Speedo error with 245/75's

daspes · **Posted:** Sat Oct 27, 2007 8:25 pm

coming from 225/75/16 to 245/75/16's, how much should i be expecting my speedo to be off at highway speed?

TIA

ATXKJ · **Posted:** Sat Oct 27, 2007 8:38 pm

it's about a 4% difference in tires - but my speedo went from low to dead on.
(Odometer appears to be a bit more pessimistic)

TheJawsOfDeath · **Joined:** Sun Nov 05, 2006 3:56 am **Posts:** 305

I found the same to be true, but check this out
http://www.miata.net/garage/tirecalcold.html

jason thompson · **Posted:** Sat Oct 27, 2007 8:50 pm

hard to say
Mine is off quite a bit according to GPS
65 on the speed O = 70ish MPH
and 1 mile on the od =1.08 on the GPS

M&P40 · **Posted:** Sat Oct 27, 2007 8:51 pm

i run 245/75/16 and there are all these signs that show your speed around when the spedo says 40 the sign says 38 when the sign says 60 speedo says 60.

daspes · **Posted:** Sat Oct 27, 2007 8:54 pm

Thanks all, I used the radar signs before the tire swap. I was nearly 5 mph low at 70ish, maybe now I'll be halfway accurate.

Joe Jeeper · **Posted:** Sat Oct 27, 2007 9:28 pm

I've got to tell you I have LT235/85R16 MTR's and my speedo is dead on. Every time I pass one of those speed check things the police have out in the street I go 25mph and it reads 25mph? Joe :roll:

I'm thinking the computer figured it out?

tjkoen01 · **Posted:** Wed Oct 31, 2007 2:21 pm

I just went from 235/70 to 245/75 and my mph went from 2 mph low on the speedo to 2.5 mph high when compared to GPS and radar feedback signs on the side of the road.

Anyone installed a correction device for the speedo? I have one coming tomorrow.

bugnout · **Posted:** Wed Oct 31, 2007 2:54 pm

odometer is 3% over 100 mile trip. Speedo can be as much as 5 miles off especially at cruising speed

65 = 69 actual

0311_DoC · **Posted:** Wed Oct 31, 2007 3:00 pm

on mine the GPS on the freeway is 2mph less then what the odo states.

tjkj2002 · **Posted:** Wed Oct 31, 2007 3:37 pm

tjkoen01 wrote:

I just went from 235/70 to 245/75 and my mph went from 2 mph low on the speedo to 2.5 mph high when compared to GPS and radar feedback signs on the side of the road.

Anyone installed a correction device for the speedo? I have one coming tomorrow.

I sure hope you didn't get this one---

Superlifts TrueSpeed recalibrator,they are junk and way overpriced.Alot of poeple here in CO(on the colorado4x4 board) that have used them hate them,they don't work right and have alot of interference issues with cell phones and garage door openers.There are better ones and cheaper than the Truespeed.Dakota digital makes one to covert just about any speed signal to anything for only $80.

tjkoen01 · **Posted:** Wed Oct 31, 2007 5:48 pm

tjkj2002 wrote:

tjkoen01 wrote:

I just went from 235/70 to 245/75 and my mph went from 2 mph low on the speedo to 2.5 mph high when compared to GPS and radar feedback signs on the side of the road.

Anyone installed a correction device for the speedo? I have one coming tomorrow.

I sure hope you didn't get this one---

Superlifts TrueSpeed recalibrator,they are junk and way overpriced.Alot of poeple here in CO(on the colorado4x4 board) that have used them hate them,they don't work right and have alot of interference issues with cell phones and garage door openers.There are better ones and cheaper than the Truespeed.Dakota digital makes one to covert just about any speed signal to anything for only $80.

Nope, I sure didn't get that one! I looked at the Dakota Digital and Jet Chip's version. I ended up getting the Jet after looking at the install instructions. Should be here tomorrow, but I won't get a chance to install it for a week.

