5 speed manual gearbox revs

CaptainK

CaptainK

CCCUK Member
Hello, after my recent long-ish drive to the BMS, it got me thinking again about RPMs and overdrive gearboxes. Its a long term thought of mine to put a 5 speed into my C3. I did originally think 6 speed, but I see its more hassle putting one in. I'm happy with my current 1-4 gearing, just want an O/D gear.

I was wondering if anyone here has a 5 speed in their C3 and could tell me your rear diff ratios, your 5th gear ratio, and what differences in RPM did the 5 speed make for you at a given speed?

My C3 currently has a 3.08 LSD rear diff and is a 4 speed manual. So in 4th its 1:1. I do around 2.7k rpm in 4th at 70mph (I say "around" 2.7k rpm as my speedo is inaccurate, so not entirely sure its 70mph), but looking at RPM calculators online adding in my tyre size and rear diff it seems about right.

I'd be really interested to know what my RPM would be with a 5 speed. I've seen online there are 0.81, 0.72 and 0.68 O/D options for 5 speeds. I'd probably want the 0.68 for maximum lower revs, but the other gear ratios I'd want for better acceleration. I have put those into online RPM calculators, but I'm not sure how I work out the "complete drivetrain ratio" for the 5th speed - for the 4 speed the gearbox is 1:1, so my drivetrain is just the rear diff ratio of 3.08. So with a 0.68 5th gear, would my "complete drivetrain" be 3.08 + 0.68 = 3.76, or 3.08 - 0.68 = 2.40 or something else? I'd assume the 2.40, in which case my revs at 70mph should drop from around 2.7k to around 2.1k rpm.

Anyway, be interested to hear results from people that have done the conversion.
 
K

Keith P

CCCUK Member
Hello Captain K,

I did this exact conversion in my 76 C3 about 15 years ago. I also thought 6 speed but 5 speed is easier to fit - I did it myself without a ramp. It's by far the best upgrade I have made to the car. Revs dropped about 1000 at 70mph and the gear change quality is vastly improved. I bought a complete kit from a US company called Keisler Engineering, but I don't think they are around now. I think the overdrive ratio is 0.68 but don't remember all the details, if I dig around I might be able to find some original info. I am more than happy to share my experience if you think it would help.

Kind regards,
Keith
 
CaptainK

CaptainK

CCCUK Member
Keith P said:
I did this exact conversion in my 76 C3 about 15 years ago. I also thought 6 speed but 5 speed is easier to fit - I did it myself without a ramp. It's by far the best upgrade I have made to the car. Revs dropped about 1000 at 70mph and the gear change quality is vastly improved. I bought a complete kit from a US company called Keisler Engineering, but I don't think they are around now. I think the overdrive ratio is 0.68 but don't remember all the details, if I dig around I might be able to find some original info. I am more than happy to share my experience if you think it would help.
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That's excellent thanks - a 1k drop in rpm at 70mph sounds really good. Would definitely be interested in hearing about the kit you got. I've just been looking at Tremec kits after doing a bit of Googling etc.
 
CaptainK

CaptainK

CCCUK Member
Oneball said:
If it’s got 0.68 as the 5 th gear ratio just times your revs that you’ve worked out for 1:1 by 0.68
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Ah good point thanks. Maths was never my strong point :ROFLMAO: . By my calculations that means a drop of about 850 ish rpm in my car. Not bad at all.
 
K

Keith P

CCCUK Member
I found some info on my gear ratios:
Original 5 speed
1st 2.52 3.27
2nd 1.88 1.98
3rd 1.47 1.34
4th 1.0 1.0
5th 0.68

I found first gear in the original box too high which hurt acceleration, and of course top was much too low. I don't know what my final drive ratio is but to be honest it could be higher now as first is a shade too low with the 5 speed, and a higher ratio would drop the revs even more at cruising speed. Pics show 60mph with 4 speed and 5 speed.
60mph-4 speed.jpg60mph-5 speed.jpg
 
Roscobbc

Roscobbc

Moderator
A 5 speed has always been on my wish list - my Vettes 3.08 rear with 27" tall tyres relates to approx 2650 rpm (if I remember correctly) - the 5th gear ratio would translate to 1750/1800 rpm - with its 600 odd ft/lbs torque the engine would be very comfortable - but the 3.27 1st gear (vs Muncie's 2.20) would be wasted in tyre smoke - tbh (for mine anyway) I'd be tempted to source a later 2.73 ring and pinion, bring the 70 mph cruise down a 150 rpm's or so but not be too much for 1st gear.
For a stock 350 cu in engine the 3.08 I'm sure would be ample and the 3.27 give a useful boost off the line as compared with the stock M20's 2.54 1st gear ratio.
 
