Speedo inaccuracy- 1JZ/R154 in NA chassis

[Dingoboy]

Hey guys... I haven't given this a tremendous amount of thought, but after doing several searches, I am having a hard time finding the exact info I need.

I just bought a 1992 MKIII 1JZ-GTE with the R154 tranny & turbo 3.73 LSD. The entire dash is still from an NA car. The speedo currently reads about 15% slower than actual speed.

Is it safe to assume that with the turbo tranny & LSD, the issue here is probably the NA speedo being geared to work with an entirely different driveline? (I am thinking the speedos are mechanical in MKIII's)

I have never removed gauges from a Supra, so I'm a little reticent to jump right in, but if slapping in a turbo speedo is likely to fix the problem, I can consider it. Only issue is that this car has a set of white face gauges with some sort of switchable green/blue "indiglo" type lighting, and I'd hate to mess that up. (The lighting controller says "TRD" on it) Can a stock turbo speedo simply be fitted with the white gauge face, or are these gauges built in a different way entirely?

Suggestions?

 

[Jeff Lange]

All speedometers are calibrated the same. All changes are made at the transmission end. What year/engine is your R154 from? You may be able to just change the driven gear. If you have the wrong drive gear, you may have to change that too.

Just need to find out where your R154 came from to figure out what is in it.

(PS: Is your odometer off the same amount as your speedometer seems to be?)

Jeff

 

[Dingoboy]

All speedometers are calibrated the same. All changes are made at the transmission end. What year/engine is your R154 from? You may be able to just change the driven gear. If you have the wrong drive gear, you may have to change that too.

Just need to find out where your R154 came from to figure out what is in it.

(PS: Is your odometer off the same amount as your speedometer seems to be?)

Jeff

Very glad I asked... did not know the speedos were the same. Yes, the odo seems to be off by approximately the same amount as the speedo.

Compared both to measurements made on Garmin GPS during the 900 mile trip from KY to FL on Friday- compared constant speed on Garmin to speedo, and compared trip miles on Garmin to odo, resetting both at each fill-up, range was 13.0-13.15%. At times, the speedo seems to have been off by a larger margin than that, but without hash marks, and with the Garmin speed not being instantaneously correct, it's hard to be certain.

Not certain where the transmission came from. Will have to contact the previous owner to see if he knows. Thanks for the info!

If all else fails, I suppose I can take it to a shop. There's one not far from me that specializes in speedo repair/adjustment.

 

[foreverpsycotic]

What is your tire size?
The only way I can see this happening is if you have a larger rear tire (27"-28"). If this is the case, you should be closer to your actual speed with a 3.91 speedo gear.

 

[gofastgeorge]

I doubt if any of the 1JZs came with the 3.7 rear end,
so if the trany came with the 1JZ, most likely it has the wrong speedo drive gear in it.
All you have to do is go get the speedo gear meant to be used with the 3.7 rear end.
(late mk3 turbo?)
I had to do this when I went from 3.9 to 4.56 gears.
Unfortunately, unlike a 3.7 speedo gear, the 4.56 was never sold over here,
so I had to order it out of Japan.

(EDIT)
You might be better off with the lower rear end ratio meant for for the 1JZ.
They (1JZs) are not knowen for having huge amounts of torque.

 

[Zumtizzle]

1JZ = 4.10 Rear Diff.

I have 1JZ trans as well; i had to purchase the speedo gear. Cost is like 30 bucks. IIRR

 

[Dingoboy]

Thanks for the additional info, guys. Much appreciated.