Engines tend to be most efficient at about 30-35% of max engine speed.  With a 6,000RPM motor you'd want your cruise RPM to be right around 2,000RPM.  A lot of rear gear selection has to do with what type of driving you do too.  A guy who only makes occassional interstate runs may be willing to deal with a little more cruise RPM to maintain punch out of the hole when he's doing most of his driving around town.  The long distance warrior may be willing to sacrifice out of the hole performance for the best cruising RPM.  With most motors in these cars you should be in the 2,000-2,500RPM range while cruising to keep from accelerating engine wear.  Like almost all things automtovie rear gear choice is all about compromise. 

Years ago I swapped my 2.73s for 3.55s.  The 3.55s had a lot better off the line performance, but it cruised a lot better with the 2.73s.  I never really had much need to drive the car on the interstate, but now I'm wanting to be able to take it on some trips to visit family and friends who live in other states.  My T5 swap will let me keep my 3.55s and still turn reasonable RPM on the highway.  I also swapped to a taller rear tire, which will also improve crusing RPM. 

Rebuilt a car that turned out to have 5.13's in it, could hit 3rd before making it through an intersection.  Story was the previous owner missed a shift and blew the engine trying to get the front wheels off the ground .. again.

First pumpkin i found was a 3.73, which was worlds better, and with maybe 50% freeway driving I had to do, they were still too short.

Next I found a 3.23, which was great on the freeway, but I thought was a little tall for the street, could not imagine going taller and definitely would have stepped up to 3.55 if I had found one.

A 3.5 was the one thing I already had decided, especially given I will have a 5th gear, not much free-way driving, and in a hilly area.

Nice to see that it seems to be the most recommended ratio.  It also looks like if you go much higher you can turn first into a granny gear.

Last edited by afnid (3/10/2015 12:38 AM)

A bud on hear is holding a set of 3.73:1 gears for my 66 T-5. Deal was....try 'em.....if you lik'em we'll talk. Since I'm the big lumpy cam guy on here 230-236....108LSA...at 70 mph(with the 3.00:1 peg leg gear) it JUST smooths out!! I sorta like that ridd'in in a motorboat feel loping along at 65 in 5th. NOW....when I need to shoot around slower traffic I downshift to 4th to avoid knock. I have the old skool 15" Torq-thrust rims with 215-65-15 tires. May try a taller rear tire some day. Love that "west-coast rake"!
6sal6

And that's exactly why FRPP offers the Z-spec T5.  The factory T5 was designed for cars that had 2.73 or 3.08 gears, so they made first gear tall to get it moving off the line.  The ratio spread between the gears was less than ideal for autocross stock too.  The Z-spec allows the installation of higher gears (numerically) and you get a better gear spread, and 1st gear doesn't feel like a granny low in a truck.

Oh, and BB, boats aren't so bad, it could be much worse, you could be into airplanes!  Price for a car part is X, same part for a boat is 2X, same part for an airplane is 10X.

That's because boats and cars don't require an FAA PMA (parts manufacturing authorization) certification. Regardless, if you can afford an aircraft, you can afford the part. Same goes for a boat. If you wanna play, you gotta pay. I know, I have owned all three.

TKOPerformance wrote:

Bullet Bob wrote:

My 90 GT had 3.73 gears with a T5Z.  The Z trans has a taller 1st and 2nd and a .63 OD.  It was, IMO, nearly perfect.  Around town it was like driveing an old close-ratio 4-spd.  Get above 60 and pop it into fifth and it went all day in the low 2k rpm range...and easily got 28 MPG...just ask Josh-Kebob.
 
A 3.55 behaves nearly the same way with the slightly lower OD of the AOD or standard T5.

BB

And that's exactly why FRPP offers the Z-spec T5.  The factory T5 was designed for cars that had 2.73 or 3.08 gears, so they made first gear tall to get it moving off the line.  The ratio spread between the gears was less than ideal for autocross stock too.  The Z-spec allows the installation of higher gears (numerically) and you get a better gear spread, and 1st gear doesn't feel like a granny low in a truck. 

Oh, and BB, boats aren't so bad, it could be much worse, you could be into airplanes!  Price for a car part is X, same part for a boat is 2X, same part for an airplane is 10X. 
 

You know, this is the first time I have seen anyone say anything about ratio spread.  First it was about getting a low gear to get one off the line the fastest.  Then the discussion moved to how high a rear end could one get and still have acceptable highway rpm.  I have always thought we need to look at the combination of all the trans ratios (1st thru High), rear end, and tire height.  Any other thinking will fall short along the line somewhere unless you're lucky.  Now with the T5Z, that was not available when I got m T5, we have something that gets us much closer to having everything we need.  Now all I need is the money to buy one.
When discussing gearing and such we also need to look at the engine rpm range.  If I build an engine that runs 3k-7k rpm, I need one set of gearing.  If I build an engine that runs 1500-5500 rpm, I need another.  One size does not fit all.
I prefer 5 speeds too.  I have driven 3, 4, 5, & 6 speed transmissions.  3 speed was fine when all I wanted to do was "go like 60".  Now we drive more like 70-80 on the highway so, we need more gears to handle that.  I feel that 5 speeds handle that spread admirably.  With todays flat torque curves& high red lines, we don't need as many gears.  In my 2013 Mustang w/6-speed, I bypass 5th much of the time both going up and down, even when driving hard.  Before that I had a Honda S2000 that had a flat torque curve from 2000 to 9000 rpm.  Even with only 165 lb-ft of torque I bypassed 5th much of the time.  Some of the German cars have 7 and 8 speed trans.  Totally useless around the town, but absolutely necessary at 160 mph.
Finally, if all you are doing is attending the local cruise night once a month, do whatever you want!  Let 6sally6 pick your cam:-)  don't worry about anything but making sure that your car looks and sounds right.  Freeway driving is different than street driving. Long distance driving is different again.  And racing?  One must consider all the the ways one will be driving.  There is nothing worse than not being able to do something one wants to do because of noise or lack af capability.  Spending extra money on a 6 speed is wasted when all I will use is a cheaper 5 speed; that is unless I do it because I want to!
Thanks for tweeking my thoughts.
(I think I just wrote a sermon.)