Gear ratio

[GregWeld]

Originally Posted by Flash68

Trans?
Rear tire height?
Engine rpm?

Another important factor to calc ----- where does your CAM begin ---- because CRUISE RPMS should be about 200 rpm under that number. Too low -- and you have sluggish throttle response -- too high (getting into the cam) will kill your gas milage on freeway cruising.

So with the tire DIAMETER --- Final drive gearing of the Tranny --- and RPM's you'd like to cruise at on the freeway using some cam info (65 or 70 or is your state 55? etc) then you have a basis to start to look at gearing.

Once you have that info --- then you have a range on either side to which you can make a judgement --- little "more" gear for better response down low and Auto X etc would be my personal choice...

Kind of a typical street gear might be a overdrive trans with .70 overdriven final --- 25.5" diameter tire -- Cam that comes in around 2400 rpm (so cam might be 2400 to 6300 rpm "type) --- then a 3.70 gear would be about right for some performance and some decent freeway cruising.

But maybe you have a 6 speed and double overdrive trans and it's final gear is .50 ----- then you might be a different gear.

What you will find is that the COMBO --- Trans -- TIRE -- Rear gears is hard to get perfect. And don't go by what someone else says they have. Do a little homework and use some charts to help you make a judgement. It depends on your tranny -- because you might have a real low 1st gear and find that your combo needs to be shifted out of 1st in about 10 feet! Okay if you can live with that -- but some trans have a little taller first gear and then some are different final drive gears... so that info is a must.

My '32 Ford has a very tall rear tire (large diameter) -- a 5 speed trans with .70 final -- and 4:30 gears! That would be crazy in a car with 26" tire diameter - but the larger diameter effectively reduces the gear. When I had 3:90's in it -- it lugged at freeway speeds at 60 mph (ever driven a '32 roadster? they're not a lot of fun at 70!).

I have to shift from 1st quicker than I like --- but everything is a trade off.

[IndyNova]

I've got a bone stock 4l80e trans, with a 275/45/18 rear tire. I haven't ordered a cam yet, but I've been given a few ideas on what I should order.

[Sieg]

Gear ratios of the trans will allow you to factor speed and rpm in each gear.

This is for a 6 spd manual but you can enter your ratios and get the answers:
http://www.tremec.com/calculadora.php

[GregWeld]

Originally Posted by IndyNova

I've got a bone stock 4l80e trans, with a 275/45/18 rear tire. I haven't ordered a cam yet, but I've been given a few ideas on what I should order.

Your tire is 27.74 inch DIAMETER


Your 4L80 is a .75 overdrive (final drive ratio)


If you have a small block -- I'd try to keep the 70 mph cruise at around 2000 to 2200 rpms.... same with a big block


3:70 = 2350rpms

3:89 = 2450rpms

4:11 = 2600rpms


These give you a rough idea of what you're dealing with