bmi2

Hello all, I thought I’d make a thought provoking thread to verify my understanding of my project and to hopefully leave a trail for anyone else going the same path I am. I posted this same thread to miata.net, but found more discussion here on the 3.3 mfactory, and figured I'd post it here and hear what you fellas had to say. Thanks!

I have a 96 M edition with 205/50R15 tires on it (2560 turbo, aiming for 250-280 hp once finished). I believe every other aspect of the car that would affect speedometer function is the same (speedo ran about 10% high all last summer). My plan is to swap the 5 speed for a six speed and change the rear end to an MFactory 3.3 gear to get a highway gear while retaining substantially similar 1-5 of the 5 speed.

(MFactory Competition Products - Performance Driveline Components)


Using the formulas from this website (Speedometer Gear Calculator and Charts - TCI® Auto) I calculated I would need a 17.3 tooth gear.

Tire Rev. per Mile*= 873.0735931
6*3.308*873.07/1001 = 17.31145322

Using the information obtained from this thread (https://forum.miata.net/vb/showthread.php?t=653657)
I determined I would need part numbers
17T M501-17-441 (17 tooth gear)
1011-17-442A Sleeve to compensate for smaller gear


And
new seal and an O-ring (1011-17-443 and 9958-60-8166).

From this thread (https://forum.miata.net/vb/showthread.php?t=266427)



Now for the more theoretical part. With the non-stock tires I have (which I believe are a common swap for many), my speedometer read about 10% high. I calculated the necessary toothed speedo gear with the same formula, and came up with 21.456
Tire Rev. per Mile =20168/23.1= 873.0735931


And,

6*4.1*873.07/1001 = 21.456





I noticed that I am running about 2% too few teeth in the stock (5 speed, 4.1, 205/50R15), but 2% too many for my proposed set up. It seems to me this swap, if my understanding and math is correct, will likely correct my speedometer?

Will the part numbers listed be all that I need for the shop to make the six speed run my speedo (fairly) accurately?

[color]If anyone notices any issues with my math, please let me know. I figure this thread may help other daily drivers looking to add a highway gear to their NA. [/color]

[color]Thank you all so much for looking![/color]

Doppelgänger

iTrader: (12)

Funny how math and calculations don't always add up (pun not intended) in the real world. When I sold my '02 and acquired the MSM, part of the deal was to swap the 3.9 from the '02 into the MSM (**** that 4.1+6-spd combination). I figured the speedometer would be a bit off, and that I'd eventually buy the 3.9 speed sensor. Now, we know the MSM comes with 17's, and I'm still running 17's....but much nicers ones. I'm not sure of Mazda's math on the MSM.... meaning I don't think they did any calculations on the 4.1 gear + larger rolling diameter (since all Miatas with a 4.1 are 5-spd and have 15" wheels).

The point? Putting the 3.9 in place of the 4.1 and leaving the 4.1 speed sensor in place has now made my speedometer dead-on accurate. Like all of those "Your speed is------" things match exactly how fast I'm going (which I imagine are calibrated the same as law enforcement). But remember, FMVSS/DOT/Whoever allows for OE speedometers to have a +/- variance of 5%. With that said, and considering the 4.1 sensor/3.9 gear...maybe a 3.6 sensor would make the speedometer accurate?

Apologies for any grammar mistakes....it's 5:30am.

x_25

Argh! THE SIZE OF THE WHEEL HAS NOTHING TO DO WITH OVERALL TIRE DIAMETER.

I keep seeing this everywhere. The 17" wheels and tires (205/40-17) that came on the MSM are only 3.2% bigger than the ones that came stock on a 90! Which are 185/60-14. 22.7" vs 23.4"

Doppelgänger

iTrader: (12)

Tire diameter of a given size does vary from manufacturer to manufacturer...it's small, but it does. I'm just saying that if one is trying to determine a specific output/gear speed, wheel/tire diameter will change the actual speed. Was just using the example that my car has 17"s because the 3.9+6spd with a 205/50-16 is what Mazda likely calibrated the 4.1 speed sensor for...and putting 17"s on the MSM would change the speedometer reading a bit between the two. Perhaps I'm not using the best terminology. One just needs to account for the actual tire diameter being used (as in looking up the specs from the manufacturer) to include in calculating actual speed.

themonkeyman

You're greatly overthinking this.

