Speedometor / Ma70 Gear ratio

[cre]

There were three different gear ratios used for the MA70's 4.30:1 3.91:1 and 3.73:1. All three were available in LSD, the latter two were ONLY available in LSD and the 4.30:1 was ONLY available in the non turbo. If you spin one wheel with the back end jacked up and the other side moves in the OPPOSITE direction it's an open LSD and thus it's 4.30:1. If both move the same way it's an LSD. If it's an LSD you may look at the charts below to easily find which one it probably is. The minor difference in gearing will make for a small offset.. so will the MASSIVE margin of error in the stock spedometer and tachometer. Best way to check is to roll the car along a measured lengthof road and check against the odometer.

75's right for 5th gear in a N/A at about 3300RPM.

W58: (that's the non-turbo transmission)

Attachment 3095

R154's:
Pre89 (3.91:1)
Attachment 3096

89+ (3.73:1)
Attachment 3097

[cre]

Originally Posted by fullthrottlesrqsupra

yep sounds right i did my first highway drive today and at 70 i was right on 3000 i think the chart might be just a little off for 3rd though because i was in 3rd at 87mph not redlining (track reading not my speedo i wasnt looking at it to know) and the chart shows that being impossible my tires are actually 1/4inch or so smaller than stock too hmmmm wonder whats up

or maybe not my car is an 86.5 so its not supposed to be a turbo but my mph to rpm match the pre 89 r154 perfect hmmmmmmmmmmm????? i know it not though because i just did a clutch on it im puzzled

It's simple... first off that chart is for a PERFECTLY dialed in FACTORY, BONE STOCK setup... even a difference in air pressure will start it moving off... second, the factory speedometer SUCKS. It can be calibrated but will not be terribly accurate except for the speed at which it was calibrated. and Third... the stock tachometer SUCKS... miles off. Mine is off by about 150RPM at idle and it gets worse the higher you rev... this could be modified so you could calibrate it, but it's not worth it in the least. Also, if you're running a R154 the numbers will be off too... their gearing is a bit different for each gear.

[cre]

Get it on a dyno and REALLY measure the speed; While you're there get a comparison on the tach readings.

Otherwise, you need to just start measuring things; pull the speedometer drive gear and cross reference the part number and find out what you REALLY have in the car. There are too many variables. If things are so far off your car's had something swapped and possibly more than one part.

You can't base any conclusions entirely on the ECU's response in most circumstances in my opinion either... Leaking capacitors, 5% tolerance resistors... skewed tachometers and speedometers...




There are two different types of offset you will see with the stock speedometer: linear and logarithmic.

I'm not going to cover how it all works... the short and simple of it is that if the speedometer is off by a constant amount regardless of speed it is due to one of the following: tire size, end gear ratio, transmission gearing, speedometer drive gear.

If it is off by a percentage it is the speedometer (the clock/tension spring inside can be used to adjust this to a degree, but it will never be accurate over the entire speedometer's scale... choose the rage where you want it accurate and accept that there will be some shift at other speeds).

If both are off it's not fun to calibrate... best is to start with physically identifying components such as the speedo drive gear and counting wheel revolutions to ID the diff.




I'm not guaranteeing the numbers in that chart are correct, but they're the output from a spreadsheet based on very simple math... if they're off I'd be very surprised, but it is possible.

[cre]

If the speedo's only a constant 2MPH off then your tach's screwy or your igniter is... Roll the car slowly across a measured one mile segment of road and see if the odometer is reading accurately. If it's right on the mark just move the speedo needle to set everything right.

And if your speedo is 2 MPH off at 10MPH and still only 2MPH off at 100MPH feel lucky, the speedo was blessed by god himself.

[cre]

If the speedometer is off but all the gearing is right you can recalibrate the gauge. There is an arm behind the face which adjusts it my percentage and flat out moving the needle is what you'll do fix a constant offset (ie: it's ALWAYS 5 MPH low).

If the gearing does NOT match you may swap out the speedometer driven gear or worst case swap out the diff to make everything right.

You may recalibrate the gauge to make the speed report accurately without changing any gearing (diff or speedo drive gear) BUT the odometer will still be off.

When changing to a different diff the proper course of action is to change the speedometer drive gear for one the appropriate size or to install small gear box which changes rate at which the speedometer cable spins (there are actually shops that do just this).


EDIT: Here's a picture showing the adjustment arm which you may use to calibrate a speedometer which is off by a percentage (ie, you're doing 30 but it reports 33 and then when doing 60 it reports 66).