switched to 3:90 Diff and speedometer is off - Miata Turbo Forum - Boost cars, acquire cats.

Welcome to Miataturbo.net   Members
 


Suspension, Brakes, Drivetrain discuss the wondrous effects of boost and your miata...

Reply
 
 
 
LinkBack Thread Tools Search this Thread
Old 06-20-2009, 01:59 PM   #1
Newb
Thread Starter
 
Join Date: Dec 2007
Posts: 36
Total Cats: 0
Default switched to 3:90 Diff and speedometer is off

I switched to a 3:90 diff for my 01 turbo Miata 5 speed and the speedometer seems to be off. It seems that I am going faster than the speedometer indicates.

Does anyone know a fix?

Is there a formula to figure out how fast I am actually going?

Thank you,

Greg
greg9ball is offline   Reply With Quote
Old 06-20-2009, 02:09 PM   #2
Boost Czar
iTrader: (61)
 
Braineack's Avatar
 
Join Date: May 2005
Location: Chantilly, VA
Posts: 72,824
Total Cats: 1,781
Default

10%

simple math can easily figure out how I came up with that number.
Braineack is online now   Reply With Quote
Old 06-20-2009, 03:19 PM   #3
Tour de Franzia
iTrader: (6)
 
hustler's Avatar
 
Join Date: Jun 2006
Location: Republic of Dallas
Posts: 29,114
Total Cats: 351
Default

yeah, be a man.
hustler is offline   Reply With Quote
Old 06-20-2009, 09:04 PM   #4
y8s
2 Props,3 Dildos,& 1 Cat
iTrader: (8)
 
y8s's Avatar
 
Join Date: Jun 2005
Location: Fake Virginia
Posts: 19,038
Total Cats: 406
Default

the speedo drive gear you have doesn't know you have a new diff. you need the new speedo drive gear that matches that diff. simple.
y8s is offline   Reply With Quote
Old 06-20-2009, 10:19 PM   #5
Junior Member
 
Join Date: Jan 2009
Location: Ashe County NC
Posts: 217
Total Cats: -1
Default

Or just use a GPS.. Thats how I check my speedometer, almost every vehicle I use it in is off 3 or 4 MPH high or low..
adamhershner is offline   Reply With Quote
Old 06-20-2009, 10:31 PM   #6
Boost Czar
iTrader: (61)
 
Braineack's Avatar
 
Join Date: May 2005
Location: Chantilly, VA
Posts: 72,824
Total Cats: 1,781
Default

interesting enough, with my 4.1 rear and taller tires, mine is spot on to my gps....
Braineack is online now   Reply With Quote
Old 06-20-2009, 10:38 PM   #7
Elite Member
iTrader: (17)
 
Join Date: Aug 2007
Location: Fort Myers, FL
Posts: 2,790
Total Cats: 128
Default

Quote:
Originally Posted by Braineack View Post
interesting enough, with my 4.1 rear and taller tires, mine is spot on to my gps....
Going from 4.30s to 4.10s put my '90 dead on as well, it read a little high stock.
pdexta is offline   Reply With Quote
Old 06-22-2009, 09:47 AM   #8
Junior Member
iTrader: (4)
 
Join Date: Jan 2008
Location: Chicago
Posts: 88
Total Cats: 0
Default

Pdexta has me confused. his speedo was reading high stock, he went from a 4.30 to a 4.10 and his speedo reading went down! Maybe its me that is confused, but his high reading speedo should be reading even higher, making the error worse not better.

Graham
FastG is offline   Reply With Quote
Old 06-22-2009, 10:26 AM   #9
Boost Czar
iTrader: (61)
 
Braineack's Avatar
 
Join Date: May 2005
Location: Chantilly, VA
Posts: 72,824
Total Cats: 1,781
Default

Quote:
Originally Posted by pdexta View Post
Going from 4.30s to 4.10s put my '90 dead on as well, it read a little high stock.
my question is, if my speedo is spot on, do i have to add in a correction value to my odometer to calculate mpg? If not I get fail for MPG.
Braineack is online now   Reply With Quote
Old 06-22-2009, 11:10 AM   #10
Senior Member
iTrader: (3)
 
Join Date: Aug 2005
Location: Gig Harbor, WA
Posts: 504
Total Cats: 3
Default

Quote:
Originally Posted by Braineack View Post
my question is, if my speedo is spot on, do i have to add in a correction value to my odometer to calculate mpg? If not I get fail for MPG.
Unfortunately not. When I went from 4.3->4.1 I checked the odometer with several mileage check sections on the highway and they were all really close. You could always verify the distance with gps as well.
TurboRoach is offline   Reply With Quote
Old 06-22-2009, 12:16 PM   #11
Moderator
iTrader: (11)
 
sixshooter's Avatar
 
Join Date: Nov 2008
Location: Tampa, Florida
Posts: 15,647
Total Cats: 1,559
Default

Quote:
Originally Posted by Braineack View Post
my question is, if my speedo is spot on, do i have to add in a correction value to my odometer to calculate mpg? If not I get fail for MPG.
If your speedo is spot-on then you are de facto calculating the number of tire rotations per mile accurately. Your odometer is likewise reading correctly. And if one was off, both would be off by the same measure.
sixshooter is online now   Reply With Quote
Old 06-22-2009, 12:46 PM   #12
Elite Member
iTrader: (17)
 
Join Date: Aug 2007
Location: Fort Myers, FL
Posts: 2,790
Total Cats: 128
Default

Quote:
Originally Posted by FastG View Post
Pdexta has me confused. his speedo was reading high stock, he went from a 4.30 to a 4.10 and his speedo reading went down! Maybe its me that is confused, but his high reading speedo should be reading even higher, making the error worse not better.

