rear end changed gearing ratio?
 

I just bought a 1986 mk III Supra. I have put a lot of work into it and have discovered it pulled 3300 rpm doing 75 miles per hour. The speedometer is reading low for some reason. Is it possible the rear differential was changed somehow, my Toyota dealer is not sure and I have to wait until Monday for them to look at it.

Hi, you can call me Ranma. My picture looks nothing like me but it symbolizes the spirt of my Supra. Am too embarrassed to post my own picture. I had a 86 mk III in Japan but could not bring it to America for some stupid reason only a government official could think of. They have to justify their salaries somehow.

Oh I need a number of the small plastic parts like the covers for the door handles, etc. Any suggestions? One nice thing all of the accessories still work unlike my old Firebird.

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

http://www.toyotaparts123.com/partlo...5450&year=1992

store to buy OEM parts.

spirit of the supra hu. cool. good luck with your build. welcome to the board.

www.toyotaworld.com (Champion Toyota) out of Texas is a favorite of mine... beat some of my local dealer's shop prices.... of course the dealers here really suck.

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 :confused: 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

Thank You for your help CRE!

I will check the speedometer

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.

i get what your saying but it says my car at fuel cut will do just at 86 i was like 500 rpm off fuel cut at 87 and my tires are almost 1 inch shorter than stock that means im revving higher to go the same speed so it just doesnt make any sense to me... but its not that important

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.

yeah my speedo is a constant 2mph fast according to a cop friend with radar equipment and it'll be on the dyno this week to make sure i dont think anything has been changed since its a constant 2mph and thats about right for my tire size idk maybe the track reads fast though my speedo was right with the track everytime i looked and it was really consistent and i know how all that works ive had to adjust speedos and what not before and ive had the tach hooked up and it reads right with my tach dwell not a bit off idk but like i said before its not that important it just seemed strange to me to go over the table speed with smaller tires