Switch rear gears?

[Daryl]

I’m running Ford Performance 3.55 rear gears with my 347 stroker & TKX 5-speed manual transmission.

Seems like I rev out pretty quickly through the gears and run a bit high on rpm’s at highway speeds. Would a taller gear set lessen the rpm’s at highway speeds and/or allow longer throws between 1st, 2nd, 3rd, 4th??

As always, I appreciate everyone’s feedback and input. Happy Holidays and New Year’s. Cheers!

 

[lwarrior1016]

Yes it would. Going down to a stock gear such as 3.08, 3.27, 3.31 will lower your rpm and allow more time in each individual gear.

However, it will also lower your acceleration speed. A gear like that doesn’t create as much mechanical advantage and may feel sluggish when compared.

You may not have an issue though, being the 347 should have great torque.

 

[Daryl]

Food for thought, thx Warrior!

 

[Daryl]

Would 3.08’s be toooo high? Or would 3.31’s be that much of a difference?

 

[Daryl]

Not sure where I should land!

 

[Mr horsepower]

The 3:55 gearset is actually a good balance between acceleration and highway cruising. On your TKX, what 5th gear ratio do you have? There are 3 options as shown below. For best highway cruising the .68 overdrive is the best, the .81 OD ratio is best for road racing applications.

 

[Daryl]

I don’t know what I have. Is there a way to tell?

 

[Mr horsepower]

I don’t know what I have. Is there a way to tell?

The simple way to tell, is to look at the part number on the invoice from where you bought it. If you did not buy it, maybe you can contact whoever bought and / or installed it. If neither is possible, there will be a metal ID tag attached to one of the bolts that holds the main case to the tail shaft together.

I've listed the part numbers for the 3 Ford TKX models in my post above. The first is for 10 spline clutches, if you have a 10 spline clutch / input shaft, then you have a .72 OD 5th gear. If you have a 26 spline input, then you could either have the .68 or .81. Based on your comments, the .68 would provide the lowest engine RPM for highway cruising. The .81 is more for road race applications.

 

[Daryl]

Thanks very much! I’ll jack it up, check it out and get back with what I find.

 

[Mr horsepower]

Thanks very much! I’ll jack it up, check it out and get back with what I find.

Also notice on the chart that the 17765 model not only have a taller OD ratio, but also a taller 1st gear, which will give you a longer runout in 1st.

 

[96blak54]

With a T45 and 2:73 rear gears that came stock, I could get 100mph out of 2nd gear. 5th gear driving torque wasnt a problem either with the 5.4l. Id imagine a 4.6l in 5th gear would have been equivalent to a 4cyl power wise. Then I swapped to 3:55 and I feel like its a good balance for an engine with mountains of torque and still rip through the gears having fun. With the 2:73 gears, mexico driving was only 2gears. I never got into 3rd on the streets funning around.

 

[Daryl]

I’m curious the rpm drop compared to what I’m pulling now.

 

[Mr horsepower]

I’m curious the rpm drop compared to what I’m pulling now.

Are you talking about rear or transmission gear changes?

 

[96blak54]

Would 3.08’s be toooo high? Or would 3.31’s be that much of a difference?

This all depends on your end goal. Such as a drag strip contender scenario most times sets the rear gear ratio to wind out the final transmission gear as you cross the finish line, whatever speed that may wind up being and of course high engine rpm utilizing every bit of available hp and not worried to much about street driving low rpm torque. Typical rear ratios above 4.11 gears.
With this said the opposite would be a higher rear ratio like 2.73 utilizing most of the engine torque and keeping out of the rpms by short shifting, street driving scenario and staying in the engines torque.

You may find yourself swapping out a few gears, testing what you like and what the engine likes, your driving style, fuel consumption ...etc. I feel youll find something in the 3.31 or 3.55 range to be in your favor.

When my car had the 2.73 rear gear along with the 5.4l off idle torque, it worked very well due to the 2valve 5.4l anemic hp beyond 4000rpm. I was able to utilize the engines torque output and keep out of the rpms where hp was pitiful.

 

[Daryl]

I have 3.55’s now. Based on the feedback here so far, I’m considering 3.08’s and MAYBE 2.73’s. I’m not convinced that 3.31’s would yield that much of a difference.
I’m looking for longer throws between gears and better mpg via less rpm’s at highway speeds.

 

[lwarrior1016]

I have 3.55’s now. Based on the feedback here so far, I’m considering 3.08’s and MAYBE 2.73’s. I’m not convinced that 3.31’s would yield that much of a difference.
I’m looking for longer throws between gears and better mpg via less rpm’s at highway speeds.

If you’re serious about it, download the tremec app. It will allow you to play with gear ratios and tire size to show you rpm at certain mph. I used that to select my gears for the drag strip.

That being said, 3.08 is the gear that came stock on manual cars. I don’t think I’d go all the way back to 2.73 in your case.

 

[Daryl]

If you’re serious about it, download the tremec app. It will allow you to play with gear ratios and tire size to show you rpm at certain mph. I used that to select my gears for the drag strip.

That being said, 3.08 is the gear that came stock on manual cars. I don’t think I’d go all the way back to 2.73 in your case.

Thx, warrior. I’ll definitely check it out online. I haven’t been able to get under the car to read the tag on the TKX to try to ascertain what gearing it has. Been raining (thank god) here in SoCal like crazy

 

[lwarrior1016]

Thx, warrior. I’ll definitely check it out online. I haven’t been able to get under the car to read the tag on the TKX to try to ascertain what gearing it has. Been raining (thank god) here in SoCal like crazy

I was looking at that picture above. There is only one option for a 10 spline clutch. Do you know if it’s got a 10 spline or 26 spline clutch? Stock came with a 10 spline.

 

[Musturd]

If you think the 3.55’s run out quick my monies on you have the 3.27 first gear . I’ve gotta tkx in my car with a 2.87 first gear and .68 overdrive and the 3.55’s are not for me at all . 4.10’s are on the way or 3.90s if I stay on a 26” tall tire I can do damn near 50mph in 1st gear

 

[Daryl]

I punched in some numbers on this calculator 3.55 gears vs 3.08 gears, to get an idea of how much rpm difference, say @ 70mph:
(the 3.08 rpm @ 70mph was roughly 400 rpm less than the 3.55’s. Picture cut off)