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

10%

simple math can easily figure out how I came up with that number.

yeah, be a man.

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.

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..

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 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

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.

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.

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*

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

I meant to write that I own billiardWarehouse.com. I didn't mean to turn it into a link. Sorry about that.

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

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.

Same here.

Another option is a Yellow Box:

MiataYellowBox.com

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.