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Old 11-08-2011, 06:41 PM   #1
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t5 swap and 373 rear question

ok so ive finally got almost everything to do my swap to a t5 but i have a question bout the speedo. when i do the swap i am gonna swap the rear end to a 373 to cause i got them both out of a 85 iroc. when i do this swap will the speedo be correct cause the t5 has a 21 tooth gear and the charts on here say this is correct but the 85 iroc had a 85 mph speedo and i have a 145 speedo? any help is appreciated!!
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Old 11-08-2011, 06:53 PM   #2
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Re: t5 swap and 373 rear question

The speedometer doesn't matter... 85mph or 145mph they both receive the same signal from the car. What matters is the transmission/rear gears. The speedometer gears in the tail of the transmission need to be properly calibrated for the rear gearing.
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Old 11-08-2011, 07:19 PM   #3
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Re: t5 swap and 373 rear question

right on man thanks for the quick reply the gears r right in the trans i already checked that just the speedo was throwing me off. now i just gotta get the flywheel and im doin the swap, even though im chatching some negative comments from friends for busting up an orignal iroc.
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Old 11-09-2011, 07:08 AM   #4
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Re: t5 swap and 373 rear question

21 tooth sounds about right for the 3.73 ratio. If your tire size is different from the donor car you pulled the trans from, it might throw off the accuracy a bit, but it'll still be pretty close. If you want it to be dead nuts accurate, go to JAM performance's website and use their calculators. They have a tire size calculator and speedo gear calculator side by side on the same webpage. So all you have to do is enter your tire size to get the outside tire diameter, then enter that into the speedo gear calculator along with how many teeth are on the speedo drive gear. Most T5's used a 7 tooth drive gear.
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Old 11-09-2011, 08:55 AM   #5
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Re: t5 swap and 373 rear question

The cars have the same size tires 245/50/16 so it all should be ok, it was all out of a 85 iroc. I checked the rpm difference between the 700r an the t5 an shouldnt be that much of a gain in rpms in the final drive.
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Old 11-09-2011, 12:53 PM   #6
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Re: t5 swap and 373 rear question

Quote:
Originally Posted by Pat Hall View Post
If you want it to be dead nuts accurate, go to JAM performance's website and use their calculators. They have a tire size calculator and speedo gear calculator side by side on the same webpage.
I know that you mean well Pat, but that's misleading.

I have yet to see a single online gear calculator that gives the same gear suggestion as what GM actually put in cars which were accurate.

See attached. 1984 Z28 came with 7/21 gears; their site errs to 7/20.

The tables out of Helms or some other GM publication with tire size/ trans/ gear counts were posted here years ago and are the way to get correct speedo gear combinations.
Attached Images
File Type: jpg never_calculate.jpg (51.6 KB, 17 views)
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Old 11-12-2011, 04:25 AM   #7
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Re: t5 swap and 373 rear question

Hey JMD, no offense taken on this end! I really shouldn't have said it'd be "dead nuts" accurate. That's taking it a bit too far on my part because you're actually right in the fact that the factory usually uses a gear that's at least one tooth different from the answers I get from Jam's calculator. What's interesting though, is I've ran the gear Jam recommended and also have ran the gear the factory used, and neither of them were right on the money. For example, 3.23 ratio with 245/50/16, the factory combo was 17/44, and Jam recommended 17/43. Just to see which combo was more accurate, my city set up one of those flashing speed things that tells you how fast you're going, and I decided to see what their gadget said with either gear combo. Well, with one combo, my speed was actually higher than what my speedometer said, and with the other combo my speed was lower than what my speedo said. So in other words, neither combo was dead on the money. With Jam's calculator, you actually end up having to round up or down to the nearest match, because it'll say something like 43.2 or 43.7 for the number of teeth required on the driven gear. It's almost never a perfectly even number, so you end up having to round either up or down to the nearest whole number. Lol. Sometimes I wonder if the newest cars out there that use an electric, pulse type speedo and a more powerful PCM have the capability to make the speedo read perfect? This simply isn't possible with a mechanical, or cable-driven speedo, but maybe it is possible with the electric ones. Whatever the case, whether I use the factory gear combo, or the Jam calculator combo, either of them comes out close enough to where I don't get busted for speeding!
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Old 11-14-2011, 05:25 PM   #8
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Re: t5 swap and 373 rear question

well i sold the 700r4 today so i guess its t5 time gonna pickup some supplies 2mar!
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Old 11-14-2011, 05:25 PM
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