synthartist69

Hey everyone,

I am going to do a T5z swap for my 4speed top loader in a 65 fastback. Right now I have 373 gears with 205/70/14 tires. My rpms are too high on the freeway and the car is drinking the gas with a hipo 289.

I would like to get the rpms down on the highway. I have an option of a 1st gear 3.35 ratio and a 5th gear .61 ratio. Is the 3.35 too low for a 3:73 rear end and do I need to go with a 2.95 1st gear instead? This car is my cruiser street racer and eventually will have a streetable 331 stroker in it. I want to go with the t5z in order to handle the extra torque of the 331. I don't see myself taking the car to the dragstrip but do want the car to have some major road muscle when needed. haha

I am not sure how many rpms I am turning right now at 70mph but it feels like it is near or over 3k and I don't like it. How much can I expect my rpms to drop by adding a tfz with a .61 5th and the 373's? How much will my highway mpg improve? I am probably around 13 or 14 mpg overall right now. Any help would be greatly appreciated! Thanks.

smittycm

Hey there. I'm planning on the same swap in my 66. Check out this RPM calculator to play around with the different possibilities...

http://www.allfordmustangs.com/Detailed/608.shtml

kalli

a T5 helps plenty with gas mileage, but can't give numbers. if you have a 3spd or 4sp the last gear is 1:1, so if you have an .061 overdrive your rpms will be 61% from what they have been at the same speed, no other changes to the car (rearend, tyres)

if you use the calculator above, the 4th gear of the T5 will be what you currently have in your highest gear, so it's easy to compare..

further with a 331 and a T5 with 3.73 the first gear will be a tyre shredder, but who wouldn't want that :-) it's still driveable though

kalli

if you have excel, I attached a calculator I made myself.
put your numbers into the green fields, i already populated most of it with your values

synthartist69

[QUOTE=smittycm;6816125]Hey there. I'm planning on the same swap in my 66. Check out this RPM calculator to play around with the different possibilities...

Wow, very nice. Thank you for the calculator link!

synthartist69

Gotcha, I appreciate the input my friend!

FoMoJoe

I installed a 347 and a t5z to replace my 289 and toploader 4 speed, rearend still an 8" with 4:11 gear. Before I was doing about 3500 rpm at 65 mph. Now I can go 65 at 1950-2000. I love it! Still thinking about going to a 3:55 though b/c I have to shift out of my 1st gear within the 1st 5 mph is seams like. Have had no problem with the transmission. You will probably need a piece of sheetmetal to fill a void in the trans tunnel since the toploader shift gears are on the side of the trans and now the t5z shifter will be on top. For me that was the case b/c a previous owner put the toploader in and had to hack an area out, but when I put the t5 in, it lined right up with the original trans tunnel whole with minimal modification. get a good tran crossmember to bolt up to, and might as well do the 331 now, build it on an 86 or newer block so you have a full rear main seal, then ditch the z bar clutch linkage (you will have no choice since newer blocks don't have the engine block z bar hook up on the side. Unless you buy a bolt in z bar connecting bracket but I think it's like $100 and really if you have to do that, might as well get the cable clutch which is i think $200 to replace all that mess.

synthartist69

Thanks for the information. My plan was to buy the t5 conversion kit and use my 289 4speed bellhousing with the conversion plate and stick with the z bar linkage, just to keep things simple. Then at a later date install the 331 in place of the 289. Will the 289 bell housing work with the 331? I am not sure yet what is available as far as 86 or newer blocks. I did find someone near Houston, Tx that I want to buy the long block 331 with pro comp heads from. I was going to put my intake and carb on that engine. Please let me know if you think I am missing something here, I want to make sure I get it right! Thanks.

Jim

Deviousfred

Wanna make a charitable donation with that toploader? I'll pay the shipping.

FoMoJoe

if you go with a newer engine, I believe it has a different bolt pattern so you'll need the newer bellhousing... "I think". It's been a couple years since I did the research. I had bought my newer bellhousing for 60 bucks I htink used on craigslist. If you talk to your engine shop they will know.