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Old 04-14-2011, 11:47 PM   #1
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Default speedo problem

Iv got a 87 MKIII with a chevy 350 engine, and th-350 trans. im trying to figure out how to run a speedo in this car. if i can use the original or if i have to use a chevy style.
I dont want to get in a debate about the swap. just looking for some advice.
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Old 04-15-2011, 03:57 PM   #2
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one way to find out is to take a drill and spin it at max rpm on this speedo and then do the same with a chevy speedo and see if they show the same reading if not then your gonna need a chevy speedo/ aftermarket
but the supra speedo/speeo cable are pretty notorious to for lack of a better word suck, so you may stray from usng them
you also have to consider your gearing though
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Old 04-15-2011, 06:48 PM   #3
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Still do not understand your thinking. The 350 engine stock does not put out that much horsepower. The figure I found was 180 horsepower vs 190 to 204. It also weighs more 575 lbs vs 440 lbs. Then the transmission weighs 125 lbs vs 77 lbs for my W58.
Wonders if the weight difference causes stability problems?
So really you have a government motors power train and I would think that only their speedo would work. About the one in my supra , well there is the skinny! Saying it is a "little off" is being kind.
Have fun dude and be careful driving that Supra of yours!
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Old 04-15-2011, 11:20 PM   #4
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I wouldn't dis you for your choice. A SBC is capable of a lot more performance than the smog-choked stocker. I've seen them punch out over 400 HP N/A with the right mods.

The trans speedo gear simply spins the speedo cable. The trick is to get the speedo cable to connect solidly to the speedo gear of the transmission. Since the Supra uses a two-piece system, you just need to get the short adapter piece modified to fit the the 350-TH

Then, you just need to figure out the proper ratio to get the speedo gear sync'ed to the supra speedo. If you can get a drill hooked up, like 'turbonicsperformance' suggested, you can determine the ratio difference between the chevy speedo and Supra speedo. Use that and the rear end gear ratio to figure the proper speedo gear set for the 350-TH and your good to go.
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Old 04-16-2011, 10:45 AM   #5
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had alot of sbc's, including my old maro with a 375 hp 360 cubic incher with a wct5 and 3.73 posi. one in a supra would be fun with all the low end torque, move the heavy chassis alot easier from a stop.

neway, you will prolly have to get aftermarket gauges and use the stock th350 cable. or see if you can find the toyota speedo gear sizes (contact dealerships) and then find corresponding chevy ones to match. that would be alot of work and research, for possible wrong end results. a full set of autometer gauges is only about 200 bux, and you could calibrate them to whatever application you have, usually.
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Old 04-17-2011, 12:17 AM   #6

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You're not going to get an accurate speed by just running the cable to the Chevy tranny... Find a shop (local or online) which specializes in speedometers (yes they do exist) and get an adapter made... It's a little gear box which you run a short cable from your tranny into and out of it you run the Toyota tranny's cable. They should have the ratios for each, if not the RPM for Toyota speedos is the same for all Toyotas of this vintage and then just get them the end gear ratio that you're running and the tooth count off the driven gear.

Good luck and have fun!
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