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Recalibration of Speedo when running smaller wheels

PeteInCT

#LS-378 - So many Porsche's, so little time....
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Connecticut
Do any of you guys know if there's a way to recalibrate the speedo when you're running with smaller tires/wheels? It's not a big deal but it would be nice if my speedo was accurate after going to 18" wheels. My calculations show that it's off by almost 8 MPH when the speedo reads 160 MPH. My guess is that this is not feasible for too expensive to ever entertain.
 
You would need to do a reflash. From what I've read it's not possible to do a reflash without overwriting TK (unless you have the dealer software and a "key") You could get a handheld tuner from SCT and reflash your stock tune onto the black key and it will allow you to adjust tire size, gear ratio, fan settings and even spark advance :eek: but you could presumably reflash with the stock tune and just change tire size. Of course you could also just order a CAI and tune and make the adjustments on the handheld when you load it ;)
 

PeteInCT

#LS-378 - So many Porsche's, so little time....
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2,848
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Connecticut
If you keep this up my wife is going to come and west and find you.... ;D LOL
 
I thought you could set the tire size in the TK tune? I think you go to the dealer and have them change the tire size in the TK tune setup and the odo will match your track tires when using TK. The silver key will match your street tires and red key match your track tires. Or maybe this is wishful thinking... :D
 
5 DOT 0 said:
I thought you could set the tire size in the TK tune? I think you go to the dealer and have them change the tire size in the TK tune setup and the odo will match your track tires when using TK. The silver key will match your street tires and red key match your track tires. Or maybe this is wishful thinking... :D
No you're correct. The only issue with that is street driving with TK. Personally I'd rather have the speedo correct on the street. They don't give speeding tickets at the track ;)
 
cloud9 said:
5 DOT 0 said:
I thought you could set the tire size in the TK tune? I think you go to the dealer and have them change the tire size in the TK tune setup and the odo will match your track tires when using TK. The silver key will match your street tires and red key match your track tires. Or maybe this is wishful thinking... :D
No you're correct. The only issue with that is street driving with TK. Personally I'd rather have the speedo correct on the street. They don't give speeding tickets at the track ;)
Same here. Where I live there's a cop on every corner and I have the paperwork to prove it. :eek:
 

PeteInCT

#LS-378 - So many Porsche's, so little time....
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Connecticut
I don't push it on the street at all actually, and it really only goes to my buddies shop or a Sunoco station. Good to know TK can adjust for that if I wish to.
 
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PeteInCT said:
Do any of you guys know if there's a way to recalibrate the speedo when you're running with smaller tires/wheels? It's not a big deal but it would be nice if my speedo was accurate after going to 18" wheels. My calculations show that it's off by almost 8 MPH when the speedo reads 160 MPH. My guess is that this is not feasible for too expensive to ever entertain.

I assume you are running 305-35X18 rear tires. My tire calculator says that's 1.68% diameter difference from 285-35x19s and would read 61 MPH at an actual 60 MPH. That would only be about a 2.7 MPH error at 160 MPH. We may be using different calculations. Here's the app I use:
http://www.1010tires.com/tiresizecalculator.asp

At any rate, I don't think there is an easy way to recalibrate the speedo.
 
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PeteInCT said:
I like your numbers better, that means i hit 157+ ;-)
I'm not too worried about my speedo at 160 MPH, there are other car components that would concern me a little more at that speed!
 
PeteInCT said:
Oh, you mean like the rear brake rotor that cracked on my second run ? LOL
A friend of mine had that happen on his '11 GT too. I have 3 days on the last set of DTC60 rears and they have hardly worn at all. I have to believe this excessive rear rotor/pad wear is happening because of a less than ideal pairing of front and rear pad compounds. This is another reason why I just stick with my DTC 70/60 setup. It works and the bias front to rear seems to be a very good match.
 
