225 to a 245

peebut · **Joined:** Thu Dec 22, 2005 2:05 pm **Posts:** 37

How much will it affect the spedo and whats the cheapest fix?

jeepkj02 · **Posted:** Tue Jul 04, 2006 3:00 pm

This will tell you:
http://www.4lo.com/calc/gearspeedo.htm

You have to find an exact diameter of the tire. Probably around 1" difference.

Video Guy · **Posted:** Wed Jul 05, 2006 10:20 am

jeepkj02 wrote:

This will tell you:
http://www.4lo.com/calc/gearspeedo.htm

You have to find an exact diameter of the tire. Probably around 1" difference.

Should you enter the diameters as inches or milimeters? When I used 215mm old and 245mm new, it shows 55 mph spedo vs. 62.7 actual. 7 mph difference? That can't be right.

Se7enLC · **Posted:** Wed Jul 05, 2006 11:40 am

peebut wrote:

How much will it affect the spedo and whats the cheapest fix?

Well, considering that 225 and 245 are tire widths, it shouldn't effect your speedometer at all!

If you mean 225/75R16 and 245/75R16, calculate out the diameters or circumference and use those numbers instead. Or use a tire size calculator that gives you a percent difference. It shows a 4% change from 225/75R16 to 245/75R16.

Code:

Specification  Sidewall  Radius  Diameter  Circumference  Revs/Mile  Difference
225/75-16         6.6in  14.6in    29.3in         92.0in        689        0.0%
235/70-16         6.5in  14.5in    29.0in         91.0in        697       -1.1%
245/70-16         6.8in  14.8in    29.5in         92.7in        684        0.7%
245/75-16         7.2in  15.2in    30.5in         95.7in        662        4.0%

For the speedometer difference calculator posted above, it expects diameter to be in inches. The 225/75R16 is 29.3" and the 245/75R16 is 30.5", so at 65mph, you are really going 67.7mph (and at 55mph you are really going 57.3).

The easiest way is to just know that there is the ~4% difference, so multiply the speedometer by 1.04 to get the actual speed (so 65 x 1.04 = 67.6)

raceinfan · **Posted:** Wed Jul 05, 2006 3:35 pm

PCTim did this also ( went to a 245 revo) from his stockers,he had his speedo recalibrated & I believe it was only 2mph off

PCTim · **Posted:** Tue Jul 11, 2006 11:29 am

I'm not sure how much it was off, but I had it done twice. Stock tires were 215/75/16. First set were 225/75/16 Destination M/T's. Had the guy at the Firestone shop calibrate the speedo for me (he used to work at a Chrysler dealer, and had the handheld computer to do it). 225/75 was not a stock size at the time, so we put in the RPM's off of Firestone's web site. Less than 200 miles later, I swapped the Destinations for 245/70/16 Revo's. I honestly forgot to have him redo the speedo again, but realized the RPM's were only off by one. Some time later I noticed that my speedo seemed to be off by a couple of MPH, so I took it to the dealer, and gave them the correct RPM to put in. I forget, but i think they chaged me $40 (1/2 hour labor). The speedo is dead on now, but my mileage sucks, and I can't stand the way the KJ shifts (can't remember what it was like stock). I'm lucky if I get 17 MPG mostly highway (got 18 on my last tank, and I was shocked). As far as shifting, I think it goes into OD too soon. I'll be going up a slight incline, or getting on to the highway, and as soon as I get to about 38 MPH, it goes into OD, and the RPM's drop to about 1200. I'm sorry, but at 1200 RPM's, the KJ cannot get out of it's own way.
Anyways, sorry for the rant, but I hope this helps you out.

