Please help me stop overanalyzing gear ratios for a street car

On a Friday afternoon, when work has finally slowed down a bit, the sun is shining, and the birds are singing, a young man's mind tends to drift...to transmission and differential gear ratios. I've been planning to solve the "strong, modern transmission that should fit decently in a small tunnel" problem on my Conquest Ecotec swap project by just hitting the easy button and paying up for a new Tremec TKX. I figure I'd better order one pretty soon to avoid getting stuck waiting in this world of backorders and shortages, so I'll have it when I need it this winter.

There are 3 versions of the TKX:

Road race:             2.87 1.89 1.28 1.00 0.81

Wide ratios:          3.27 1.98 1.34 1.00 0.72

Close ratios:         2.87 1.89 1.28 1.00 0.68

They're all pretty similar, the 1-4 spacing is pretty good on all of them, but depending on which rear gear ratio I pair them with, they have different strengths and weaknesses.

Road race: With my stock 3.54 rear gears, I get a 10:1 first gear, 2,700RPM at 70MPH in 5th, and 2,800RPM at 60MPH in 4th, which I think puts the engine in a nice happy place on both the interstate and 2-lane highways. This would be the easiest to install because I can leave the rear end alone, and if I want to upgrade it someday, I can get a Ford 8.8 straight out of the junkyard with exactly the same ratio. The only thing I'm worried about is whether the 10:1 first gear will be annoying on the street for a 4-cylinder with a 17lb flywheel. The T56 in my Corvette has a 9:1 first gear, but it's a V8 with a 30lb flywheel, pretty big difference. GM's stock first gear ratio for the Ecotec-powered Solstice/Sky is 14:1, which I would consider stupid short. I still think the road race gear set is my first choice because I don't feel like the 10:1 first gear ratio will be that big of a deal, and everything else about it seems pretty close to ideal.

Wide ratios: With 3.73 rear gears, which are not available for the factory rear diff, I get a 12:1 first gear, 2,500RPM at 70MPH in 5th, and 3,000RPM at 60MPH in 4th. This would be the hardest to install because I would need a non-Mitsubishi rear diff immediately, with all the fabrication work and expense associated with that. I'd probably get another 1-2MPG on the interstate at 2,500RPM, but who really cares? This isn't a road trip car. I think this is my third choice, mostly because I want to avoid scope creep by not also committing to install a new rear diff right away. 12:1 is the same as the factory first gear ratio for the car, for what it's worth.

Close ratios: With 3.91 rear gears, which are available for both my factory rear diff and others, I get an 11:1 first gear, 2,500RPM at 70MPH in 5th, and 3,100RPM at 60MPH in 4th. I think this one ranks somewhere between the other two. Swapping gears in the Mitsu rear diff is extra work, but it's less difficult than building mounts and axles for something completely different. Good gas mileage on the interstate, but 3,100 in 4th is creeping into the "buzzy" range. 11:1 first is a compromise between the other two. 3.91 does get me the closest 1-4 spacing, but the difference isn't really that drastic, about 3MPH per gear on average.

For anyone who wants to see the full extent to which I've overanalyzed this, here's the Google Sheet I put together to calculate everything, complete with decision matrix: https://docs.google.com/spreadsheets/d/1I1t8Q8uCUjx9K7T3VU1I__5hlU5KEt-cuAoUCjD71cs/edit?usp=sharing

I'm just looking for some feedback--does my decision make sense? Anything else I'm overlooking? Grateful as always for the knowledge of the hive, and excited to finally get started on this project over the winter.

What's you're primary use for the car?
 

Have you driven it with the stock drivetrain, and if so, how do you like the gearing now?

I will offer no feedback other than... lets make that spreadsheet WAY more complicated by putting in your predicted powerband and graphing rpm vs. speed for all gears overlayed, as well as tractive power in each gear so the gaps can be better analyzed.

Then we need to go into Assetto Corsa and make a variant of the car with each transmission for extensive testing.

why wouldn't you use the wide ratio trans with your stock rear gear? Better highway cruise in 5th and in between 10 and 12 to 1 in first, which seem to be your two hold ups with the road race gearset. 

