Newbie needs help with Speedometer Prob

HBBOWLVT · April 16th, 2010, 01:51 PM

Hello everyone. Just bought a 2002 GT Convertible-automatic. I drove it home to Vermont from Florida. Ran great but did notice the speedometer was reading about 9 miles per hour faster than speed showing on the TomTom navigation unit. When traveling at 80 mph on the speedo (probably closer to 70)the tach was about 2400 rpm. I also noticed when the car idles the tach shows about 700-800 rpm, but the engine sounds like it is revving much higher. The only mods that I have seen so far are a KN Induction, nonstock exhaust, wheels are 17 inch cobra. Any help would be greatly appreciated.

dcook_14 · April 16th, 2010, 01:58 PM

I'm guessing you have a higher than stock gear ratio and it's throwing off your speedo. Jack the back tires up and spin the wheel one rotation while having someone else count how many times the driveshaft goes around. If it's about 3 1/4 turns of the driveshaft you have stock gears, if it's more like 3 3/4 then you have 3.73s and if the driveshaft turns a little more than four times for every one rotation of the tire, you have 4.10s. Do that and let us know what you find.

dcook_14 · April 16th, 2010, 02:01 PM

According to the halfass calculator that I built, if you were in fifth gear at 2400 rpms with stock tires and 3.73s, you'd be going 70. The speedometers on GPS units are pretty damn accurate, so I'd be willing to say it was right and your car speedo was wrong. I'd be willing to bet you have 3.73s in your car and you're not tuned for it, so it's throwing your speedo off

F8L GT · April 16th, 2010, 02:16 PM

If it does happen to have gears in it and you arent tuned, get a tuner asap since you are an auto. Having gears and not being tuned for it is doing more than just throwing your speedo out of wack

casper gt · April 16th, 2010, 09:55 PM

Originally Posted by F8L GT

If it does happen to have gears in it and you arent tuned, get a tuner asap since you are an auto. Having gears and not being tuned for it is doing more than just throwing your speedo out of wack

+1^ get this sorted out right away before you end up putting a trans in.

ESABATM · April 17th, 2010, 05:52 AM

Gears won't mess up a transmission. What should be looked at though is the tailshaft lube mod being done. When I had my 95 Tbird I just used a speedcal and called it a day. That's my 2cents though.

3sportbob · April 17th, 2010, 07:38 AM

Originally Posted by ESABATM

Gears won't mess up a transmission. What should be looked at though is the tailshaft lube mod being done. When I had my 95 Tbird I just used a speedcal and called it a day. That's my 2cents though.

Originally Posted by casper gt

+1^ get this sorted out right away before you end up putting a trans in.

I'm thinking he was referring to shift points and stuff due to the ecm tune not the gears causing the trans issues.

ESABATM · April 17th, 2010, 10:43 AM

Originally Posted by 3sportbob

I'm thinking he was referring to shift points and stuff due to the ecm tune not the gears causing the trans issues.

Yea you're probably right. I didn't think of it in that fashion.

F8L GT · April 17th, 2010, 11:41 AM

Originally Posted by ESABATM

Gears won't mess up a transmission. What should be looked at though is the tailshaft lube mod being done. When I had my 95 Tbird I just used a speedcal and called it a day. That's my 2cents though.

gears do so mess up an auto trans. The readings from the VSS are not correct when you dont reprogram for the different gears and makes the tranny shift at times when its not suppose to. Thats why everyone says to get a tune asap when you do gears with an auto

ESABATM · April 17th, 2010, 02:33 PM

Originally Posted by F8L GT

gears do so mess up an auto trans. The readings from the VSS are not correct when you dont reprogram for the different gears and makes the tranny shift at times when its not suppose to. Thats why everyone says to get a tune asap when you do gears with an auto

From personal experience I ran my T-Bird for over 6 years with no tune. It is still on the original trans that I J-modded before the 3:73's went it. It shifted fine AFTER the speedcal. While shifts weren't optimal before I fixed the speedometer they weren't of the magnitude to grenade it.

You do realize that a speedcal does correct the input to the computer right? That is all that matters really. A tune is nice if you did other mods to warrant is fine but not really needed.

F8L GT · April 17th, 2010, 07:50 PM

exaclty, you corrected the problem with a speedcal. Without a speedcal or tune to give the vss the correct reading it will hurt the trans. Not saying its going to grenade it, but it does do other harm

casper gt · April 17th, 2010, 09:34 PM

Originally Posted by ESABATM

From personal experience I ran my T-Bird for over 6 years with no tune. It is still on the original trans that I J-modded before the 3:73's went it. It shifted fine AFTER the speedcal. While shifts weren't optimal before I fixed the speedometer they weren't of the magnitude to grenade it.