Jeepjeepster · **Posted:** Tue Jul 11, 2006 4:45 pm

PCTim wrote:

I'm not sure how much it was off, but I had it done twice. Stock tires were 215/75/16. First set were 225/75/16 Destination M/T's. Had the guy at the Firestone shop calibrate the speedo for me (he used to work at a Chrysler dealer, and had the handheld computer to do it). 225/75 was not a stock size at the time, so we put in the RPM's off of Firestone's web site. Less than 200 miles later, I swapped the Destinations for 245/70/16 Revo's. I honestly forgot to have him redo the speedo again, but realized the RPM's were only off by one. Some time later I noticed that my speedo seemed to be off by a couple of MPH, so I took it to the dealer, and gave them the correct RPM to put in. I forget, but i think they chaged me $40 (1/2 hour labor). The speedo is dead on now, but my mileage sucks, and I can't stand the way the KJ shifts (can't remember what it was like stock). I'm lucky if I get 17 MPG mostly highway (got 18 on my last tank, and I was shocked). As far as shifting, I think it goes into OD too soon. I'll be going up a slight incline, or getting on to the highway, and as soon as I get to about 38 MPH, it goes into OD, and the RPM's drop to about 1200. I'm sorry, but at 1200 RPM's, the KJ cannot get out of it's own way.
Anyways, sorry for the rant, but I hope this helps you out.

Haha, that normal, and no it cannot get out of its own way. My kj with stock tires does the same thing(235/65/17). Ive learned to lock out O/D until I get up to 50-55mph then I let it go into O/D. Its my second nature now to turn O/D off when i get in, I dont even know that I do it anymore.

pixeldzn · **Posted:** Tue Jul 11, 2006 5:11 pm

Here's an interesting one:

My buddy has an '03 Renegade with 245/75-16 MT/Rs and a Frankie

I have an '05 Renegade with 245/75-16 MT/Rs and a Frankie.

Basically smae setup, just different model years.

We were on our way up to Road America north of Milwaukee for a Camp Jeep pre-run, and he had the speedo on his GPS going. The GPS read 70 mph and his speedo read something like 64 or 65. He got on the CB and asked what my speedo read, and mine was accurate to the GPS, reading 70 mph. We were definitely synced up and I was travelling behing him with 2 car lengths between us. I wonder if the speed sensors in the '05 are smart enough somehow to compensate where the '03 is not, and thats why dealers claim to not be able to recalibrate that model as easily.

the1jferg · **Posted:** Fri Jul 14, 2006 11:17 am

the 245/70 and 225's are the same diam and less than a half inch tread width.

Jeepjeepster · **Posted:** Fri Jul 14, 2006 12:04 pm

the1jferg wrote:

the 245/70 and 225's are the same diam and less than a half inch tread width.

No. A 245/70/16 is bigger than a 225/70/16. The 70 is the percent of sidewall to the section width. So if the tire is wider, the sidewall will be taller. 70% of 245mm is more than 70% of 225mm. :wink:

InCommando · **Posted:** Sun Jul 16, 2006 3:09 am

I think maybe he meant a 245/70 vs. a 225/75.

Also, try to get the actual tire height for your brand of tire. The metric sizes seem to run a bit truer, but it is not unusual to see a 1" difference in brands on , say, a 33x12.5x15.

RFCRD · **Posted:** Sun Jul 16, 2006 6:50 pm

Jeepjeepster wrote:

the1jferg wrote:

the 245/70 and 225's are the same diam and less than a half inch tread width.

No. A 245/70/16 is bigger than a 225/70/16. The 70 is the percent of sidewall to the section width. So if the tire is wider, the sidewall will be taller. 70% of 245mm is more than 70% of 225mm. :wink:

Yes and No. Working with the wrong base tire size, need to look at 225/75/16. Tire manufacturers have already done the math for you, just check rev/mile charts. Difference in rev/mile between 225/75/16 and 245/70/16 is insignificant. Try this link, will get you to numerous charts to compare. One thing that is noticable is the 225/75/16 is a real oddball size & spec, limiting tire chioces.

http://www.bridgestonetire.com/tireselector/index_bs.asp

Jeepjeepster · **Posted:** Sun Jul 16, 2006 8:48 pm

Ohh, didnt know he was talking about a 225/75.

PCTim · **Posted:** Mon Jul 17, 2006 8:23 am

Yes, my 225/75 Destinations were listed at the same diameter as my 245/70 Revo's, however, the RPM's were different by 2 (according to the MFG website). I forget the actual numbers though.

225 to a 245

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