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PeteInCT said:
Oh, you mean like the rear brake rotor that cracked on my second run ? LOL
Yeah, that would concern me more than a speedo. One time about 12 years ago in my Porsche I was entering a very fast (90+) downhill right hand kink ("The Chute") at Summit Point when the left stub axle snapped because the axle bolt was loose. The only thing that kept the left rear wheel on the car was the caliper around the rotor. I went down the track sideways to the bottom of the hill and stopped (right side up, fortunately) off track at the bottom of the hill. After that I invested in a 6 foot long torque wrench to apply the 390 ft/lbs required to the axle bolts and put paint marks on the nuts to make sure they did not move.
 
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cloud9 said:
PeteInCT said:
Oh, you mean like the rear brake rotor that cracked on my second run ? LOL
A friend of mine had that happen on his '11 GT too. I have 3 days on the last set of DTC60 rears and they have hardly worn at all. I have to believe this excessive rear rotor/pad wear is happening because of a less than ideal pairing of front and rear pad compounds. This is another reason why I just stick with my DTC 70/60 setup. It works and the bias front to rear seems to be a very good match.
So far can't complain about the DTC70/DTC60 combo. However, a couple of weekends ago I was working a combined SCCA/PCA race at Laguna Seca. I found out a lot of the Miata guys (there must have been 80 of them there!) found out that the hot setup for them was DTC60s front and DTC70s rear. Of course it's a little hard to compare a Miata to a Mustang but that was interesting.
 
ChuckP said:
cloud9 said:
PeteInCT said:
Oh, you mean like the rear brake rotor that cracked on my second run ? LOL
A friend of mine had that happen on his '11 GT too. I have 3 days on the last set of DTC60 rears and they have hardly worn at all. I have to believe this excessive rear rotor/pad wear is happening because of a less than ideal pairing of front and rear pad compounds. This is another reason why I just stick with my DTC 70/60 setup. It works and the bias front to rear seems to be a very good match.
So far can't complain about the DTC70/DTC60 combo. However, a couple of weekends ago I was working a combined SCCA/PCA race at Laguna Seca. I found out a lot of the Miata guys (there must have been 80 of them there!) found out that the hot setup for them was DTC60s front and DTC70s rear. Of course it's a little hard to compare a Miata to a Mustang but that was interesting.
That is interesting. They don't make a DTC70 for the rears on our cars. Of course with most of the braking being done with the fronts on our cars, I don't know why you'd go the other direrction.
 

PeteInCT

#LS-378 - So many Porsche's, so little time....
Moderator
2,848
13
Connecticut
cloud9 said:
PeteInCT said:
Oh, you mean like the rear brake rotor that cracked on my second run ? LOL
A friend of mine had that happen on his '11 GT too. I have 3 days on the last set of DTC60 rears and they have hardly worn at all. I have to believe this excessive rear rotor/pad wear is happening because of a less than ideal pairing of front and rear pad compounds. This is another reason why I just stick with my DTC 70/60 setup. It works and the bias front to rear seems to be a very good match.

I guess anytings possible, but the RS-29's I have up front are doing a heck of a lot of the stopping - if I was braking mostly with the rears I'd feel that easily. I also do not need much pedal pressure in the brake zone even at very high speeds, so again I can see reasons why the DTC-60's are those rotors other than that's just what they do based on their compound. Keep in mind I had 6 track days on those pads/rotors. With the Pagids I expect to get a lot more track days out of both th epads and the rotors. I have DBA 4000's on the rear with the new RS-55 compound in the rears. Up front I still have the stock rotors and the RS-29 compound and after 8 track days they still have a heck of a lot of life left in them.
 
Hopefully the new setup will work better and be easier on your rears. It just seemed rather quick to crack a rear rotor. I've typically gotten at least 15-20 days out of rear rotors, but it's been a long time since I've used the stockers. Maybe the DBA 4000s just hold up a lot better than the stock rotors.
 

PeteInCT

#LS-378 - So many Porsche's, so little time....
Moderator
2,848
13
Connecticut
I do think the fact that I was on stock rotors has a lot to do with it. My experiences then go to cryo treated rotors ast at least twice as long. However my front rotors are also stock. I think the dba 4000 I have in the new years now are cryo treated. The coleman rotors should be here in a couple of weeks and yhose will be treated.
 

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