I'm assuming fun street and light sporting use? It's also very hard to say without having back-to-back compared similar overall setups to each of your options, so grains of salt and all that.

First of all, I'm skeptical that you'd actually need to run 4th @ 60mph for any of those options, unless you never want to have to downshift even for moderate grades or passing. Larger and heavier modern I4T cars, like EB Mustangs, are running like 2300rpm on the highway just fine.

I do enjoy running through the gears a bit on the street though. So that would incline me away from the Road Race, due to those tall lower gears.

I'm also not the biggest fan of the large rpm drop from 4th to 5th that's common to 'old fashioned' domestic 5-speeds, as seen on the Close Ratio. Without a low revving engine, it has more potential to get caught feeling a bit 'between' 4th and 5th in a common speed range. I'm sure It would ultimately perform just fine overall, and maybe even the 'best' in the real world depending how your engine runs... At least for other people, but maybe even for myself as well too.

For me personally though, I think I might actually be inclined towards the Wide Ratio box but with the 3.91. Yes it would have a short 1st gear, but I don't spend much time there anyway. Maybe just need to run it out a little further around town, which isn't a terrible side effect. However, the spacing is more of a steady progression and the gearing would be fun on the street, while also making 5th more usable on the highway without getting too buzzy. You could even still get it up and running initially with the 3.54 and see how you like it that way first too. While it may provide the closest equivalency to the Close Ratio box, I'm just really not convinced the 3.73 is that valuable unless the diff upgrade itself is warranted.

Here's another way to look at it...

RPM after shift from redline: (1-2), (2-3), (3-4), (4-5)

Road Race: 4280, 4400, 5080, 5270

Wide Ratio: 3940, 4400, 4850, 4680

Close Ratio: 4280, 4400, 5080, 4420

Pete Gossett (Forum Supporter) said:

What's you're primary use for the car?

Oh yeah, that would help, huh? Realistically, it'll spend 99% of its time on the street. It's basically just a restomod, so since I'm not competing in a particular class or optimizing for a specific track, design targets are harder to nail down, but it's good for me to have to stop and think about what I actually want. As far as the engine and transmission are concerned, I think it's just two things:

• Fun rowing gears around town and on local driving roads. I have two opposite ends of the spectrum as benchmarks for this, my C5 Corvette and my buddy's Subaru. The C5 has long gears and makes >300ft*lb just off idle. 2nd gear will take you to 80MPH, 3rd gear will take you to jail in most states. The Subaru has the 6-speed and 1-2-3-4 are really closely spaced, so you're hearing/feeling the turbo spool, shifting, and hearing the BOV all the time. It's really fun. He used to have a 5-speed with 4.44 gears in it and I hated that setup, but I drove it again recently and was surprised by how much I like it now with the 6-speed/3.91 combo. Some of that is definitely due to how much nicer the 6-speed is to shift, but it really changed my mind from believing I didn't like short gears. I think there's a sweet spot between the two cars that I'd like to try to hit with the Conquest.
• Better than factory gas mileage. The '88 Conquest gets around 20MPG on the highway, which is abysmal for a 4-cylinder, so this is easy. I'll hit this target just by swapping the lower-BSFC Ecotec, but I'd like to drop the RPMs at 70MPH from 2,800 down to something more reasonable. I don't mind having to downshift to pass and climb hills. A Solstice/Sky with the 2.4L Ecotec turns 2,500 RPM on the highway and gets about 26MPG, so that's about what I've been shooting for. Gas mileage doesn't really matter, this isn't a daily driver economy car, but c'mon, I have to do better than 20MPG!

Have you driven it with the stock drivetrain, and if so, how do you like the gearing now?

Yeah, I've driven it quite a bit with the stock drivetrain. The only things I find annoying are in 1st and 5th. The 12:1 first gear is kind of awkward in parking lots, which is at least partially due to how unhappy the Mitsu engine sounds at light load at 3000RPM. In a car with a longer first gear, like my 9:1 C5, it feels better turning lower revs at lower speeds. In a car with a shorter first gear, like the 14:1 Subaru, you just get rolling in 1st and then immediately shift to 2nd, which is actually the same ratio as 1st in the C5! So I think I'd be fine going either up or down in 1st gear ratio.