Graham
We both went with taller gears. For you, if 4300rpms = 80mph actual speed, now 3900rpms = 80mph actual speed. Your speedometer still tells you 3900rpms is 72mph (when you are traveling 80mph), so it is reading low. In my case I was reading around 5% high before the swap, after the swap it made the reading accurate.

*Pretty sure I've got this right, but I've been wrong before*
pdexta is offline   Reply With Quote
Old 06-22-2009, 04:44 PM   #13
Elite Member
iTrader: (13)
 
Join Date: Nov 2006
Location: Tigard, Oregon
Posts: 3,908
Total Cats: 0
Default

Driving impressions with the new diff?
cueball1 is offline   Reply With Quote
Old 06-24-2009, 01:50 PM   #14
Newb
Thread Starter
 
Join Date: Dec 2007
Posts: 36
Total Cats: 0
Default

Quote:
Originally Posted by cueball1 View Post
Driving impressions with the new diff?
I really like the new diff. 1st gear is much more usable and the car revs lower on the highways. I did the swap primarily for the track. I will be going to VIR on 7/2, which will be my first time with this turbo Miata. I can't wait to see how it does.

My last turbo Miata, a silver 1991, caught on fire on the track at VIR last december. I'm hoping for a bit better results this time around!

I'm looking for a higher top on the back straight at VIR. With my 1991 turbo Miata, I was redlining around 122 or so in 5th. I want more!

I see we have something in common other than Miatas. I own Pool Cues and Billiard Supplies at Discount Prices.

Are you going to the BCA? If so, let me know. Perhaps we can meet for a drink.

Greg Savoie
greg9ball is offline   Reply With Quote
Old 06-24-2009, 01:51 PM   #15
Newb
Thread Starter
 
Join Date: Dec 2007
Posts: 36
Total Cats: 0
Default

I meant to write that I own billiardWarehouse.com. I didn't mean to turn it into a link. Sorry about that.
greg9ball is offline   Reply With Quote
Old 06-24-2009, 05:17 PM   #16
Junior Member
iTrader: (4)
 
Join Date: Jan 2008
Location: Chicago
Posts: 88
Total Cats: 0
Default

I also swapped from a 4:30 to a 3:91 in a 1990 and love it. Better on the highway, and around town, 4th is a useful gear now. I think it helped at an AutoX I did last weekend, 1st gear just goes a lot further, I spend far less time searching for the correct gear.

Graham
FastG is offline   Reply With Quote
Old 07-03-2009, 03:11 PM   #17
Elite Member
iTrader: (13)
 
Join Date: Nov 2006
Location: Tigard, Oregon
Posts: 3,908
Total Cats: 0
Default

Damn Greg, I didn't make it back to this thread before leaving for the BCA. Didn't see your responses. Wish I had! I was there Thursday and Friday. Sorry we didn't hook up.
cueball1 is offline   Reply With Quote
Old 07-03-2009, 09:08 PM   #18
Elite Member
iTrader: (9)
 
Join Date: Aug 2006
Location: Concord, North Carolina
Posts: 4,172
Total Cats: 5
Default

Quote:
Originally Posted by pdexta View Post
Going from 4.30s to 4.10s put my '90 dead on as well, it read a little high stock.
Same here.
miatamania is offline   Reply With Quote
Old 07-14-2009, 01:38 AM   #19
Supporting Vendor
 
MiataRoadster's Avatar
 
Join Date: Dec 2006
Location: SoCal
Posts: 271
Total Cats: 4
Default

Another option is a Yellow Box:

MiataYellowBox.com
MiataRoadster is offline   Reply With Quote
Old 07-14-2009, 10:56 AM   #20
Junior Member
 
Join Date: Aug 2008
Location: MD/DE/PA
Posts: 337
Total Cats: 0
Default

Quote:
Originally Posted by sixshooter View Post
If your speedo is spot-on then you are de facto calculating the number of tire rotations per mile accurately. Your odometer is likewise reading correctly. And if one was off, both would be off by the same measure.
No. In many cars (and, FWIW, motorcycles) the odo tends to read dead on while the speedo is a bit fast. I assume the reasoning for this is that if the speedos were to read slow and people got tickets, they'd wind up suing the manufacturer for having faulty equipment, so they just err on the side of having them read fast. Likewise, imagine the class-action suits if people weren't getting their full, advertised warranties because the odos were reading fast. Just because the two instruments take the reading from the same spot on the trans doesn't mean that they are necessarily calibrated equally and giving the same readout on the other end.

Speedos in Miatas are long noted for reading high, this is easily checked instantaneously via GPS. What's more difficult is the odometer, which is best checked over a longer distance to expose minute error that presents itself on a larger scale over longer distances, and may not be obvious using mile markers on the freeway.
the_man is offline   Reply With Quote
 
 
Reply

Related Topics
Thread Thread Starter Forum Replies Last Post
91 1.6l turbo complete part out. Tony the Tiger Miata parts for sale/trade 63 12-29-2016 03:23 AM
Another Alternator thread ihiryu General Miata Chat 9 09-28-2015 11:22 AM
mm0105 TABLE SWITCHING WIRING? slomiata MEGAsquirt 2 09-15-2015 01:54 PM
Noob from Orlando Marcdasharq Meet and Greet 2 09-11-2015 01:21 AM


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



All times are GMT -4. The time now is 03:23 PM.