Pros and cons of rear end choice

[Kmcallahan]

Team,
I am looking for feedback on changing my rear gears from 3.55:1 to 3.08:1. I have the TKX 600 with .68 final drive. The engine I have is a Mike Forte built 351w stroked to 427ci. The power plant is pushing out 502 HP @5700 rpm and 542ftlb torque @ 3700 rpm. At cruise speed of~78-80 mph I am turning ~2700 rpm.
I want to bring down the rpms some @ cruise speeds. I hat are the cons of going to 3.08:1?
Also what are some of you guys experience with this change?

Thanks

Callahan

[Jim1855]

Callahan,
Tires will matter but perhaps not much.
I ran a 427w with similar torque but a bit more HP.
3.27 gears, same OD as you. 315/35-17 2,200 at 80mph. I cruised cross country with this setup multiple times, liked it.
Current build keeps everything the same except the rear gears change to 3.31.
Had a chance over the weekend to drive a roadster with a similar motor and 3.31s, wouldn't change a thing.
Jim

[Jim1855]

To add.
Two friends, one a similar 427w the other an almost 500 ci FE, ran 3.08s. They seemed to like the gears but I'm not sure on their OD ratios.
Jim

[edwardb]

I don't have experience with 3.08:1, but can offer this FWIW. Using a nominal tire size of 25" and this https://www.tremec.com/calculadora.php, your experience with RPM's at around 80 is roughly correct with the 3.55. Changing to 3.08 would reduce it to around 2250. Is that enough and worth it? You have to decide. You can enter your actual tire size, if different, and evaluate both setups. I find this kind of exercise to be useful.

[jamminj]

what size tire are you running
with a 25.5" tall tire with your 3.55 and .68 final at 80 mph you will be at 2547rpms
with a 26.5" 2450 rpms
are you sure your ratio is 3.55
3.73 with a 24.5 tall tire 2785 rpms at 80 mph
https://spicerparts.com/calculators/transmission-ratio-rpm-calculator

[Kmcallahan]

what size tire are you running
with a 25.5" tall tire with your 3.55 and .68 final at 80 mph you will be at 2547rpms
with a 26.5" 2450 rpms
are you sure your ratio is 3.55
3.73 with a 24.5 tall tire 2785 rpms at 80 mph
https://spicerparts.com/calculators/transmission-ratio-rpm-calculator


Sorry for not adding tire size.
I am running 315/30/18 with a height of 25.44.

Also the numbers I am giving are my actual #’s

[jamminj]

sorry the numbers are not adding up that combo 3.55 should be right at 2552 rpms
and with 3.08 at 2215 rpms at 80mph
I think I'm running the same tire on my mustang Toyo r888r

[Kmcallahan]

what size tire are you running
with a 25.5" tall tire with your 3.55 and .68 final at 80 mph you will be at 2547rpms
with a 26.5" 2450 rpms
are you sure your ratio is 3.55
3.73 with a 24.5 tall tire 2785 rpms at 80 mph
https://spicerparts.com/calculators/transmission-ratio-rpm-calculator

Jamming,
According to my delivery sheet I have 3:55 dead end.

Callahan

[Kmcallahan]

sorry the numbers are not adding up that combo 3.55 should be right at 2552 rpms
and with 3.08 at 2215 rpms at 80mph
I think I'm running the same tire on my mustang Toyo r888r

Jamminj,
I agree with this. According to the tremendous gear cal calculator I should be @ 100 mph. But if I change the calculator to the fix with the .81 OD I come the to same speed I am running now. I called the person I got it from and the serial # of the trans matches the .68 Or trans. I also checked the tach and it is correct. So I am leaning toward changing the rear end gears.

Callahan

[Kmcallahan]

I don't have experience with 3.08:1, but can offer this FWIW. Using a nominal tire size of 25" and this https://www.tremec.com/calculadora.php, your experience with RPM's at around 80 is roughly correct with the 3.55. Changing to 3.08 would reduce it to around 2250. Is that enough and worth it? You have to decide. You can enter your actual tire size, if different, and evaluate both setups. I find this kind of exercise to be useful.

edwardb,
I have been using the same calculator and wondering if the transmission I received has the gears I ordered. Just wondering. I need to check some how.

Callahan

[bobl]

Get some data in 4th gear, which you know is 1:1. You know the tire size, and use gps speed. then assuming the tach is accurate(verify accuracy with another source) you can calculate rear gear ratio. Once you have that, use 5th gear data and confirm the overdrive ratio.

[Kmcallahan]

Get some data in 4th gear, which you know is 1:1. You know the tire size, and use gps speed. then assuming the tach is accurate(verify accuracy with another source) you can calculate rear gear ratio. Once you have that, use 5th gear data and confirm the overdrive ratio.

bobl,
Good idea. I work on that thanks.