The gear spacing from 1-4 is fine, it's actually almost identical to the wide ratio TKX. 5th is 0.86:1, which is just not enough overdrive. Too close to 4th. All the TKX options improve that to different degrees.

Any time 1st gets awkward at parking lot speeds, there's always 2nd available. So that's never been something I have typically paid much attention to.

Robbie (Forum Supporter) said:

why wouldn't you use the wide ratio trans with your stock rear gear? Better highway cruise in 5th and in between 10 and 12 to 1 in first, which seem to be your two hold ups with the road race gearset. 

Yeah, that's a good point. That's actually the combo I looked at first. 11.6:1 first, which is so close to stock I might not notice the difference. 1-4 are actually all basically identical to the factory gearing for the car. 0.72 5th would put me at 2400RPM on the highway, which I started out thinking would be a little low, but is actually probably pretty close to optimal. Not a bad setup at all.

Get the "wide" ratio.

IMO the "close" ratio sets are not anywhere near close, but first is still kinda steep to pull out without a lot of clutch slip, so it's like the worst of both worlds.

Driven5 said:

I'm assuming fun street and light sporting use? It's also very hard to say without having back-to-back compared similar overall setups to each of your options, so grains of salt and all that.

Yeah, exactly. I'd like it to be competent enough to be fun at 1-2 HPDE or AutoX events a year, but not really competitive, primarily just street driven.

First of all, I'm skeptical that you'd actually need to run 4th @ 60mph for any of those options, unless you never want to have to downshift even for moderate grades or passing. Larger and heavier modern I4T cars, like EB Mustangs, are running like 2300rpm on the highway just fine.

That's really good to know. I haven't driven any turbo 4-cylinders that revved that low on the highway. The LE5 dyno chart from GM claims that even without the turbo, I'll have 150 ft*lb of torque from 2,000RPM and up, which seems a tad optimistic, but is still kind of reassuring.

I do enjoy running through the gears a bit on the street though. So that would incline me away from the Road Race, due to those tall lower gears.

I'm also not the biggest fan of the large rpm drop from 4th to 5th that's common to 'old fashioned' domestic 5-speeds, as seen on the Close Ratio. Without a low revving engine, it has more potential to get caught feeling a bit 'between' 4th and 5th in a common speed range. I'm sure It would ultimately perform just fine overall, and maybe even the 'best' in the real world depending how your engine runs... At least for other people, but maybe even for myself as well too.

Yeah, the big gap between 4th and 5th is one thing I was trying to mitigate by pairing the Close Ratio with the 3.91. There's actually a factory 4.22 gear set that's kind of hard to find, but if I could get my hands on one, I think it (or a 4.11 or 4.30 Ford 8.8) would work even better with the Close Ratio box, and would bring 2nd down low enough to make it more usable at lower speeds.

For me personally though, I think I might actually be inclined towards the Wide Ratio box but with the 3.91. Yes it would have a short 1st gear, but I don't spend much time there anyway. Maybe just need to run it out a little further around town, which isn't a terrible side effect. However, my gut feel is the rest of the gears have pretty good spacing and the gearing would be fun on the street, while making 5th more usable on the highway without getting too buzzy. You could even still get it up and running initially with the 3.54 and see how you like it that way first too. While it may provide the closest equivalency to the Close Ratio box, I'm just really not convinced the 3.73 is that valuable unless the diff upgrade itself is warranted.

That's a great point. Being pretty well matched with the stock 3.54 with the option to either keep that ratio jump to the 3.91 down the road if I want more gear seems like a win/win. The Mitsu diffs are strong, but they get sloppy over time and the parts to fix the slop are NLA, so the drivetrain gets annoyingly clunky. Most of my reason for moving to a Ford 8.8 would be to fix that, and also for more ratio choices.

Here's another way to look at it...

