Do I need to adjust my speedometer for 30" tires? - Page 2 - JeepForum.com
Search  
Sign Up   Today's Posts
User: Pass: Remember?
Advertise Here
Jeep Home Jeep Forum Jeep Classifieds Jeep Registry JeepSpace Jeep Reviews Jeep Gallery Jeep Clubs Jeep Groups Jeep Videos Jeep Events Jeep Articles
Go Back JeepForum.com > Models > Jeep Cherokee & Comanche Forums > XJ Cherokee Technical Forum > Do I need to adjust my speedometer for 30" tires?

The Original 3/8" Ruffstuff Diff Cover!Building a Bumper?Introducing MONSTALINER™ UV Permanent DIY Roll On Bed Line

Reply
Unread 04-22-2013, 01:56 PM   #16
Charley3
Web Wheeler
1999 XJ Cherokee 
 
Join Date: Mar 2013
Location: Longview, WA
Posts: 3,366
I had a couple numbers wrong in prior 2 posts. I corrected them. Please read again.

He won't be speeding, if he does what I suggested.

For my 30" tires, I don't need to worry about it. All I need to do is drive with speedo reading the speed limit. I'll really be going around 2 to 3 mph faster than speed limit, but cops don't care (except for school zones). I'll go extra slow in school zones.

I agree that with 31" tires on an XJ it'd be a good idea to check using GPS speedometer.

Quote:
Originally Posted by habbyguy View Post
Be VERY careful. You can't tell a cop an anonymous person on a Jeep forum told you you weren't really speeding....

When I put 31's on my XJ, I was going 10% faster than the speedo indicated (so when the speedo said 60, I was actually going 66mph). My wife got a ticket doing about 50 in a 45 while "driving the speed limit" according to the speedo. Changing to a 27 tooth from my 30 tooth gear exactly corrected the error, which (if you can divide 30 by 27) is that 10% I was seeing.

But this really isn't rocket science. Borrow a GPS, or drive by one of those speed-registering signs, and you'll know what's REALLY going on. Or check your odometer against interstate mileage signs (I was registering 9 miles when I was driving 10, for example).

Or just hope you don't get a ticket, I suppose...


__________________
Warning: Sometimes I edit a post a few times to get it how I want it.
Charley3 is offline   Reply With Quote
Unread 04-23-2013, 10:35 PM   #17
sAe23
Registered User
1995 XJ Cherokee 
 
Join Date: Apr 2009
Location: Los Angeles / SFV, California
Posts: 2,329
Found a chart that specifies the percent change in the speedometer readout with various tire sizes and gear ratios:
speedocalibration.jpg  
sAe23 is online now   Reply With Quote
Unread 04-24-2013, 12:25 AM   #18
Charley3
Web Wheeler
1999 XJ Cherokee 
 
Join Date: Mar 2013
Location: Longview, WA
Posts: 3,366
Does your chart mention that on stock Jeeps speedo reads 2 to 3 mph faster than you are really going?

Increasing tire 1 size corrects that and makes speedo accurate.

I've already stated that in 3 prior posts.

Do you acknowledge having read that in my posts?

Your chart does not take that in to account.
__________________
Warning: Sometimes I edit a post a few times to get it how I want it.
Charley3 is offline   Reply With Quote
Unread 04-24-2013, 09:59 AM   #19
habbyguy
Registered User
1996 XJ Cherokee 
 
Join Date: Feb 2010
Location: Mesa, AZ
Posts: 574
Just to be anal about this, the speedo error out of the factory shouldn't be discussed in mph terms, but in % terms. A 3mph error at 35mph is serious, at 85, it's a lot more trivial. And as I said previously, there is so much variability in tire size (actual vs. advertised, and new tire vs. worn out tire) that it's always a good idea to measure the error instead of assuming. There's also the possibility that a previous owner had changed something, or that your particular speedo has other accuracy issues. Speeding tickets are expensive, checking is easy. And on the other side of the same coin, you don't want to be driving around a few mph under the speed limit when you really want to be going the speed limit.

Bottom line, I can't imagine why someone wouldn't want to know how fast they're really going if it's cheap (free) and easy (trivial) to do so. Guesstimating is fine for a starting point, but when you're coming up behind that cop in the right lane, don't you want to know if you're going to be going the speed limit, or 2mph over or 2mph under if you pass him? I know if my speedo tells me I'm going 75mph, and the speed limit is 75mph, I can cruise on by him safely. If you don't know, you can get stuck behind him for 20 miles doing 72, or maybe you can get a ticket for doing 77mph if he's in a bad mood or behind in his quota.
habbyguy is offline   Reply With Quote
Unread 04-24-2013, 01:21 PM   #20
Charley3
Web Wheeler
1999 XJ Cherokee 
 
Join Date: Mar 2013
Location: Longview, WA
Posts: 3,366
Of course checking with a GPS is a good idea, and I will when I have one available to me.

But in meanwhile, I'm confident that with 30" tires I won't get any tickets if I keep speedo saying speed limit.

To clarify, when I said I think 2 to 3 mph off, I meant when speedo says 70 mph, which would mean less off at slower speeds.