You do realize that a speedcal does correct the input to the computer right? That is all that matters really. A tune is nice if you did other mods to warrant is fine but not really needed.

speed cal's don't work on 99-04 auto cars so that point is out. the VSS in these cars run the trans (ie. shift points/etc) & you most defenately CAN blow one up with out the proper tune (99-04 cars).

dcook_14 · April 19th, 2010, 10:46 AM

Ownage

Anyway OP, did you get a chance to check your ratio?

April 16th, 2010, 01:51 PM	#1
HBBOWLVT Newbie 2002 Mustang GT Vert Join Date: Apr 2010 Location: Vermont Posts: 22 iTrader: 0 reviews	Newbie needs help with Speedometer Prob Hello everyone. Just bought a 2002 GT Convertible-automatic. I drove it home to Vermont from Florida. Ran great but did notice the speedometer was reading about 9 miles per hour faster than speed showing on the TomTom navigation unit. When traveling at 80 mph on the speedo (probably closer to 70)the tach was about 2400 rpm. I also noticed when the car idles the tach shows about 700-800 rpm, but the engine sounds like it is revving much higher. The only mods that I have seen so far are a KN Induction, nonstock exhaust, wheels are 17 inch cobra. Any help would be greatly appreciated.

April 16th, 2010, 01:58 PM	#2
dcook_14 Banned 2004 Mustang GT 12.35@114.08 w/ 1.88 60' Join Date: Feb 2009 Location: Waterloo, IA Posts: 7,015 Blog Entries: 2 iTrader: 7 reviews	I'm guessing you have a higher than stock gear ratio and it's throwing off your speedo. Jack the back tires up and spin the wheel one rotation while having someone else count how many times the driveshaft goes around. If it's about 3 1/4 turns of the driveshaft you have stock gears, if it's more like 3 3/4 then you have 3.73s and if the driveshaft turns a little more than four times for every one rotation of the tire, you have 4.10s. Do that and let us know what you find. __________________ Do the Right Thing, Cook. ~Zip.

April 16th, 2010, 02:01 PM	#3
dcook_14 Banned 2004 Mustang GT 12.35@114.08 w/ 1.88 60' Join Date: Feb 2009 Location: Waterloo, IA Posts: 7,015 Blog Entries: 2 iTrader: 7 reviews	According to the halfass calculator that I built, if you were in fifth gear at 2400 rpms with stock tires and 3.73s, you'd be going 70. The speedometers on GPS units are pretty damn accurate, so I'd be willing to say it was right and your car speedo was wrong. I'd be willing to bet you have 3.73s in your car and you're not tuned for it, so it's throwing your speedo off __________________ Do the Right Thing, Cook. ~Zip.

April 16th, 2010, 02:16 PM	#4
F8L GT !FACEPLATED! 2002 Mustang GT 7.84 @ 86.01 Join Date: Jul 2008 Location: Upper Michigan Posts: 12,917 iTrader: 13 reviews	If it does happen to have gears in it and you arent tuned, get a tuner asap since you are an auto. Having gears and not being tuned for it is doing more than just throwing your speedo out of wack __________________ MM Bracket Racing Crew Co-Founder ~Its a Drag Car, Not a Show Car~ LINK-->Custom Race Transmission Build<--LINK

April 17th, 2010, 07:50 PM	#11
F8L GT !FACEPLATED! 2002 Mustang GT 7.84 @ 86.01 Join Date: Jul 2008 Location: Upper Michigan Posts: 12,917 iTrader: 13 reviews	exaclty, you corrected the problem with a speedcal. Without a speedcal or tune to give the vss the correct reading it will hurt the trans. Not saying its going to grenade it, but it does do other harm __________________ MM Bracket Racing Crew Co-Founder ~Its a Drag Car, Not a Show Car~ LINK-->Custom Race Transmission Build<--LINK

April 17th, 2010, 05:52 AM	#6
ESABATM Regular 2000 Mustang GT SLOW :( Join Date: Apr 2010 Location: GA Posts: 194 iTrader: 0 reviews	Gears won't mess up a transmission. What should be looked at though is the tailshaft lube mod being done. When I had my 95 Tbird I just used a speedcal and called it a day. That's my 2cents though.

April 19th, 2010, 10:46 AM	#13
dcook_14 Banned 2004 Mustang GT 12.35@114.08 w/ 1.88 60' Join Date: Feb 2009 Location: Waterloo, IA Posts: 7,015 Blog Entries: 2 iTrader: 7 reviews	Ownage Anyway OP, did you get a chance to check your ratio? __________________ Do the Right Thing, Cook. ~Zip.

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