RPM after shift from redline: (1-2), (2-3), (3-4), (4-5)

Road Race: 4280, 4400, 5080, 5270

Wide Ratio: 3940, 4400, 4850, 4680

Close Ratio: 4280, 4400, 5080, 4420

Driven5 said:

Any time 1st gets awkward at parking lot speeds, there's always 2nd available. So that's never been something I have typically paid much attention to.

With the stock Conquest gears, 3,000 in 1st feels like too much, but 1,800 in 2nd feels like it's just not quite enough. Yeah, I'm being picky, but I think moving either direction, so 1st is longer, or both 1st and 2nd are shorter, would help.

In reply to obsolete :

Close ratio is race track friendly except the hardest thing to do is get a car moving. Consequently the 3.27 first gear ( wide ratio ) will work for theJaguar  race car because I use a triple disk 7&1/4" clutch.  That will keep clutch maintenance  to a reasonable level. 
  The slightly wider ratios means most corners will be taken in 3 rd gear. (5th isn't used )  Once the team is up to speed using syncro's I'll replace it with a dog ring Seinz that has gears that are easily swapped out and replaced.  On the bench it's about a 10 minute  job to replace all 5 sets.  ( no overdrive )   That will help in tight uphill corners like turn 5 at Road America. Since no clutch is used the moments where the car is coasting waiting for the syncros to adjust gear speed  will be eliminated. 

Close ratio,  the 1-2 shift on the wide ratio will be horrible for any kind of fun driving.

ProDarwin said:

I will offer no feedback other than... lets make that spreadsheet WAY more complicated by putting in your predicted powerband and graphing rpm vs. speed for all gears overlayed, as well as tractive power in each gear so the gaps can be better analyzed.

Then we need to go into Assetto Corsa and make a variant of the car with each transmission for extensive testing.

 

I am up for that!!!!

https://docs.google.com/spreadsheets/d/1arZTH5ZtwjjiYQVtRfkEtCz5vxOIjVHFXbjx9I7n5fE/edit?usp=drivesdk

here is the excel file i use.

RossD said:

https://docs.google.com/spreadsheets/d/1arZTH5ZtwjjiYQVtRfkEtCz5vxOIjVHFXbjx9I7n5fE/edit?usp=drivesdk

 

here is the excel file i use.

Thanks. Looks like it's not public, though.

it shiuld be open to everyone now

I think the wide ratio box is completely pointless on a street car.  That 1-2 ratio change is just hideous, and with 3.54s you'll just waste your torque into tire smoke.  Those are light-duty towing ratios, not street car ratios.  I would completely eliminate the wide ratio box from my thoughts.  To me, the wide ratio box is just a consolation for people who mismatch a lumpy cam in a heavy vehicle and large wallets.

That makes the choice easy.  The other two boxes only differ in OD ratio.  I would honestly keep the factory rear ratio and run the close ratio box.  That gives you a 10:1 first and a 2.40 OD.  The only reason to do the road race box would be if you actually need a properly-spaced 4-5 ratio change for high speed things.  You're driving on the street, so your OD ratio just needs to match your cruise RPM goals, not be used for acceleration.

AnthonyGS (Forum Supporter) said:

Close ratio,  the 1-2 shift on the wide ratio will be horrible for any kind of fun driving.

Street cars have a camshaft that has a wide power band, unlike race cars that trade bottom end  for power higher up in the rev band.   Most street cars are daily commuters which means traffic jams. That's a lot o extra rowing  as traffic comes to a stop every block or so. Your clutch takes the biggest hit wear wise starting.     

dean1484 said:

ProDarwin said:

I will offer no feedback other than... lets make that spreadsheet WAY more complicated by putting in your predicted powerband and graphing rpm vs. speed for all gears overlayed, as well as tractive power in each gear so the gaps can be better analyzed.

Then we need to go into Assetto Corsa and make a variant of the car with each transmission for extensive testing.

 

I am up for that!!!!

Cool, thanks guys, let me know when you have completed your research :)

Curtis73 (Forum Supporter) said:

I think the wide ratio box is completely pointless on a street car.  That 1-2 ratio change is just hideous, and with 3.54s you'll just waste your torque into tire smoke.  Those are light-duty towing ratios, not street car ratios.  I would completely eliminate the wide ratio box from my thoughts.

