Rear end question.

[Malice]

I posted this is the Drivetrain section but decided I'd get more responses in the general section. Need some advice from the Forum.


I am currently in the planning stage of my build.
I am having a very hard time with deciding what rear end gear ratio i should be using. My build is going to go a little something like this.

Art Morrison Chassis.
5.3l (550 - 600 hp goal)
4l65e built transmission (currently considering monster transmission)
rear end ?.??:1

I look at Art Morrison's farm truck. They used a 3.51:1.
The Icon Thriftmaster. was a 4.09:1 295/45zr18 rear tires.

Which i know this all depends on was gear ratio of the transmission and the tire size.

End result is i want this truck to run like a corvette. It won't be a zr1 corvette but c5 or c6 range sure. Good low end torque but want it to run nice and smooth at 75mph. This will be a daily driver but i want it to burn the tires when i want to.

So I am hoping for some suggestions or if someone has some experience in this area.
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[solidaxel]

I like a higher rear gear with the first gear ratio of a 4L 60/65E so low
.
Remember you are driving a pick up with a very light back half of the truck.
"Posi" for sure, as tire spin can get out of control and not putting anything to the ground.

All of my "challenges" on the street have "gone up in smoke"

I like the 3:25-1 for a low 1st gear and a good with overdrive it is a good hiway gear.
I drive one of each, but my favorite is the 3:25/4L65E

[yossarian19]

Step one is pick your tire size or rolling diameter anyway.
I'd be looking for OE cars with similar power & transmisison gearing, then duplicate their rear end ratio. It's a bit of homework but probably no more so than other strategies for picking a ratio.
Consider a Winters or something with a dropout 3rd (like a Ford 9") - things change.

[Malice]

Tire size is 28.5". I did look at a lot of corvettes and that they have really low gearing. So maybe that is a good route. A lot different than what I was looking at.

[Malice]

Ok so maybe I'm wrong. What is a 295/45?

[OrrieG]

There was recent post about how to get a truck with 6 to 100 mph.

http://67-72chevytrucks.com/vboard/s....php?p=6623367

I posted two calculators that show variables for rpm, trans ratio, rear end ratio, tire diameter and speed. Change any variable and it shows how the others change. The second calculated how much horsepower is required to acheive a mph using weight, frontal area and resistance. Tirerack has a good tutorial on tire sizes and nomenclature and a calculator to check diameter for different wheel diameters and aspect ratio. Using those tools you can estimate what your drive train and tires need to be.

[_Ogre]

and my post in that thread

Originally Posted by _Ogre

truk has no problems driving with the speedo pegged and ogre enjoys driving it there
but it also has 4 wheel disc power brakes, front and rear sway bars, ifs, 700r4 and 3.27 rear gear

the gm ramjet350 has 350hp and 400ftlb of torque, these trucks are light anything over 250hp will scorch the tires. endlessly
but my 3.27 rearend with 28'' tires gets 20 mpg cruising at 80 mph
that to me was my basis for setting up truk for hitting the highways

your problem will be hooking up, with that hp you could probably use a 3.00 or 2.70 gear

[Malice]

hey thanks for that help. That makes a lot of sense. That also helps out a ton seeing your set up and what you are getting out of it. 20 mpg sounds pretty good. I guess i didn't realize just how light the back of those trucks are.

[1project2many]

It's not just the light truck but the weight distribution that matters. They're front heavy in a big way. The engine should really be set back 3-4" which suggests a LCF cab instead of traditional cab.
My truck might make 300hp out of the 302 and it will light up the skinny stockers... and haul a trailer full of car parts and mechanics tools across the country at 70-75 mph with no problem. In fact, the poor old drum brakes were giving up long before the end of the offramps since the trailer had no brakes. But then the wheel bearing on the trailer let go and the wheel-n-hub sailed off the highway just outside Indy. That incident got me a free "upgrade" to a tandem axle trailer with brakes and just might have extended my life by many years. But that's a story for a different thread.

Anyway, where's the power curve of your engine? A 5.3 in the 500-600 hp range is likely to have a fairly high rpm power peak and you generally want to match the cruise rpm to the power curve. Low rpm cruising is great for mileage if the engine makes low rpm power because the peak torque is where fuel efficiency is best. Check your cam's torque range and try and set cruise to cover that rpm if it's not too high. There are plenty of online calculators to help with that task. Don't forget to include the overdrive ratio of the trans... .72 if my memory is working tonight. My '57 will reach 18 mpg in lean cruise mode with the stock 4 speed and it was built long before I knew how to build an efficient engine. A good setup with a 5.3 should surpass that mark.