teamzr1

teamzr1

Supporting vendor
Keith P said:
I found some info on my gear ratios:
Original 5 speed
1st 2.52 3.27
2nd 1.88 1.98
3rd 1.47 1.34
4th 1.0 1.0
5th 0.68

I found first gear in the original box too high which hurt acceleration, and of course top was much too low. I don't know what my final drive ratio is but to be honest it could be higher now as first is a shade too low with the 5 speed, and a higher ratio would drop the revs even more at cruising speed. Pics show 60mph with 4 speed and 5 speed.
Click to expand...

Running through my model/simulator

Using those ratios for a 5 speed, but using a 3.08 diff gear using a 1982 C3 stock engine

5speed308.jpg

From a standing start till running out of gear and RPMs

topend.jpg
 
CaptainK

CaptainK

CCCUK Member
Keith P said:
I found some info on my gear ratios:
Original 5 speed
1st 2.52 3.27
2nd 1.88 1.98
3rd 1.47 1.34
4th 1.0 1.0
5th 0.68

I found first gear in the original box too high which hurt acceleration, and of course top was much too low. I don't know what my final drive ratio is but to be honest it could be higher now as first is a shade too low with the 5 speed, and a higher ratio would drop the revs even more at cruising speed. Pics show 60mph with 4 speed and 5 speed.
View attachment 24148View attachment 24150
Click to expand...
That's excellent thanks. I was hoping to find somewhere with that gearing too, but nowadays I can only find these two gearing options:
Wide Ratio – 3.27, 1.98, 1.34, 1.00, 0.72
Close Ratio – 2.87, 1.89, 1.28, 1.00, 0.68 OR 0.81 5th

So if I wanted my ideal 0.68 5th gear, I'd have to go with the Close Ratio option. Which to be fair in first and second is still more accelerative than my Muncie 4 speed, which would be handy for my little 327. Ideally I'd like your gearing (aka the "wide" option above), but with the 0.68 fifth. Probably doable if I asked them nicely when ordering etc.

Still, I've got time as this is early days and saving money - still got my FTO to fix up first, then save for the 5 speed.
 
GiuG

GiuG

CCCUK Member
Anyone consider to use the TH200R4, to have a overdrive instead the Expensive Tremec?! They say that is easier to fit
 
CaptainK

CaptainK

CCCUK Member
GiuG said:
Anyone consider to use the TH200R4, to have a overdrive instead the Expensive Tremec?! They say that is easier to fit
Click to expand...
Isn't the TH200R4 an automatic gearbox? In this thread specifically I'm talking about manual gearboxes. For me, an automatic is not an option.
 
S

spog1

CCCUK Member
Ill be doing mine in the spring, Modern Drive line do the complete kit. I've got 3.08 in the rear but wanted something a bit more punchy from the go but also have the eco of the 5th. I did the same on my camaro Night and day compared to what i had, although i had 3.50 gears in the rear.
 
CaptainK

CaptainK

CCCUK Member
spog1 said:
Ill be doing mine in the spring, Modern Drive line do the complete kit. I've got 3.08 in the rear but wanted something a bit more punchy from the go but also have the eco of the 5th. I did the same on my camaro Night and day compared to what i had, although i had 3.50 gears in the rear.
Click to expand...
Excellent stuff. Do you have a link to the kit? And what ratios does your chosen 5 speed box have?
 
GiuG

GiuG

CCCUK Member
CaptainK said:
Excellent stuff. Do you have a link to the kit? And what ratios does your chosen 5 speed box have?
Click to expand...
Roughly, how much is the cost, we taking about?:) I mean, for the upgrade from star to finish, I suppose labour does not come cheap....
 
S

spog1

CCCUK Member
CaptainK said:
Excellent stuff. Do you have a link to the kit? And what ratios does your chosen 5 speed box have?
Click to expand...
Okay just pulled the trigger on the 5 speed tkx
I have 3.08 rear gear ratio so ordered the TCET18083 (GM) 1st 3.27, 2nd 1.98, 3rd 1.34, 4th 1.00, 5th 0.72, Reverse 3.00
Cost was £4150 plus duty and tax
This is the complete kit, from drive shaft to 5 speed decal on the console
Purchased from SST transmission on a black Friday deal, fitted one in my Camaro and was well impressed
 
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