When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Anyone really good at figuring best rear axle ratios?
Hey guys, little overwhelmed with reading other threads and online articles so wanted to put my thoughts here and see what you think!
Wondering if the 3.42 gears I have in my Firebird are optimal for the short range daily driving I do. Car never sees highway and I'm looking for the best off-the-line acceleration I can get with this n/a V6. Don't care about gas mileage changes. I'd like a posi rear anyways so it's swap time!
Re: Anyone really good at figuring best rear axle ratios?
If you are never on the highway and want a V6 to feel strong then a 4.10 might work for you. Also look into the Eaton Posi, Yukon Dura Grip or Eaton Truetrac. Don't bother with the Auburn or Governor Lock. They are junk. You will also need a good installation kit. Tom's Differentials has the best kits, and US strange gears. He is also a Eaton distributor and has good prices on their products.
Re: Anyone really good at figuring best rear axle ratios?
I typically look at 1st gear x rear ratio. The "performance" gear offerings from the factory end up in the 10:1 range... Manual = 2.95 x 3.42 = 10.0, Auto = 3.08 x 3.23 = 9.94. The "standard" 2.77 rear gear factors out to 8.2 and 8.53 respectively... At least when applying to a V8's greater torque.
Your T5's 4.03 1st gear gives you a big boost already. It factors to x2.77 = 11.16, x3.42 = 13.78, x4.10 = 16.52.
Find a gear/tire/rpm/speed calculator and plug your redline (5000?) in with these numbers to see where you have to shift out of FIRST gear. Also plug in the RPM of your average shift (3000?). What is the MPH for both of those? With 4.10s and the 16.5:1 first ratio, do you really want to shift out of 1st gear at a ridiculous low speed?
I guess the point I am driving: Your V6 has the performance boost of a 4:1 first gear. The 3.23-3:43 might be snappy enough, and the 4.10 too much - a chore to drive from the other end of the argument.
Re: Anyone really good at figuring best rear axle ratios?
Problem probably starts here:
245/50-16 tires, 25.6" tire diameter
Don't outsmart yourself. Go pull a measuring tape from the center of a rear wheel to the ground. I'm almost (but not quite, being a statistics professor) willing to bet that the "diameter" you're putting into your calculator is WRONG.
Diameter = 2 times what the tape measure reads.
Then, use something accurate for RPMs instead of the wavy-pointer trim package thing in the dash.
Put those NEW REAL REALITY ACTUAL TRUE numbers into a "calculator" and see what you get. Just like you don't have to worry about checking all the calculators on the shelf at the office supply place to make sure you pick up one that says that 2+2=4.
Last edited by sofakingdom; 05-17-2016 at 01:34 PM.
I hadn't noticed your 4.03 first gear. A 4.10 or 4.56 would make 1st gear almost useless. A 3.42 would be much better. Be careful with that 4.03 transmission. They are not very strong.
Last edited by big gear head; 05-17-2016 at 02:41 PM.
Re: Anyone really good at figuring best rear axle ratios?
Thanks for the replies guys, got me headed in the right direction now.
Originally Posted by big gear head
If you are never on the highway and want a V6 to feel strong then a 4.10 might work for you. Also look into the Eaton Posi, Yukon Dura Grip or Eaton Truetrac. Don't bother with the Auburn or Governor Lock. They are junk. You will also need a good installation kit. Tom's Differentials has the best kits, and US strange gears. He is also a Eaton distributor and has good prices on their products.
I thought about the 4.10, but was worried about losing 1st gear as it has such a high ratio already! As for the posi unit, for now, I'll just be finding a good lower-mileage stocker in the ratio I decide on and refreshing it.
Originally Posted by MoJoe
I typically look at 1st gear x rear ratio. The "performance" gear offerings from the factory end up in the 10:1 range... Manual = 2.95 x 3.42 = 10.0, Auto = 3.08 x 3.23 = 9.94. The "standard" 2.77 rear gear factors out to 8.2 and 8.53 respectively... At least when applying to a V8's greater torque.
Your T5's 4.03 1st gear gives you a big boost already. It factors to x2.77 = 11.16, x3.42 = 13.78, x4.10 = 16.52.
