Gear Ratio Change

[bobl]

There are many threads discussing gear ratios. Seems that the majority recommend 3.55:1 ratio, so that’s what I installed. After driving my car for 5 years and 12,000 miles I came to some realizations. I spend probably 95% of my driving time well below 70 mph. We have a lot of hills and twisty roads so that’s where I enjoy driving. Also, a lot of time idling around at events and such, which required pushing in the clutch a lot to keep the speed down. So, I decided to switch to 4.10:1 gears. The car is much more enjoyable to drive! It will idle at a much slower speed and no more down gearing when pulling hills. Of course an added benefit is better acceleration. The trans has a 2.95 first and .63 overdrive and turns 2300 rpm at 70, 2600 at 80(300 more than the 3.55’s). I can easily live with that, even if I lose a bit of fuel economy. I am running a 500+ HP 347 that revs to 6800 rpm. I Certainly wouldn’t recommend this ratio for a high torque, low rpm engine, but with a small block or Coyote it might be just the ticket. I’m really happy with the change.

Bob

[GoDadGo]

Looks like a great choice to me for your high winding SBF or any Coyote Motor.


You'll hit your 6,800 RPM Redline at 126.81 MPH at the top 4th Gear so it looks like you've got a real 1/4 Mile Killer.
For the record, I agonized over the rear gears and went with 3.73's when I really wanted to go with 3.90's, but I chose a more conservative ring and pinion.
6,000 RPM yields 123.94 MPH in the top of 4th which drops to 4,500 RPM in 5th and then down to 3,000 once I'm in 6th, which 40 MPH is faster than I care to cruise.

[CraigS]

Good point bobl. How it will be driven, what is the torque curve like, what are the rpm limits, etc all play a part in choosing rear ratio.

[Railroad]

I totally agree with the gear change on the Coyote. I have 3.55s and can tell 3.73s would really be ideal.
I have 3.73s in a 13 GT500 and a 99 Mustang Cobra with supercharger.
I end up running in 3 rd gear in the F5 Cobra, just to be in the power band.
I got the IRS gears in my deal on my anniversary edition, so I put them in.
Maybe I will get brave, one day and pull the chunk and put in the right gears.

[Mike DiGiorgio]

.63 overdrive is a great ratio, Most cars have a .72 -.78 top gear. The longer .63 overdrive lets you move into the high 3s, low 4s rear axle ratio without issue.

the math isn't past any high school level. Or you can take the easy way out at use the calculator on the Tremec site linked below

https://www.tremec.com/calculadora.php

[tonywy]

As I said in another post here, I had just switched from 3.08 to 3.73 and very happy. I had considered 4.10 but it would of put me outside of where I need to be. Big block with a .68 fifth gear. With the cam that is in this engine it starts to really pull at 2000 rpm.

[J R Jones]

Mike, the calculator is a terrific reference, thank you.
Back in the day my 66 GT350 street car had a 3.89 and it was terrific.
I bought a SCCA race prepared GT350 and ran the upper midwest. The 3.89 worked at Elkhart Lake and Brainerd, a 4.11 at IRP and Mid-Ohio, and a 4.56 at Blackhawk Farms.
I did run a Boss 351 in a Trans Am at Elkhart and had to set-up a 3.50. It clocked 164MPH through the traps.
Ford top loader.
jim

[NC Cobra]

I will add the following tool to the discussion: https://www.blocklayer.com/rpm-gear.aspx This seems to take the Tremec data and make it much more intuitive in my opinion.

[CobraboyDR]

IMO, for a SBF, 3.73 gears with a .63OD is an ideal set-up for quick stoplight blasting and decent slab cruising.