After I use a GPS, I'll post results in this thread.
__________________
Warning: Sometimes I edit a post a few times to get it how I want it.
Charley3 is offline   Reply With Quote
Unread 04-24-2013, 03:44 PM   #21
Kevin108
Registered User
2000 XJ Cherokee 
 
Join Date: Aug 2009
Location: Portsmouth, VA
Posts: 3,777
Charley, the charts aren't always as accurate as one would like.

There's a couple formulas that let you determine exactly what speedo gear you need. Like you, mine was spot on with some worn 235s.

I made it a little overly complicated but at $40 a gear, you definitely want to get it right the first time.

Read through http://www.jeepforum.com/forum/f11/s...tener-1482679/
__________________
My build thread has pics again. http://www.jeepforum.com/forum/f177/kevin108s-2000-xj-build-867428/
Kevin108 is offline   Reply With Quote
Unread 04-24-2013, 03:51 PM   #22
sAe23
Registered User
1995 XJ Cherokee 
 
Join Date: Apr 2009
Location: Los Angeles / SFV, California
Posts: 2,329
Quote:
Originally Posted by Charley3 View Post
Does your chart mention that on stock Jeeps speedo reads 2 to 3 mph faster than you are really going?

Increasing tire 1 size corrects that and makes speedo accurate.

I've already stated that in 3 prior posts.

Do you acknowledge having read that in my posts?

Your chart does not take that in to account.
What is this, the Spanish Inquisition? I wasn't expecting it (insert Monty Python retort here).
It's not "my chart". It's a chart I found that I though might be useful and informative.
And I'm sure it's not Jeep specific so it OBVIOUSLY doesn't take anything specific to a Jeep into account.
I don't know why it shows 30" and 28" tires with 3.55 gears to be within calibration but at a glance the rest of the chart seems right
sAe23 is online now   Reply With Quote
Unread 04-24-2013, 04:05 PM   #23
sAe23
Registered User
1995 XJ Cherokee 
 
Join Date: Apr 2009
Location: Los Angeles / SFV, California
Posts: 2,329
Quote:
Originally Posted by Kevin108 View Post
Charley, the charts aren't always as accurate as one would like.

There's a couple formulas that let you determine exactly what speedo gear you need. Like you, mine was spot on with some worn 235s.

I made it a little overly complicated but at $40 a gear, you definitely want to get it right the first time.

Read through http://www.jeepforum.com/forum/f11/s...tener-1482679/
Agreed. Specific calculations are usually best when it comes to choosing the speedometer gear. Generic charts simply give you an idea where you are.
sAe23 is online now   Reply With Quote
Unread 04-24-2013, 04:27 PM   #24
habbyguy
Registered User
1996 XJ Cherokee 
 
Join Date: Feb 2010
Location: Mesa, AZ
Posts: 574
I thought that formula was interesting, but it too relies on some assumptions I don't like to (like that the speedo was spot-on to begin with). My "formula" uses actual speed, obtained with a GPS or a drive-by "You are going XX MPH" sign.

For the sake of this example, let's say the vehicle is a 5-speed, 3.07 geared XJ with 31" tires (mine, that is...).

Here's how you do it...
1) Get your Jeep up to 60mph (any speed will work, but that's kind of in the middle of the "speeds that matter", and at least my speedo is pretty much spot-on at every speed if it's right at 60mph)
2) Check your ACTUAL speed with a GPS or drive-by sign. Let's say it says 66mph.
3) Divide your ACTUAL speed (66) by INDICATED speed (60). This time, it comes out 1.10
4) Count the teeth on your current speedo gear. In this case, it was 30
5) Divide your teeth count by the result of step 3. In this case, that ends up being 30 divided by 1.10, or 27.27
That's the number of teeth you need. Obviously, you can't get a gear with 27.27 teeth, or even 27.3 teeth, so you settle for 27 teeth (which will be within 1% of accurate, or 1mph at 100mph).

School zones and speed traps suddenly hold no unnatural fear, and your orienteering will be much improved (when following directions like "drive 5.2 miles and turn left").
habbyguy is offline   Reply With Quote
Unread 04-24-2013, 05:05 PM   #25
lostatindo
Registered User
2000 XJ Cherokee 
 
Join Date: May 2011
Location: Astoria, NY
Posts: 113
Quote:
Originally Posted by Charley3
Does your chart mention that on stock Jeeps speedo reads 2 to 3 mph faster than you are really going?

Increasing tire 1 size corrects that and makes speedo accurate.

I've already stated that in 3 prior posts.

Do you acknowledge having read that in my posts?

Your chart does not take that in to account.
Not sure why speedometer manufacturers would publish inaccurate data.