6sp cars (MSM excepted) came with the 3.9 final drive, and the appropriate speedo drive gear. You are going to a 3.3 final drive, which is roughly ~10% heavier so if the speedo reads roughly 10% high now, you should be right in the ballpark post swap.

x_25

The non MSM 2004s came with a 205/45-16 from the factory. Which is 0.8% smaller than the 205/40-17 that came on the MSM. Your difference lies elsewhere. Putting 17s on will not change the speedometer reading any apriciable amount if you put the correct size tires on.

themonkeyman

Enjoy some cats my friend.

Doppelgänger

iTrader: (12)

Likely...im also brainfried at this point in the day. Depends on the % one is chasing... if it's coming down to a couple of % determining one thing or another, tire diameter difference of even 0.3" can make a difference in the long run. Apologies for any frustration.

Do think it's kinda funny that swapping the 4.1 to a 3.9 and leaving the 4.1 sensor in did make the speedometer accurate haha.

themonkeyman

That was more direct towards the OP, I think you posted your initial reply as I was typing that.

The thing to remember here in terms of chasing single percentage inaccuracies is that even if you can pinpoint the exact percentage of error, we are constrained by available tooth count gears, especially considering we can only change one of the two meshed gears that ultimately sends signal to the dash (we don't change the tailshaft gear drive gear, just the driven "signal" gear). A 17t gear to an 18t is a ~5.5% change, so you have to balance your percentage of error versus percentage change of stepping up or down in tooth counts.

Doppelgänger

iTrader: (12)

No problem dude S'all good!

bmi2

Is there anywhere to buy 18 tooth gears anymore?I called up Mazda and asked about part numbers, but it had been discontinued.

shuiend

iTrader: (24)

Which Mazda part store did you call? I would give Tommy@Priority Mazda Parts a call and talk to him. If there are any left in stock anywhere, he is the guy who can find it.

hks_kansei

My calcs below, all rpm are at 60mph (obviously it's a guide, manufacturing tolerances tyre pressures and all that ****)

stock 195/50/15 + 5speed (in 5th) + oem 4.1 = 2968rpm
205/50/15 + 5speed (5th) + oem 4.1 = 2917rpm
205/50/15 + 5speed (5th) + mtuned 3.308 = 2354rpm
205/50/15 + 6speed (6th) + oem 4.1 = 3021rpm
205/50/15 + 6speed (6th) + Mtuned 3.308 = 2438rpm


You can compare RPM to work out the % change in speed from your current and planned setup.






Also, were US spec 96's mechanical or electronic speedo? (they were mechanical on Aust spec ones, but not sure if Mazda changed to electronic early for the US market with the weird OBD rules you had)
If it's an electronic sender just buy one of the speedo adjuster boxes and use that.

shuiend

iTrader: (24)

All NA's in America are mechanical speedo.

themonkeyman

I've actually got my old speedo sender/drive gear with me to look at maybe designing 3d printable ones. I figure since the gear holder is eccentric, rotating it should allow it to mesh even with a larger/smaller gear on it, just need to figure out how to affix it since the mounting tab is designed for being clocked to the original gear size. Have to test it on the spare 5sp in the basement, both for mounting and gear mesh. I'm looking into it for a similar problem but on the other end of the spectrum; I have a 4.77 R&P from a Kia so my speedo reads way high (in the neighborhood of 20%) so I am prototyping a 24 or 25t gear up from my 21.

Does anyone know if the gauge itself has any calibration dials on the back of it? Might be the simpler solution here for all of us.

sixshooter

iTrader: (12)

The easy button for me was getting rev limiter to make a custom gauge face that was correct for the Speedo gear I already had when I switched to 3.6 gears