by "MM54" (mm54mk2)
Published 08/19/2017 at 22:07
by "MM54" (mm54mk2)
Published 08/19/2017 at 22:07
Tags: '71Chevelle
STARS: 0
I spent most of the afternoon finishing prepping the Muncie to go into the Chevelle, which included investigating what speedometer drive gear is in it so I can get the appropriate driven gear. To make a long story short, I cannot get the tooth count I need (19) in the size that will correctly mesh with the (1.76") drive gear.
Option 1: Remove tailhousing, remove drive gear (pressed onto shaft), press on new drive gear. This is probably the ‘right’ way to do it, but I really don’t want to pull the transmission apart and attempt to remove and install a press-fit gear on a shaft still installed in the transmission. Cost: not much; Labor: Oodles.
Option 2: Run a 22 or 23 tooth gear (the closest I can get for the drive gear I have) and run an adapter box. Cost: ~$90; Labor: Not Much
Option 3: Get a GPS-based mechanical speedometer drive I found on the internet and use that, making it irrelevant what gear set is in the transmission (and later axle ratio changes, etc). Cost: ~$350; Labor: A little more than Option 2 but nowhere near Option 1.
All this assumes that the article I read where it said the 1.76" drive gears are .531" wide, while the 1.84" are .610" is correct - I made a .53" wide piece of cardboard and stuck it in through the driven gear hole and found it to be the same width as the drive gear.
"crowmolly" (crowmolly)
08/19/2017 at 22:17, STARS: 0
What’s your tire size and rear axle ratio?
If it’s a factory 27 spline output shaft I believe 1.76" is correct.
"TylerLinner" (tylerlinner1)
08/19/2017 at 22:21, STARS: 0
Wait until winter and do Option 1. You might be bored enough that the labor won’t matter.
"MM54" (mm54mk2)
08/19/2017 at 22:42, STARS: 0
235/60R15 (so 26" tall) and a 3.08 axle. It is the earlier 10-spline input, so 27 spline output sounds right.
"MM54" (mm54mk2)
08/19/2017 at 22:44, STARS: 0
The goal is to get the trans in before winter so I can actually drive it some this year, and to do bodywork all winter, though. Right now would be the most ‘reasonable’ time to do it, since the transmission is sitting on the ground. I need to look into what’s actually involved in changing that gear out before I decide to actually start pulling the trans back apart (it’s all together and sealed up as of about 5pm today).
"crowmolly" (crowmolly)
08/19/2017 at 22:47, STARS: 0
You do have an Option 4: higher performance rear axle ratio. A 3.55 gear would allow you to use the 22 tooth green gear. Just sayin’. LOL.
That aside, I’d swap out the gear for a 1.84" one. They’re available for the 27 spline mainshaft. I don’t know if they’re a GM part though.
"TylerLinner" (tylerlinner1)
08/19/2017 at 22:48, STARS: 1
Tough call. I would always try to do it the right way as opposed to some bandaid fix. It’s too bad the interrim solutions aren’t cheap enough to use for a season and do the right way later.
"crowmolly" (crowmolly)
08/19/2017 at 22:50, STARS: 0
You can use a cheap Garmin or Android/iOS GPS speedometer on your dash in a pinch. You’d have to pull the trans back out to swap gears though.
"MM54" (mm54mk2)
08/19/2017 at 23:10, STARS: 0
Haha I was about to come back and make an edit that says “I suppose option 4 is put 3.73s in the back”
Nothing quite like 3400RPM down the highway!
"MM54" (mm54mk2)
08/19/2017 at 23:12, STARS: 0
For sure. If there was an adapter to get me even close that was cheap, I’d do that and come back to it later (at some point it’ll change again when I do the axle). I may just stick a 22 or 23 tooth (closest I can get) in there and deal with it for the time being. Maybe do some math and leave sticky note next to the tach :)
"TylerLinner" (tylerlinner1)
08/19/2017 at 23:24, STARS: 1
Yeah, Post-it life FTW. I’ve been dealing with a similar issue on my mountain bike. It’s been geared slightly high since I swapped to the internal gear hub with 32/23 gearing. I can’t get a larger rear cog, and a smaller front chainring (a very boutique 30 tooth) is out of my budget now. So, I ride and I deal with it.