This thread makes me want to stab myself in the eye with a speedo gear.
lostatindo is offline   Reply With Quote
Unread 04-24-2013, 06:21 PM   #26
mschi772
Web Wheeler
 
mschi772's Avatar
1997 XJ Cherokee 
 
Join Date: Apr 2012
Location: Racine, WI
Posts: 1,889
I simply have to chime in here about all this talk of speedos deliberately reading high from the factory. That's simply not a reliable enough rule to operate with, so stop spreading crummy info like that. I'm running 225/75r15 which is my original factory tire size with the original 35 tooth speedo gear--my speedo is accurate and has been since it left the factory in Toledo, OH in September of 1995 (with slight variation due to all 225/75r15s being slightly different circumferences). This thread really shouldn't require so much discussion and disagreement. Use a reliable speedometer such as GPS tracking to test current accuracy. If it is off, use the appropriate conversion chart and/or math (even better than just relying on a chart) to determine the correct tooth-count for your new tire size and change it out.
mschi772 is offline   Reply With Quote
Unread 04-25-2013, 01:15 AM   #27
Charley3
Web Wheeler
1999 XJ Cherokee 
 
Join Date: Mar 2013
Location: Longview, WA
Posts: 3,366
Quote:
Originally Posted by sAe23 View Post
What is this, the Spanish Inquisition? I wasn't expecting it (insert Monty Python retort here).
It's not "my chart". It's a chart I found that I though might be useful and informative.
And I'm sure it's not Jeep specific so it OBVIOUSLY doesn't take anything specific to a Jeep into account.
I don't know why it shows 30" and 28" tires with 3.55 gears to be within calibration but at a glance the rest of the chart seems right
All right. I apologize.
__________________
Warning: Sometimes I edit a post a few times to get it how I want it.
Charley3 is offline   Reply With Quote
Unread 04-25-2013, 01:22 AM   #28
Charley3
Web Wheeler
1999 XJ Cherokee 
 
Join Date: Mar 2013
Location: Longview, WA
Posts: 3,366
Quote:
Originally Posted by lostatindo View Post
Not sure why speedometer manufacturers would publish inaccurate data.

This thread makes me want to stab myself in the eye with a speedo gear.
I'm not saying the chart is wrong. I'm just saying it doesn't take into account that one tire size increase often results in an accurate speedo because most speedos (certainly Jeeps) read fast with stock tires. In some cases, maybe even two tire size increases will read accurate on speedo.

If you stab yourself, please video tape it and upload.
__________________
Warning: Sometimes I edit a post a few times to get it how I want it.
Charley3 is offline   Reply With Quote
Unread 04-25-2013, 01:25 AM   #29
Charley3
Web Wheeler
1999 XJ Cherokee 
 
Join Date: Mar 2013
Location: Longview, WA
Posts: 3,366
Quote:
Originally Posted by mschi772 View Post
I simply have to chime in here about all this talk of speedos deliberately reading high from the factory. That's simply not a reliable enough rule to operate with, so stop spreading crummy info like that. I'm running 225/75r15 which is my original factory tire size with the original 35 tooth speedo gear--my speedo is accurate and has been since it left the factory in Toledo, OH in September of 1995 (with slight variation due to all 225/75r15s being slightly different circumferences). This thread really shouldn't require so much discussion and disagreement. Use a reliable speedometer such as GPS tracking to test current accuracy. If it is off, use the appropriate conversion chart and/or math (even better than just relying on a chart) to determine the correct tooth-count for your new tire size and change it out.
I'll get a GPS, test it, and post result.
__________________
Warning: Sometimes I edit a post a few times to get it how I want it.
Charley3 is offline   Reply With Quote
Unread 04-25-2013, 01:28 AM   #30
Charley3
Web Wheeler
1999 XJ Cherokee 
 
Join Date: Mar 2013
Location: Longview, WA
Posts: 3,366
Quote:
Originally Posted by habbyguy View Post
I thought that formula was interesting, but it too relies on some assumptions I don't like to (like that the speedo was spot-on to begin with). My "formula" uses actual speed, obtained with a GPS or a drive-by "You are going XX MPH" sign.

For the sake of this example, let's say the vehicle is a 5-speed, 3.07 geared XJ with 31" tires (mine, that is...).

Here's how you do it...
1) Get your Jeep up to 60mph (any speed will work, but that's kind of in the middle of the "speeds that matter", and at least my speedo is pretty much spot-on at every speed if it's right at 60mph)
2) Check your ACTUAL speed with a GPS or drive-by sign. Let's say it says 66mph.
3) Divide your ACTUAL speed (66) by INDICATED speed (60). This time, it comes out 1.10
4) Count the teeth on your current speedo gear. In this case, it was 30
5) Divide your teeth count by the result of step 3. In this case, that ends up being 30 divided by 1.10, or 27.27
That's the number of teeth you need. Obviously, you can't get a gear with 27.27 teeth, or even 27.3 teeth, so you settle for 27 teeth (which will be within 1% of accurate, or 1mph at 100mph).

School zones and speed traps suddenly hold no unnatural fear, and your orienteering will be much improved (when following directions like "drive 5.2 miles and turn left").
Great post. I'll do those things.
__________________
Warning: Sometimes I edit a post a few times to get it how I want it.
Charley3 is offline   Reply With Quote
Reply
Thread Tools






Jeep, Wrangler, Cherokee, Grand Cherokee, and other models are copyrighted and trademarked to Jeep/Chrysler Corporation. JeepForum.com is not in any way associated with Jeep or the Chrysler Corp.