS to see what it is now.
>> I have found that Jeep speedometers are calibrated for the largest
>> stock tire size so yours might be right on now.
>>
>> Our XJ's became right on with 235's which are almost 30" tall.
>>
>> Mike
>> 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
>> 88 Cherokee 235 BFG AT's
>> Canadian Off Road Trips Photos: Non members can still view!
>> Jan/06 http://www.imagestation.com/album/pi...?id=2115147590
>> (More Off Road album links at bottom of the view page)
>>
>> tk wrote:
>>> I upgraded to alum canyon wheels and 30" tires from 195/75R15 steel
>>> wheels and tires. Does my speedometer need to be adjusted?
>>>
>>> Has anyone done this?
>>>
>>> Tom
>>>
>

mark diegel

Speedometer ratio adjustment calculation.
Ever wonder how far off your speedometer is with your new bigger tires? By
using a simple ratio calculation, this info will only approximate your
corrected speed and depends on the accuracy of tire size diameters (assumimg
nothing but tire size has changed). If you know specific information about
your vehicle (gear ratios, RPM, etc.) then use the Gear Ratio calculator,
it's probably a little more accurate.

Formula used

(New Tire Diameter / Old Tire Diameter) * Speedometer MPH = Actual MPH



Another way of looking at this relationship would be to figure what the
indicated speed would be if you were actually going 60 mph. In this case,
the tire diameter relationship is flip-flopped to:

Indicated Speed = old tire diameter x actual speed
New tire diameter

Using the previous example, your speedometer reading at an actual 60 mph is:

28 x 60 = 48 mph

35

mark diegel

Speedometer ratio adjustment calculation.
Ever wonder how far off your speedometer is with your new bigger tires? By
using a simple ratio calculation, this info will only approximate your
corrected speed and depends on the accuracy of tire size diameters (assumimg
nothing but tire size has changed). If you know specific information about
your vehicle (gear ratios, RPM, etc.) then use the Gear Ratio calculator,
it's probably a little more accurate.

Formula used

(New Tire Diameter / Old Tire Diameter) * Speedometer MPH = Actual MPH



Another way of looking at this relationship would be to figure what the
indicated speed would be if you were actually going 60 mph. In this case,
the tire diameter relationship is flip-flopped to:

Indicated Speed = old tire diameter x actual speed
New tire diameter

Using the previous example, your speedometer reading at an actual 60 mph is:

28 x 60 = 48 mph

35

mark diegel

Speedometer ratio adjustment calculation.
Ever wonder how far off your speedometer is with your new bigger tires? By
using a simple ratio calculation, this info will only approximate your
corrected speed and depends on the accuracy of tire size diameters (assumimg
nothing but tire size has changed). If you know specific information about
your vehicle (gear ratios, RPM, etc.) then use the Gear Ratio calculator,
it's probably a little more accurate.

Formula used

(New Tire Diameter / Old Tire Diameter) * Speedometer MPH = Actual MPH



Another way of looking at this relationship would be to figure what the
indicated speed would be if you were actually going 60 mph. In this case,
the tire diameter relationship is flip-flopped to:

Indicated Speed = old tire diameter x actual speed
New tire diameter

Using the previous example, your speedometer reading at an actual 60 mph is:

28 x 60 = 48 mph

35

mark diegel

Speedometer ratio adjustment calculation.
Ever wonder how far off your speedometer is with your new bigger tires? By
using a simple ratio calculation, this info will only approximate your
corrected speed and depends on the accuracy of tire size diameters (assumimg
nothing but tire size has changed). If you know specific information about
your vehicle (gear ratios, RPM, etc.) then use the Gear Ratio calculator,
it's probably a little more accurate.

Formula used

(New Tire Diameter / Old Tire Diameter) * Speedometer MPH = Actual MPH



Another way of looking at this relationship would be to figure what the
indicated speed would be if you were actually going 60 mph. In this case,
the tire diameter relationship is flip-flopped to:

Indicated Speed = old tire diameter x actual speed
New tire diameter

Using the previous example, your speedometer reading at an actual 60 mph is:

28 x 60 = 48 mph

35

mark diegel

http://www.offroaders.com/info/tech-...ng/mph-rpm.htm

Here's the site.