revs per mile?

bgrd351

Member

Jun 10, 2005

109

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17

Jacksonville, Fl

Nov 29, 2006

#1

• Nov 29, 2006

• #1

I just replaced my Nitto's with sumitomo tires in the same 315/35/17 size but now my speedo is way off. It reads 10-15 mph too high. So I went to the website to get the RPM and put them in my Xcal2 and tried it again...even WORSE?!?!?! So right now I have the stock value of 811 RPM in the car and I am still about 10 mph off.

So anyone that is running these tires...what number did you put into the car to fix the speedo?

note: the Nittos were dead on with 806 RPM (verified by a cop ), just like their site says

 

hotmustang331

Active Member

Apr 29, 2004

2,967

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48

Bastrop,TX

Nov 29, 2006

#2

• Nov 29, 2006

• #2

They are the same zize as the nittos +/- 1MPH difference at BEST. There is something else wrong, or you dont have the tires listed.

 

bgrd351

Member

Jun 10, 2005

109

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17

Jacksonville, Fl

Nov 29, 2006

#3

• Nov 29, 2006

• #3

I just thought of something....do our cars read speed from the transmission like most cars or the front wheels? Reason I ask is that my front tires are of a different size and this might explain it. hmmmm.

But I do have the tires listed and it is strange that this is happening.

 

Matt's 03 GT

Member

Aug 28, 2006

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17

Orange County

Nov 29, 2006

#4

• Nov 29, 2006

• #4

There is something else wrong. If the tires are replaced with the same size as you say the diameter is the same and your spedo should be the same. The things that can make your spedo read wrong are Different Wheel size (going from 17" rims to 18"), changing the rear gears, worn spedo gear??, etc. I see you have 3.73's... maybe this happened when you put the gears in and just noticed now???

Nothing you did in changing brands of tires should make a difference of +- 1% as stated before.

 

bgrd351

Member

Jun 10, 2005

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Jacksonville, Fl

Nov 29, 2006

#5

• Nov 29, 2006

• #5

well, I got the spedo checked after the 255 Ft 315 Bk Nitto's and 3.73's by getting a ticket

The only thing I have changed is the tires 275 Ft 315 Bk Sumo's. The RPM for Nitto's is 806 and according to the Sumo site their's is 787.

 

jstreet0204

Active Member

Jun 26, 2003

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Winston Salem, NC

Nov 29, 2006

#6

• Nov 29, 2006

• #6

bgrd351 said:

well, I got the spedo checked after the 255 Ft 315 Bk Nitto's and 3.73's by getting a ticket

The only thing I have changed is the tires 275 Ft 315 Bk Sumo's. The RPM for Nitto's is 806 and according to the Sumo site their's is 787.

Click to expand...

Just measure the diameter of your tire, then use this formula to get the rpm where ht = your tire height.
rpm = 63360/(3.14*ht)

Make sure your gear ratio is applied in the tune also.

 

Kurgan

Founding Member

Jan 31, 2001

3,028

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58

Palm Beach Gardens, FL

Nov 29, 2006

#7

• Nov 29, 2006

• #7

Vehicle speed sensor measures speed from the transmission output shaft.

315/35/17 tires are ~25.68 inches in diameter which should be the same as 245/45/17 (~25.68") and 275/40/17 (~25.66") tires. Your speedo should not have changed because of this tire if you were using one of the other two tires before.

jstreet0204 posted the correct formula for determining the number of revolutions per mile.

25.68" diameter calculates out to 785.3289981152328905339214204537 revolutions per mile.

 

bgrd351

Member

Jun 10, 2005

109

0

17

Jacksonville, Fl

Nov 29, 2006

#8

• Nov 29, 2006

• #8

ok...here is what Sumo has on their site for info:

P315/35ZR17 --- ht:25.7 rpm:787

my measurement just gave me ht:25 3/16 (25.1875) rpm:801.125 (63360 / (3.14 * 25.1875) = 801.125318)

I will give this a try on the way home tonight. Also, I have the right gear ratio under scalars>axle>axle_ratio

thanks for the help

 

bgrd351

Member

Jun 10, 2005

109

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17

Jacksonville, Fl

Nov 29, 2006

#9

• Nov 29, 2006

• #9

ok...I was just looking at my SCT Pro software and found that it has the trans gear ratios at (2.86,1.91,1.41,1.10,.87) which seem way off!!

my searching I have found these (3.38,2,1.32,1,.68) which when plugged into EA equal up to what I normally see driving. I dont understand what happened. hmmm.

so what are the correct ratios for the t3650?

 

hotmustang331

Active Member

Apr 29, 2004

2,967

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48

Bastrop,TX

Nov 29, 2006

#10

• Nov 29, 2006

• #10

5th gear is eithe .62 or .68. But that depends on which site you go to. I PERSONALLY believe its .62, as I know the T-45 tranny is .68 5th and the T-3650's 5th is higher.

The rest you stated sounds correct for the gears. The SCT tunes gear setting were ALL off. So yea reset all the gear ratios to what you have + .62 for 5th.

 

bgrd351

Member

Jun 10, 2005

109

0

17

Jacksonville, Fl

Nov 29, 2006

#11

• Nov 29, 2006

• #11

update:

1. I made a new tune in the software when I got my new tires and saw that they are of a different size plus I wanted to start from scratch...one problem...I forgot to to make the EEC use my settings for axle/tire instead of oem...DUH!!

2. I found that by going to several places on Ebay and such that the correct 5th is .68 ...at least that is what I found so far.

Outcome --with the gears put in and the EEC using my settings and using Sumo's 787 for rpm my car seemed to be dead on for speed.

Thanks you for all the help and if anyone can figure out for sure what the O/D ratio is for the t3650 for 2k2 that would be great!