Thanks. You and AnthonyGS have me taking another look at the close ratio box. I don't think the larger 1-2 gap with the wide ratio box is a deal breaker, but I do like the idea of reducing it. What I have eliminated from my thoughts at this point is the road race box, so I think it's just down to close vs. wide. I'm not so sure I'll be struggling for grip all the time with around 350whp, but if I am, that's a problem I'd be happy to work on solving.

To me, the wide ratio box is just a consolation for people who mismatch a lumpy cam in a heavy vehicle and large wallets.

Well, what about people with turbo 4-cylinders? Not an over-cammed V8, but similar lack of low-RPM torque.

That makes the choice easy.  The other two boxes only differ in OD ratio.  I would honestly keep the factory rear ratio and run the close ratio box.  That gives you a 10:1 first and a 2.40 OD.  The only reason to do the road race box would be if you actually need a properly-spaced 4-5 ratio change for high speed things.  You're driving on the street, so your OD ratio just needs to match your cruise RPM goals, not be used for acceleration.

Yeah, the road race box is out. So, the real question, am I worrying about nothing with the 10:1 first gear being too steep for a 2.4L Ecotec? I guess I could try it, and if I hate it, a set of 3.91-4.22 rear gears with the close ratio box may be the way to happiness.

In reply to obsolete :

I suppose I should look up a torque curve for the Ecotec, but I can't imagine needing more than 10:1 reduction unless you're running really big tires or more weight than I'm imagining.

I'm just looking at that wide ratio box and that ridiculous 1-2 ratio split.  That's like 700r4 kind of numbers but you don't have the advantage of a torque converter.  If you wind it out in first to 7000, the 1-2 shift will take you down to 4100 rpms.  If I had a 4 cyl turbo without the advantage of low end torque, I might not be okay with that, but compare it against your torque curve.

If anything, I would invest the time and money in a different rear ratio than intentionally sabotage it by trying to get a proper first by just making a massive gap in the 1-2 shift.  This was the same problem GM had in the 70s with the wide ratio M's.  They put wheezer engines in heavy cars and had to widen the ratios to get them moving which, with only 4 gears to play with, made things not so great.

If I were doing this right now today, I would buy the close ratio box all day every day.  If I don't have enough multiplication in first, I would change the rear, but I would never intentionally put a wide 1-2 split like that unless I was using it to tow 3000 lbs.

Th only difference between road race and close ratio is the overdrive ratio. If this isn't a race car, go for the longer OD for relaxed touring, otherwise just opt for the close ratio.

I much prefer close ratio boxes. My most shift-fun street car has 2.50:1, 1.66:1, 1.23:1, 1.00:1, 0.85:1, :1

Okay, I added RPM splits to my spreadsheet and came up with the same numbers that Driven5 posted earlier, and I think I'm finally getting it. No matter what I do with the rear diff, if I have the wide ratio box, I'm always starting over at 3940RPM in 2nd after a redline 1-2 shift. With the close ratio box, it's always going to be 4280RPM. Changing the rear gear ratio changes the road speed that happens at, but the RPM splits are always the same. That's the key piece of info that I was missing by only looking at MPH splits before.

It's only 340RPM, but it could be the difference between full boost and not quite full boost depending on my turbo choice. Maybe it won't matter, could be 10HP difference between the two that I would never notice, but 4280 will always be better than 3940.

I think I'm pretty much sold on throwing the close ratio in with my factory rear diff. If I don't like it, I will go looking for a rear gear ratio that starts with a 4.

Thinking about your engine more, I think I may be coming around to the close ratio camp too. It's not redline shifting that's making me reconsider though. Full boost should be no problem at 3.9k rpm. However, it's more the simply playful driving, where you're maybe shifting more in the 4k rpm range. The extra 200rpm after the 1-2 shift from there might be more helpful overall than the narrowing the 4-5 spacing, which might not actually prove to be an issue for your car anyway.