Speedo Error and Od. Error Any Help Please

SoDakJeep · **Posted:** Mon Nov 12, 2007 10:14 pm

This weekned I went to see the Chief vs Broncos in Kansas City. According to mapquest it is Total Est. Distance: 578.17 miles from my house to the hotel. Well I left the hotel this morning and when I made it home I had 556 miles on the odometer. Now I'm not to good at math but can anyone tell me how off Speedo is or maybe its just the Odometer. Havent been pulled over yet but also no signs to check the speed around. I have 245/75/16 MTR's so I know it should be off but just how much. Like I said math was never my strong point so any help would be greatful hahaha.

Also how can I go about fixing this? I called the dealer and they said for $60 bucks they can change it so it reads right. But I thought I read somewhere that you can not change the speedo and odometer.

myjeepwalks · **Posted:** Mon Nov 12, 2007 10:21 pm

mapquest is about as accurate as you measuring that amount of distance with a tape measure. :lol:

did you take the exact route as mapquest. with my 245/75-16 mtr my speedo is only off 1 or 2mph according to my gps.

SoDakJeep · **Posted:** Mon Nov 12, 2007 10:37 pm

almost positive i took the exact route. except maybe a little off when I got off the interstate for gas and food.

jason thompson · **Posted:** Mon Nov 12, 2007 10:52 pm

it is going to vary as far as error
best thing to do is get some one with a good GPS and drive for 10 miles then compare
we found ours to be WAY off
doing 70 on the speedo and 75 or so on the GPS ,IIRC
milage was about the same
the good news was that our fuel milage was not as bad as we thought
like you we were at 13 mpg truth is we are at 19+/- on the highway with the AC on and I can get
20+ with NO AC and cruise at 65 on the speedo

SoDakJeep · **Posted:** Mon Nov 12, 2007 10:59 pm

any idea on how to correct it?? :?:

501 · **Posted:** Mon Nov 12, 2007 11:16 pm

Here's a site with calculators to calculate Gear Ratio, Speedometer, Crawl Ratio, RPM, Tire size Calculators

http://www.rocky-road.com/calculator.html

Hope it helps.

jason thompson · **Posted:** Mon Nov 12, 2007 11:22 pm

SoDakJeep wrote:

any idea on how to correct it?? :?:

nope
I just drive 5 mph slower than the posted speed
or keep up with the flow
no tickets yet

Guest · **Posted:** Tue Nov 13, 2007 12:24 am

SoDakJeep wrote:

any idea on how to correct it?? :?:

we told you before in another thread... your speedo will be accurate with the 245/75 MT/R's but your odometer will be incorrect. Call your local stealership and ask how much ti'd be to calibrate your rig.

jason thompson · **Posted:** Tue Nov 13, 2007 9:05 am

ThunderbirdJunkie wrote:

SoDakJeep wrote:

any idea on how to correct it?? :?:

we told you before in another thread... your speedo will be accurate with the 245/75 MT/R's but your odometer will be incorrect. Call your local stealership and ask how much ti'd be to calibrate your rig.

not true
read above
it will vary
I do not know why
I have 31's on the TJ it is WAY off
also 31's on the KJ ,245/75's, it is off but not as bad as the KJ
both according to GPS my F250 with stock tires ,along with 2 other cars, seem to be dead on with the GPS

meh_kick · **Posted:** Tue Nov 13, 2007 10:41 am

Caution Math Ahead:

Stock 235/70 r16 Height= 235*.70*2/25.4 + 16 = 28.9. Therefore circumference = 28.9*3.14 = 90.7
Aftermarket 245/75 r16 = 245*.75*2/25.4 + 16 = 30.5. Therefore circumference = 95.7

Now difference in circumference is 5 inches, divide by original = 5/90.7 = .056 = 5.6% difference in circumference.

Say an accurate Odometer read 578 miles on stock tires, it would only read (1 - .056)*578 = 545.6.