Find a gear/tire/rpm/speed calculator and plug your redline (5000?) in with these numbers to see where you have to shift out of FIRST gear. Also plug in the RPM of your average shift (3000?). What is the MPH for both of those? With 4.10s and the 16.5:1 first ratio, do you really want to shift out of 1st gear at a ridiculous low speed?
I guess the point I am driving: Your V6 has the performance boost of a 4:1 first gear. The 3.23-3:43 might be snappy enough, and the 4.10 too much - a chore to drive from the other end of the argument.
Ok, that is very helpful and confirms what I was worried about if I ran the 4.10 gears. Redline is 5500 and when getting on it (usual driving :P) I shift around 4500.
That said, I used those numbers and got very wrong numbers out of the calculator (like a 64.1 ratio...). Here's the link, maybe I'm just missing something here. http://hotrodworks.net/hotrodmath/quarter.html
And I guess this is why they stuck the V6 with the 3.42 rear from the factory! Just thought I could gain some acceleration at the same time as swapping in a posi rear. If I have to find a posi 3.42, so be it!
Originally Posted by sofakingdom
Problem probably starts here:
245/50-16 tires, 25.6" tire diameter
Don't outsmart yourself. Go pull a measuring tape from the center of a rear wheel to the ground. I'm almost (but not quite, being a statistics professor) willing to bet that the "diameter" you're putting into your calculator is WRONG.
Diameter = 2 times what the tape measure reads.
Then, use something accurate for RPMs instead of the wavy-pointer trim package thing in the dash.
Put those NEW REAL REALITY ACTUAL TRUE numbers into a "calculator" and see what you get. Just like you don't have to worry about checking all the calculators on the shelf at the office supply place to make sure you pick up one that says that 2+2=4.
Ruler says 24" nearly on the dot. So yes, off 1.6" doesn't help with calculations.
I hadn't noticed your 4.03 first gear. A 4.10 or 4.56 would make 1st gear almost useless. A 3.42 would be much better. Be careful with that 4.03 transmission. They are not very strong.
Thanks for the link, good to have another tool to try. Seems I should stick with the 3.42 ratio I already have and just find a posi version to refurb & swap in!
As for the strength, if you're referring to the V6 transmission being weaker than the V8, that's been proven to be semi-false. The fault is the high 1st gear ratio paired with the torque of the V8; just too much stress. But the casing & gears are the same strength as the V8 variants.
Re: Anyone really good at figuring best rear axle ratios?
I've broke 5 of those transmissions with my 3.4 V6. The 4.03 first gear ratio makes the transmission weaker than other T5 transmissions. It's due to the size of the input gear, not the strength of the case or anything else. Any manual transmission gets weaker as the first gear ratio gets numerically higher. A 2.20 first gear is much stronger than a 4.03. Same thing goes for rear end gears. A 2.73 is stronger than a 4.56 due to the size of the pinion gear head.
Re: Anyone really good at figuring best rear axle ratios?
4.10s
4.33s
Something similar.
If you want low speed "max" performance and don't care about higher speed there's no reason not to go with 4.10s.
If you are concerned with the track then there's quite a bit more to take into account. Only concern is a short 1st gear. It's quite obnoxious shifting before you even get out of an intersection.
Re: Anyone really good at figuring best rear axle ratios?
4.10s is too much for a T-5...
Great for a T-56 though.
With a motor with good power, you'll be out of 1st gear in less than a second.
Rule of thumb is, you want your final drive ratio to be somewhere 10 ± 1 kind of range in 1st gear. There's nothing empirical about it; just kinda a rule of thumb. Of course tire size figures into it as well which further muddies the picture, but with the small tires these cars have, you want to keep from going too far off the high end. With a 2.95 1st, 3.73 rear gears put you at almost exactly 11 (2.95 x 3.73); perfect. 4.10s put you at over 12; a bit too short but possibly OK if you're willing to accept the amount of shifting you'll be doing while just driving around, and the constant high RPMs.
The ratios you gave sound like a 6-cyl transmission. You will HATE that thing. Besides just the extreme low 1st, the ratios are ALL real wide spread. Yuo'll have the feel of the 700 1st - 2nd shift in EVERY gear. I'd STRONGLY suggest putting the rear in as-is and just see how you like it. With that, and 4.10 in the rear, you'll end up with over a 16:1 final drive ratio; which renders 1st gear all but useless.