Larger wheels/tires, possibly speed sensor swap?

[MrMazda92]

I'm hoping to run MazdaSpeed6 brakes on my '92 this coming year, and the only thing holding me back at this point is the thought of riding around on 17" wheels with low profile tires... I'd rather not dent expensive wheels, and the car is bumpy enough with 15s.

My thinking is... Why not use a 17" wheel, with say... a 215/50-17 tire. It'd have a TINY bit less sidewall than our stock size, and throw the speedo off by ~6.6%(according to miata.net's tire size calculator).

That being the case... What exactly would it take to either recalibrate, or replace the speedometer sensor in the transmission? Would it be as simple as finding a vehicle with that exact tire size as OEM equip, and making that work, or would an aftermarket option be simpler?

[MrMazda92]

The only issue I can see at all, will be ride height adjustment.

I won't be able to drop my car as low as many members do... However, I never intended to in the first place!!
This has me kind of excited, not gonna lie. I will likely need to roll my fenders, and run stiff settings on my coilovers though...

Edit:
Heavy a-- wheels will be another issue for me, hmm.

[MrMazda92]

Epic excel document w/ wheel weights, listed by Brand/Size/Weight/Manufacturing Process on this page:
http://www.wheelweights.net/

[Nd4SpdSe]

http://www.dakotadigital.com/index.cfm/ ... prd126.htm

[MrMazda92]

Yeah, that'd work... It's a fair bit more costly than my idea though.

Our speedometer sensors are calibrated specifically for our stock 15" wheel with the 205/55-15 tire size, and I know very well that the 2 tire sizes I'm considering(215/45-17 and 215/50-17) are OEM spec on a few other vehicles out there.

The GS has an electronic speedo, IIRC, whereas the RS(92-93?) had a cable driven one... It's safe to assume that the sensor itself is calibrated for the OEM wheel/tire size combo, and that the cluster simply receives a signal based on those readings. Simply enough, I'm wondering if anybody has had success in the past swapping other OEM speedometer sensors into the G series transmission?

How does the speedometer sensor work in the G series transmission anyways? I haven't put much thought into transmissions in the past... What part of the transmission is the sensor actually monitoring? Please forgive my ignorance on the topic!

[Daninski]

I think the 215/45-17 would be perrrrrrfect. Nice chunkey look without lookin like your goin 4 X 4in. My car has a 1.5 inch drop and those are the next size tire I'll be buyin.

PS Screw the speedo, just adjust your speed accordingly.

[Nd4SpdSe]

Our speedometer sensors are calibrated specifically for our stock 15" wheel with the 205/55-15 tire size

Speedometers do the calculation based on the circumference of the wheel. The rim is not part of the calculation, but the tire mounts to the rim, and tire options change with the rim size. Changing the rim doesn't change anything, but you need to make sure the new tire has the same circumference that the old one did on the new rim.

You gotta know about this:
http://www.miata.net/garage/tirecalc.html

Going on a 17, 205/45 is 1.6% bigger, 205/40 is 1.8% smaller, 215/40 is 0.4% smaller. These are negeligable changes to your speedometer that would change with natural tire wear from new to bald.

I'm wondering if anybody has had success in the past swapping other OEM speedometer sensors into the G series transmission? How does the speedometer sensor work in the G series transmission anyways? I haven't put much thought into transmissions in the past... What part of the transmission is the sensor actually monitoring?

Trying to do something and especially, change something, it helps to understand how it works.

Speed is calculated on the gear on the differential,



and the speedo gear driven by it,

[Daninski]

Who cares, do the math and enjoy your tires. When your doing 60mph your really doing 63,,,,,great.

[MrMazda92]

Dan,
I agree but it'd be a little less cushion on western Oregon's terrible roads...

Nd4SpdSe,
I know the rim isn't taken into consideration, as the tire determines the outside circumference and diameter... My issue is that I want to go to a 17" wheel, without giving up the protection offered by a "taller" sidewall. Hence the speedometer calibration issues...

The entire point is to have a larger outside diameter, so I can fit larger brakes without denting $600+ worth of wheels on the terrible local roads. I WANT to change the outside circumference of the tire, and have an accurate speedometer while I'm at it. Call me crazy, but I prefer immediate feedback to calculating the discrepancy between given speed and actual speed. I also mentioned that calculator in my original post, as well as the 6.6% difference when using 215/50-17 tires.

Thank you very much for the speedo gear images, that helps a lot! Seeing that, I imagine it'd be far simpler(and likely cheaper) to go with your original suggestion, although I'd still prefer to avoid adding more electronics, that is likely the route I'll go with this.

[RobMinhas]

Not sure if this:
http://www.youtube.com/watch?v=bMs7w72QTVM

Will work for ya but it's a thought

[MrMazda92]

Ding Ding Ding!!!!

http://forum.mazda6club.com/speed6-susp ... ost3473875

16x8 RX7 FD wheels... Which happen to be VERY affordable, and VERY light for 16x8s! Minor modification on the caliper required... Boohoo, reduced un-sprung mass as well as clearing 16" wheels? Hell yes.

RobMinhas,
Thank you for that, I'm looking into that as an option now.

[Josh]

17's are not that bad... Just don't buy some cheep a-- ebay wheel and they wont bend or break. I would run a 215/45/17 that is what I run on my RS. And I am going to run 18's on it soon enough. 16's are ideal IMO but nothing wrong at all with 17's. BTW my RS is so much harsher than your car. I am running stiffer spring rates than your GC's and Koni inserts adjusted to full. it is kind of a kidney killer. I think you will be fine.

[crazycanadian]

http://www.dakotadigital.com/index.cfm/ ... prd126.htm

Not to sure if that will work, but this is the idea of what you need.. Every G series transmission I have pulled apart had the same tooth count on both the diff gear and the speedo gear.. I haven't seen any different tooth counts yet..

I would message them an ask if it work for an A/C sign wave signal though... That set up looks like its for a digital square wave signal... Our sensors produce and A/C sign wave and sent that directly to the cluster...

[Nd4SpdSe]

I would message them an ask if it work for an A/C sign wave signal though... That set up looks like its for a digital square wave signal... Our sensors produce and A/C sign wave and sent that directly to the cluster...

The Dakota Digital SGI-5 is designed to recalibrate a speedometer signal or correct sinewave(AC)/squarewave(‘oc’) signal incompatibilities.

APPLICATION #1
Recalibrate a high speed (64,000ppm – 250,000ppm) signal for an OEM speedometer or engine/transmission computer. Do not use this unit to adjust a signal going to an anti-lock braking system. Anti-lock braking systems may not operate correctly or behave erratically due to the signal processing done to recalibrate the speed signal.

These speed sensors have a two-pin connector that plugs into the transmission or transfer case. One of the wires will be a ground and the other will be the signal wire. The wires will usually go up under the dash and into the speedometer, vehicle speed buffer, or engine/transmission computer. The signal wire will need to be cut so the SGI-5 can recalibrate it. The sensor side of the wire will go to the SIG IN terminal. The speedometer or buffer side will go to the OUT1 terminal. If the speedometer does not operate correctly after installation of the SGI-5 you may need to switch to OUT2 instead of OUT1. Connect the PWR terminal to a 12-volt accessory wire and connect the GND terminal to a good ground location.

You can determine how far the speedometer is off by having it checked with radar or following another vehicle going at a set speed. Once you know how far it is off at a certain speed, you can use the push button switches to adjust the speedometer while you drive or use the following equation and then look up the calibration setting in the table.

Actual speed
------------------------------- x current Cal ratio = new Cal ratio
speedometer reading