Callahan

[edwardb]

edwardb,
I have been using the same calculator and wondering if the transmission I received has the gears I ordered. Just wondering. I need to check some how.

Callahan

That was my first thought. That you really had the .82 OD gear. The calculation exactly matches that. I see from another post you confirmed .68. Sure it's not .64? That's the TCET5008 which is the most common for these builds, including the 2.87 1st gear. But to the point, there should be a model and serial number metal tag under one of the housing bolts. I would trust that before anything else. Assuming it's there. If it's not, that would suggest the transmission has been opened because it left the factory with that tag. I have no idea how to check what 5th great ratio you really have. Maybe one of the Tremec dealers could help, e.g. Modern Driveline, Liberty Gears, Mike Forte, etc. Personally I'd check that before tearing into the diff.

[jamminj]

the one constant is 4th is 1:1
so if you have 3.55 gears with 25.44 tall tires and you drive at 60 your rpms will be 2815
if that is the case your tachometer and your rear end gear are as stated and correct
this gives you a good starting point

[J R Jones]

When I doubt the specifications and calculations I put down the calculator, get under the car and turn the shafts to count the revolutions, then do the math.
The differential is easy, counting fifth or sixth gear overdrive is not. One test is worth a thousand opinions.

[Kmcallahan]

153464
That was my first thought. That you really had the .82 OD gear. The calculation exactly matches that. I see from another post you confirmed .68. Sure it's not .64? That's the TCET5008 which is the most common for these builds, including the 2.87 1st gear. But to the point, there should be a model and serial number metal tag under one of the housing bolts. I would trust that before anything else. Assuming it's there. If it's not, that would suggest the transmission has been opened because it left the factory with that tag. I have no idea how to check what 5th great ratio you really have. Maybe one of the Tremec dealers could help, e.g. Modern Driveline, Liberty Gears, Mike Forte, etc. Personally I'd check that before tearing into the diff.

Edwardb,
I have the TKX 600 this is a pic of the box and I confirmed the tags on the side when I performed in inventory when I received. I agree with you and at this point still doing investigation and home work on options. Thank you I will keep all posted on out come.

TCET17765

Ford

600 lb-ft

26

2.87

1.89

1.28

1.00

0.68

2.56

Callahan

[GoDadGo]

3.08 gears with a .64 O/D coupled with a 25.9" tall rear tire equates to 70.36 MPH at 1,800 RPM.

[edwardb]

153464

Edwardb,
I have the TKX 600 this is a pic of the box and I confirmed the tags on the side when I performed in inventory when I received. I agree with you and at this point still doing investigation and home work on options. Thank you I will keep all posted on out come.

TCET17765

Ford

600 lb-ft

26

2.87

1.89

1.28

1.00

0.68

2.56

Callahan

Oops. Missed that it was a TKX. On a side note, interesting you have it on the road already. Several have been asking about reviews and/or first impressions. Might want to comment about that. But I'm still wondering about the metal tag on the transmission itself. Assuming they're doing that with the TKX same as the TKO. Transmission could have been put in the wrong box. It happens.

[Kmcallahan]

My findings with comparing to pm and speeds in 4th and 5th. Also gps app on phone showed to be 2-3 mph slower than speedo.
4th gear
40 mph - 2000 rpm
50 mph - 2415 rpm
60 mph - 2800 rpm
70 mph - 3300 rpm
80 mph - 3900 rpm
5th gear
50 mph - 1600 rpm
60 mph - 2000 rpm
70 mph - 2300 rpm
80 mph - 2650 rpm
90 mph - 3000 rpm

[jamminj]

your 5th gear ratio is exactly what you thought .68
i sent you a private message

[jamminj]

___3.08__3.31
50 1384 1488
60 1661 1785
70 1938 2083
80 2214 2380
the 3.31s will not need to be down shifted on the freeway at 60 if you want to accelerate
I think the 3.08s will be dogging out at 60 a require a down shift
I love to shift so it wouldn't matter to me
in fact if you were in 5th@50 mph and down shift to 4th you would be at 2036 rpms and by the time you hit 60 you would already be at 2443 rpms

[Kmcallahan]

Oops. Missed that it was a TKX. On a side note, interesting you have it on the road already. Several have been asking about reviews and/or first impressions. Might want to comment about that. But I'm still wondering about the metal tag on the transmission itself. Assuming they're doing that with the TKX same as the TKO. Transmission could have been put in the wrong box. It happens.

edwardb,
When received I did confirm the pn & sn’s on the tags on the trans matched the invoice & box.