TAC Tech => Mechanical => Topic started by: Mileswinger on October 17, 2021, 07:25:44 PM
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I need a new transmission for my 403 1977 trans am I was wondering what my options are for better gas mileage and price and what fits with my engine and mounting if anyone has any knowledge that would be awesome I have no idea where to start
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Well the factory TH350 when rebuilt is an invincible transmission, what's going to get you better fuel economy would be your rear end gearing.
You could also consider a 200R4, not a direct fit some adjustments required, but will provide that OD gear that will help on the highway.
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I would definitely find something with an overdrive.
You could then drop a rear gear too
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Even with $3.25 per gallon, fuel economy is not a priority with most owners. I would get another TH350, easy to find and cheap.
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It’s more than fuel economy. It’s performance having a lower rear gear then the drivability and comfort on the highway.
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Best place to start is figuring out what your intentions are for driving the car? Also, it would be helpful for us to know your starting point. What is your current transmission and rear gearing? If you have an auto, are you planning to stay auto or possibly swap to a manual?
My suggestions would be to get a transmission with an overdrive and a 3.23ish gear if fuel economy is a concern. As mentioned the 200r4 has a dual bolt pattern so it will be a direct fit behind a BOP engine and give you the overdrive you want. It would be a good match behind a stock 403, but if it's built or you plan to build it I'd suggest sending it out to be beefed up. You'll also have to get a kit to hook up the TV cable and lockup for the shifts to work properly.
Or alternatively if you plan to go with a manual, I'd go look at either the TKO500/600 or even the new TKX trans. They offer nice ranges in gear ratios and will work great behind a 3.23 or 3.42 gear.
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Ryan,
All good points that you mentioned. Knowing the final drive ratio or overdrive of the transmission in order to figure out what axel ratio. Changing rear gears is something you don’t want to do more than once. I know I have seen some calculators using the tire diameter and axel ratio to determine engine RPM.
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So where's the OP? No more info, opinions, or questions?
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sorry for the delay my buddy had an extra th 400 trans he just rebuilt that he offered to give to me i figure that would be easiest and for fuel economy i ordered a holley efi sniper figure that could get me up at least a few mpg while upgrading to EFI
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There are many ways to put a th400 in your (never offered f-body)
1st is crossmember? hack yours or buy one..$$$$
2nd speedo gear combo?...take a look at what color speedo is inside the tranny
3rd new driveshaft yoke. beware there are "short" and "long" yokes
4th with the "short" yoke you need to cut 3/4" off driveshaft...not sure with the "long" yoke.
Also note: 1st gear is a little bit taller on the th400 vs th350 @ 2.48 vs 2.52 so it is not EXACTLY apples to apples swap.