68 cutlass convertible, 350 4bbl original 4spd muncie.
Installed TKX now considering rear gear ratio. Im going to have JD race convert my open 3.08 rear to either 3.42 or 3.90 rear posi.
So ? is which ratio? motor will be built eventually to around 375 to 400hp, edelbrock heads, cam etc.
TKX has 2.87 first gear ratio vs the muncie 2.52
Leaning towards the 3.42 rear as the tkx has steep first gear making the 3.42 about the same as a muncie with a 3.90 in first.
3.90 too much for the tkx?
On freeway 70mph roughly 27 inch tire
3.42 rear about 2000rpm
3.90 rear about 2300rpm

Whats your opinion? 3.90 would be "fun" but maybe too much, overkill?
Car will be street cruiser with mainly car show hops. Maybe some freeway trips to shows.

TKX install, for those that might be considering
I used Silver Sport and am overall satisfied with the kit. My car is original 4spd console so the TKX fit ok but I did have some interference at the rear top side of shifter cover plate. I had to cut about a 3" x 3" hole at this spot, bent the edges up and welded in new sheet metal patch to add about 1/4" clearance. My body mounts are original so maybe with new mounts it would have fit? New mounts would need to add around 3/8" lift to provide 1/4" clearance that silver sport recommended.
Also I needed to drill one set of holes and trim my cross member to fit the longer TKX. The cross member just contacted the trans at the front. Cross member has to move back, one factory hole on each side lined up but had to add one new hole on each side. Little difficult as the access holes on bottom of frame were not exactly below the new holes. Overall not bad spent one good weekend installing.

Thanks
Mark

 

You'd think this is an easy decision, but there is a lot to it! I went through the same thing when I installed my Keisler RS-600 (0.67 overdrive) 10 years ago.

I had a 400hp small block and went with 3.90 gears and it was a blast to drive around town. The highway was fine but the RPM was a bit on the high side at 75-80 mph, which around here (Chicago) you need to be doing 75 mph or you become a hazard. 2,600 RPM was about 80 mph, not terrible but not great.

However, the bigger issue I had was at lower speeds like 50 MPH I'd be at 2,400 RPM which is a touch more than I'd like, but shifting into OD put it down to 1,600 RPM which was too low and bogged it down. This is where a 6-speed with two OD gears is beneficial.

When I had Brian Trick build me a rear end, I went with 3.73 gears. My thinking is that it would reduce my around town RPM just enough to basically drive it like a 4-speed, and then OD is a highway only gear. I will say I did the rear end swap over winter time, so it was months between driving it with the 3.90 and then the 3.73, but I felt no loss of acceleration from the change.

Now that I'm having a SBO stroker built that should be over 500hp, I'll drive it with the 3.73's but I'd be open to going down to 3.55 or 3.42 to mellow out the RPM's even more.

I drive my car a lot, often during rush hour traffic both on the highway and side roads, during peak times on the weekend as well. If around town fun is your priority over "maybe highway trips to shows" then get the 3.90s. They won't be overkill at all, and will really shine if you're willing to make some 5,500 RPM pulls on occasion. If it truly will be used as a mild cruiser and you rarely go WOT, then do the 3.42's. The lower gears will make the exhaust slightly quieter at any given speed as well.

Helpful? More confused?

 

Rule of thumb is about 10:1 effective ratio in first. 2.87 with 3.90 rear gets you 11.2:1 off the line. Not awful, but you'll be upshifting early. 3.42 gears give you 9.82 off the line. Personally I'd go with the steeper gears, but that's how I drive.

Which OD ratio does your trans have? 0.81 would make the 3.90 gear equivalent to 3.16:1, 0.68 would make it 2.65:1.

 

When I built my Vista Cruiser 20-some years ago, I chose a rear gear that would get me 10.9:1 effective ratio in first.

With my 410/510 hp/tq 455, my heavy car feels like I could turn 11s until I shift out of first. It's been so much fun that I would do that again, even if I didn't have deep overdrive 6th gear.

 

I'm seemingly leaning with everyone else for the steeper 3.91

(I've noticed threads that should have a poll don't, and the ones that have polls usually don't need them ...)

Mhollida, your ride maxed from the factory, 4 speed 3.91 rear would be turning 3400 rpm with a 27 inch tire at 70 mph. That being optionally available at the time means the factory says its fine. So your close ratio TKX is sleepwalking in comparison at the same speed. Whether its overdrive is 0.68 or 0.81 and being stick allows you to pick your poison if you want better traction on the street from the jump. 2nd and its 1.89 ratio means a starting line ratio of 7.4, less wheelspin still quick with 400 hp. But being that its intended usage is not as a daily driver that's just a bonus.

 

Tire size also affects rpm a bit as well. For a mostly around town with only occasional high speed runs, the 3.90 is for you. A .68 and 3.42 is a great highway gear choice.

 

A lot less than some people think. The OP says he has a 27" tire. Going to a 28" tire makes less than a 4% reduction in RPMs. Going to a 29" tire is only a 7.5% reduction.

 

Thanks all for the advise. My TKX is the .68 5th so I think 3.90 is the winner. Cant wait tor springtime to try it out.

 

A late vote for 3.91, though it’s unlikely that you’d be disappointed at 3.73. With 3.91, 2300 @ 70, 2600 @ 80 (ish). That’s a beautiful thing. First gear will be a bit short but hardly annoying for routine driving and when given the spurs, it’ll be niiice.

​​​​​​…

 

I have the same question and I think I will go 3.73.

 

Notice I said, a bit, usually 100 to maybe 200 rpm difference, less than a lock up vs non lock up converter. I'm going from a 28" to a 26.5" tire, equals 100 highway rpm, matches the orginal tire size for an actual factory 3.08 gear ratio. I miss the old Olds Gmail calculator, super accurate with a lock up torque converter or a 3 speed, 4 speed and 5 TKX/TKO and 6 speed manual. With a manual, the online calculators are quite, a no lock up 3 spd auto add 200 to 300 rpm for most street converter. A rare highway car, a 3.90 is a no brainer. For me, a 3.42 would have been my choice with the .68 OD, doing hours of highway miles. l
miles. I ran that combo with a 2004R, the best of both worlds.

 

That's called Excel. I made up a spreadsheet with a handful of formulas years ago for this.

 

But your calculator didn't have useless Oldsmobile knowledge included or a really nice tire size calculator built in.

 

I'll take that bet. My spreadsheet also calculates the tire dimensions given the size. That really isn't rocket science.

 

Joe P, Like taking candy from a Babyyyyyyyyyy.

 

Bah, sure🙄, only for you, means ZERO to me. Either way, a really cool and convenient online tool gone, just like Oldsmobile. Nothing lasts forever, will probably completely disappear like many old websites and once we are all gone, any interest in Oldsmobile at all.

 

😅🤣🤣🤣🤣😂