Now each tire varies in true circumference, and will continue to vary based on treadwear. But you can see where the mileage went to.

meh_kick · **Posted:** Tue Nov 13, 2007 10:53 am

meh_kick wrote:

= 5.6% difference in circumference.

This number will also correct your speedo issues. Use mental math.

If your speedo reads 70mph with the 245/75s, subract about 5% or 3.5 mph from the total. At 50 mph, subtract 2.5 mph, at 30mph subtract 1.5 mph...

Hope this helps.

Cowcatcher · **Posted:** Tue Nov 13, 2007 12:04 pm

I don't know why you gasser guys should be special and have speedometers or odometers that are accurate and are accurate the same way. My CRD reads about 5% fast on the speedometer and it used to read about 6% slow on the odometer. I put on larger tires and expected the odometer error to increase as a result but it went from 6% error to 5% error. The speedometer did not seem to change.

I did my tests early on against freeway mileposts on multiple roads over long distances for the odometer and against radar speed limit signs for the odometer. I have graduated to GPS now and find similar results. I live with it. I figure if Jeep wants to give me 5% more on my warranty it works for me.

Guest · **Posted:** Tue Nov 13, 2007 5:29 pm

jason thompson wrote:

ThunderbirdJunkie wrote:

SoDakJeep wrote:

any idea on how to correct it?? :?:

we told you before in another thread... your speedo will be accurate with the 245/75 MT/R's but your odometer will be incorrect. Call your local stealership and ask how much ti'd be to calibrate your rig.

not true
read above
it will vary
I do not know why
I have 31's on the TJ it is WAY off
also 31's on the KJ ,245/75's, it is off but not as bad as the KJ
both according to GPS my F250 with stock tires ,along with 2 other cars, seem to be dead on with the GPS

Jason, I meant since the KJ's speedometer reads high with 235/70s or 225/75s, it's dead on with the 245/75s (mine is), but the odometer will be inaccurate, since the odometer is calibrated for the 235/70s. I think they intentionally make the speedo read high so you don't go gettin tickets and whatnot;)

SoDakJeep · **Posted:** Tue Nov 13, 2007 6:43 pm

WELL THANK YOU FOR ALL THE REPLYS I WONDER WHEN THE DEALERSHIP CHANGES THE OD. IF IT WILL MAYBE BRING BACK SOME POWER. I KNOW ON MY OLD CAR THE COMPUTER WOULD READ THE RPM VS THROTTLE VS SPEEDO TO HELP WITH TIMING AND POWER. BUT ALSO.....NOW THAT YOU MENTION THAT MY SPEEDO IS ON NOW....WOULDNT IT BE SET OFF WHEN THEY RECALIBRATE THE OD. IF THE OD. WAS ORINGALLY SET. SO THAT IT READS 5MPH SLOWER THEN THE SPEEDO? WOULDNT THAT CAUSE THE SPEEDO TO GO BACK UP 5MPH AS WELL???

USMarine1171 · **Posted:** Tue Nov 13, 2007 7:19 pm

OK,

So you drove 578 miles but your speedo says 556.

SOUNDS LIKE A GOOD DEAL TO ME!

EDIT: If you want to calculate your actual speed under real world conditions read the last post in this thread:

http://www.lostjeeps.com/forum/phpBB3/vie ... hp?t=25893

I don't know about correcting the whole speedometer thing. Let us know what the stealership says...I'm curious.

SoDakJeep · **Posted:** Tue Nov 13, 2007 7:31 pm

HAHA TRUE I GET MY WAR. A LITTLE LONGER AND FEWER MILES PUT ON MY CAR HAHAHA

cmohr · **Posted:** Tue Nov 13, 2007 8:06 pm

I've had my speedo recalibrated, yes to get the speedo giving you the right speed, but, more importantly for the petrols, it really helps out the ECU and you'll find the car runs better when you put on big tyres.

Heres a link to a thread I started about it in the Downunder section..

http://www.lostjeeps.com/forum/phpBB3/vie ... hp?t=21020

Speedo Error and Od. Error Any Help Please

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