Rear End Gear Ratio

[jayh]

My Factory Five currently does not have posi, going to install tru loc. Just wondering how much additional kick (fun factor) I can expect by changing gears from 3.27 to 3.77 while I'm at it. Running a 302 with Trick Flow top end kit, Cam, aluminum heads Trick Flow intake. All feedback welcome.

[Ace23]

Originally Posted by jayh

My Factory Five currently does not have posi, going to install tru loc. Just wondering how much additional kick (fun factor) I can expect by changing gears from 3.27 to 3.77 while I'm at it. Running a 302 with Trick Flow top end kit, Cam, aluminum heads Trick Flow intake. All feedback welcome.

Don’t fear the gear as they say. I went from a 3.31 to a 4.30 with a 427 Windsor.....went with taller tires so it was more like going to a 4.10. Run the second strike gearing calculator and see what you get. I would say go with a bit more than you think.

[t walgamuth]

What is your final gear ratio in your tranny?

My car has the six speed Tremec with a .5 sixth gear ratio and a 4.10 winters rear end so my final drive is 2.05. I imagine I'll have all the gear I want for acceleration or fuel economy.

[Ron61]

I run the 3:77 in my Factory Five Coupe and with my transmission I have more acceleration in 1st, 2nd, and 3rd than I can really use. If I mat it the tack pegs before I can get my foot off and slow down. But 4th and 5th are really good for driving down the road.

Ron

[Grey 65]

Originally Posted by Ron61

I run the 3:77 in my Factory Five Coupe and with my transmission I have more acceleration in 1st, 2nd, and 3rd than I can really use. If I mat it the tack pegs before I can get my foot off and slow down. But 4th and 5th are really good for driving down the road.

Ron

I went from 3.46 to 3.15 gears in my 527hp SBF using a TKO-600 and still lite up the tires in 1st, 2nd or 3rd if I am not careful..... FYI tire O.D. is 25.9"

[Kit Coyle]

If you go too low in the rear you may render your 1st gear useless. Who wants
a granny gear...not fun.

[t walgamuth]

With a 7 liter the first three or four gears are optional in any case.

[Gaz64]

A diff ratio change has the biggest impact on driveability, compared to any other component.

I changed from 3.08 to 3.36 as my first experience of changing diff ratios back in 1987. This was only about 9% lower.
That feeling of torque increase in each gear, engine revs more easily in each gear, push in the back etc, I was very happy.

I'm sure you will be happy when you drop your ratio, yours is about 15.3%.

Of course take transmission ratios and tyre rolling diameter into account.

Gary

[Cruisin428]

You should tell us how the car drives now, before asking if you should change something!

How well does it launch now? Changing gears means shifting faster. Smoking tires is smoking tires, making it easier to do may not add anything to the joy of driving it!

Do you find when cruising that one gear has too high of revs and the next to low so changing ratios may move to a nice in between?
When you are in"fun mode" how many gears do you go through before you get traction?
Also the posi will help with launch and traction so consider that before changing ratios.
As stated above, Look up how to calc the rpms and mph for each gear or use one of the online calculators for both ratios. That way you will be able to judge where you will move the mph and rpms to!

[Snake2998]

You will gain roughly 15% more performance - e.g. 0 to 60 MPH will drop from say 5.5 sec to 4.8 sec depending on all sorts of factors such as traction etc. eating away from the 15%.

[jayh]

thanks for all input. I value your thoughts and opinions.

[jayh]

Thanks Gary.

[joyridin']

What transmission are you using and what is the final drive gear? I have 3.70 in my car and I love them, but my final drive is .54, so I can cruise the highway at 1900-2200 RPM without a problem. That is with roughly 575hp and FI.

I would do it without issue unless you had a 4 speed or a trans with a 1:1 final drive ratio.

[sdonnel]

I made the swap from 3.31 to 3.73. The change in acceleration was very obvious. Cruising at 70 with my T5 has me at 2,050 RPM. No need to downshift to pass. Just apply some pressure to the loud pedal. As it was said before, you will have to watch yourself when running hard, the rev limiter has caught me off guard a couple of times. Was it worth it. Absolutely.

Scott

[Kit Coyle]

What is the rough cost to change the rear gear ratio with
a Jag rear end set up? It would by fun to try a few more.
I raced a 1958 live axle Corvette once and had 3 different
"pumpkins" or rear ends for the car. I could swap them in
about an hour as I remember and did so for each race track
I went to. 3.70 for the long tracks and 4.56 for the short
ones with 4.11 for the others.

[joyridin']

Originally Posted by Kit Coyle

What is the rough cost to change the rear gear ratio with
a Jag rear end set up? It would by fun to try a few more.
I raced a 1958 live axle Corvette once and had 3 different
"pumpkins" or rear ends for the car. I could swap them in
about an hour as I remember and did so for each race track
I went to. 3.70 for the long tracks and 4.56 for the short
ones with 4.11 for the others.

If you switch the gears to a Dana 44 instead of messing with the Salisbury gears, it is not overly expensive.

I have a set of Jaguar 4.56 gears if you are interested.

[MKS427]

Transmission gear ratios and tire height our big factors here. You will need to get some type of a mile an hour calculator to figure this out. Ideally, you want to reach about 40 to 45 in first gear, 70 to 75 in second, 100+ in third. You will want to make sure you have ideal cruising RPM for fifth gear.
I run a 3.08 with 17” wheels and TKO 600 with wide ratio 5th gear. I have 585 lb/ft torque. Having less torque, if you have a close ratio fifth gear less gear may work well.
Again, use a MPH calculator. Look at 400 rpm below peak torque and shift point for first through fourth. Also, look for your ideal cruising rpms for fifth.
Good luck

[Frank Messina]

You need to provide more information about your car (trans and tires) before you can expect any relevant guidance.

Frank

[1795]

Originally Posted by Kit Coyle

What is the rough cost to change the rear gear ratio with
a Jag rear end set up? It would by fun to try a few more.
I raced a 1958 live axle Corvette once and had 3 different
"pumpkins" or rear ends for the car. I could swap them in
about an hour as I remember and did so for each race track
I went to. 3.70 for the long tracks and 4.56 for the short
ones with 4.11 for the others.

Kit,

Not sure about the cost for having different gear ratio setups. Having installed a Jag rear end in my cobra last year, I can tell you that you will not be changing out pumpkins in an hour. There is not a lot of room to work with in there and you would have to remove the half-shafts to get the rear end out and back in again.

Jim

[Dwight]

torque of the motor is also a factor.

Cam profile is another. Will the motor (cam/torque) cruise smoothly at 2,000 - 2,200 rpms with the rear end gears and tire size?

I have worked on and driven at less 20 different Cobras. Most had gears in the 3.25 to 3.54. Tire size 26" to 27". Motors 302, 351w, 408s 427s and a few with 482s.
Most could cruise at 2,000 with out a trailer hitch jerk.

We have changed a couple of rear end ratios to get a better driving Cobra. I have removed several 3.73 gears and installed 3.55s.

IMHO

Let me add that these are all street cars not race cars