Green Renegade · **Posted:** Wed Oct 31, 2007 6:28 pm

You can just take your jeep into the dealer to have the computer recalibrated, they just need the revs per mile of your new tires. My dealer did it for me for free.

GilaMonster · **Posted:** Wed Oct 31, 2007 6:46 pm

My speedo is constantly 3 mph faster than my GPS measures, even when going to 245/70/16s. And, my odometer is slower than GPS readings.

tjkoen01 · **Posted:** Thu Nov 01, 2007 8:57 am

Green Renegade wrote:

You can just take your jeep into the dealer to have the computer recalibrated, they just need the revs per mile of your new tires. My dealer did it for me for free.

i tried that route, they didn't seem to enthusiastic about it....more-less they told me they were going to charge me an hour's worth of labor which is $95. So I figured I'll go with the aftermarket device so when I go to 4.10's i can calibrate it again for no additional cost.

chefdab · **Posted:** Thu Nov 01, 2007 11:49 am

Also remember that the speedo is calibrated to the average size of a tire over it's life. As it wears it will actually become smaller. A tire whose tread is worn almost to the point where it needs to be replace could be close to 1/2 inch less diameter which means the tire will turn more times per mile than a brand new tire. Hence the differences even with GPS.

Rush345 · **Posted:** Fri Nov 02, 2007 2:50 am

For the past couple of weeks I drove pass on real close to my house and everytime it read dead on. I thought cool and then today I was on the other side of town, and drove down a stretch of road that turned out had a total of 5 radar signs as I drove down that stretch of road . I found every one of each sign showed my speedo of by anywhere from 2 mph off to 7 mph off. Not a single one gave me the same reading :shock:

KJ Taz · **Posted:** Fri Nov 02, 2007 9:17 am

Those signs are not acurate at all, never trust them unless they are the ones that give you a ticket, then they are calibrated regularly.

Guest · **Posted:** Fri Nov 02, 2007 8:06 pm

tjkoen01 wrote:

Green Renegade wrote:

You can just take your jeep into the dealer to have the computer recalibrated, they just need the revs per mile of your new tires. My dealer did it for me for free.

i tried that route, they didn't seem to enthusiastic about it....more-less they told me they were going to charge me an hour's worth of labor which is $95. So I figured I'll go with the aftermarket device so when I go to 4.10's i can calibrate it again for no additional cost.

You don't need to recalibrate the KJ's speedo when swapping gears

USMarine1171 · **Posted:** Mon Nov 05, 2007 6:03 pm

Be skeptical of the GPS. If you read your manual chances are it will state the accuracy is only like 2 meters at best. They are not accurate for speed...unless maybe you have some sweet gajillion dollar one. I know I've marked several fishing spots w/ my Garmin and then w/ my Lowrance and everytime I go back a year later it never puts me back on the spot. This year when I went back I was on the fishing spot and my Lowrance was saying I was on land? They're not reliable. If you really want to know if your speedo is off your going to have to do some physics calculations(one of my most hated experiences in college).

Get on the freeway with a stopwatch of some sort. Start the stopwatch when you pass a mile marker....the thing is you have to keep your speed *constant* so make sure you are on a flat, straight stretch. The longer distance you can time over the more accurate the math will be. Stop the watch at the next mile marker or a couple down the road at exactly the time you pass the marker while going constant speed. Go to this website: http://www.csgnetwork.com/csgtsd.html

Now, type in your values in the "Speed Calculated by Distance and Time" section. You want to select "Statute Miles" and enter the miles you timed your speed over...1 Mile, 2, 3...whatever. Then enter your time in the blanks. Next select "Statute Miles per Hour" and all you do is select "Calculate." It's pretty easy to do considering it cuts out all the physics calculations for you. Just make sure you travel at a constant speed. Then compare the calculated speed with what your speedo was saying when you were driving.

Speedo error with 245/75's

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