Speed odometer off

gragra73 · May 17th, 2008, 05:36 PM

When I purchased my 89tq tune from Doug, I had 3.55 gears and my tires were 255/50/17. Since then I changed my tires and gears. My tires are now 245/45/17 and I now have 4.10s. I modified my custom tune. I adjusted my tire revs to 785 per. min and gears to 4.09 (diablo doesn't have 4.10). When I compare my speed odometer to my GPS my speed odometer reads about 5mph than the GPS. When I modify my tune to 4.30 gears my speed odometer is off by only 2 mph. Does Doug need to re-send a new tune to reflect my new gears and tires?

TheUNZippee! · May 17th, 2008, 05:38 PM

I dunno. When I set mine with the Predator yesterday to 4.09 after having the 4.10's installed, it was bang on, as checked by the dyno computer.

Might wanna drop Doug an email and ask him what gives.

What was your original tire size so I can check that for you?

EDIT: just saw that in your OP. hang on two secs.

TheUNZippee! · May 17th, 2008, 05:40 PM

Here's what I came up with:

Stock Tire - 255/50R17 >Search Tires Tire 1 - 245/45R17 >Search Tires Section Width:10.03 in255 mmSection Width:9.64 in245 mmRim Diameter:17 in431.8 mmRim Diameter:17 in431.8 mmRim Width Range:7 - 9 inRim Width Range:7.5 - 9 inOverall Diameter:27.03 in686.56 mmOverall Diameter:25.68 in652.27 mmSidewall Height:5.01 in127.25 mmSidewall Height:4.34 in110.23 mmRadius:13.51 in343.15 mmRadius:12.84 in326.13 mmCircumference:84.91 in2156.7 mmCircumference:80.67 in2049.0 mmRevs per Mile:769.5Revs per Mile:809.9Actual Speed:60 mph100 km/hSpeedometer1:63.1 mph105. km/hSpeedometer Difference: -Speedometer Difference: 5.250% too fastDiameter Difference: -Diameter Difference: 5%

TheUNZippee! · May 17th, 2008, 05:41 PM

Bottom line: your new rubber is too short. Almost an inch and a half smaller in overall diameter.

gragra73 · May 17th, 2008, 05:52 PM

I thought by modifying the tune for 785 revs per. min 20168/25.7, it would correct the speed odometer problem. I guess not.

TheUNZippee! · May 17th, 2008, 06:03 PM

Originally Posted by gragra73

I thought by modifying the tune for 785 revs per. min 20168/25.7, it would correct the speed odometer problem. I guess not.

That I am unfamiliar with. Maybe Doug will be able to tell you how to set it up properly.

gragra73 · May 18th, 2008, 12:13 AM

I think I found the problem. I had the tire revs set for 785 per. min when it should have been 810 per. min.

TheUNZippee! · May 18th, 2008, 12:20 AM

Cool

gragra73 · May 18th, 2008, 08:22 PM

set the revs to 810, but my speed odometer still reads about 2 MPH faster than the GPS. At this point I am not going to worry about it. Aren't most speed odometers off by 2-3 MPH anyway?

TheUNZippee! · May 18th, 2008, 08:57 PM

Yes, they are. 2 mph is not a big deal.

Jucostud · May 18th, 2008, 11:59 PM

maybe the gps is off by 2mph

May 17th, 2008, 05:36 PM	#1
gragra73 Regular Join Date: Feb 2008 Posts: 259	Speed odometer off When I purchased my 89tq tune from Doug, I had 3.55 gears and my tires were 255/50/17. Since then I changed my tires and gears. My tires are now 245/45/17 and I now have 4.10s. I modified my custom tune. I adjusted my tire revs to 785 per. min and gears to 4.09 (diablo doesn't have 4.10). When I compare my speed odometer to my GPS my speed odometer reads about 5mph than the GPS. When I modify my tune to 4.30 gears my speed odometer is off by only 2 mph. Does Doug need to re-send a new tune to reflect my new gears and tires?

May 18th, 2008, 11:59 PM	#11
Jucostud Hardcore Enthusiast 2007 Mustang Join Date: Nov 2006 Location: North of Dallas, TX Posts: 2,559	maybe the gps is off by 2mph __________________ GT500 Spoiler Club Join the club and get your painted spoiler shipped for $120!

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May 17th, 2008, 05:52 PM	#5
gragra73 Regular Join Date: Feb 2008 Posts: 259	I thought by modifying the tune for 785 revs per. min 20168/25.7, it would correct the speed odometer problem. I guess not.

May 18th, 2008, 12:13 AM	#7
gragra73 Regular Join Date: Feb 2008 Posts: 259	I think I found the problem. I had the tire revs set for 785 per. min when it should have been 810 per. min.

May 18th, 2008, 08:22 PM	#9
gragra73 Regular Join Date: Feb 2008 Posts: 259	set the revs to 810, but my speed odometer still reads about 2 MPH faster than the GPS. At this point I am not going to worry about it. Aren't most speed odometers off by 2-3 MPH anyway?

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