Thread: Ok time to find out my correct gearing needed for rear end

Tweet

First I need to find out whats in there. I jack up the 89 suburban and mark the drive shaft and tire and count the number of times one turns compared to the other , then divide??

I have made some up grades in performance and added some 30" tall tires and have a 700r4 for a trans. I need to find out what gears I need to make all this work better with each other. This surbuban will mainly be used for high way trips pulling a trailer. Once I get the gears in there I will start to get the computer tuned to get the most mileage I can out of it but thats another story. Any links on calculators that will get me the gearing close to where I need to be? Or any advice, thanks

First off, let's get the nomenclature nailed down for those who are just learning on this board. We normally refer to rear differential gears as being "tall" or "short". For instance, a very tall gear would be a 2.50:1 (two-fifty to one), while a very short gear would be 5.13:1 (five-thirteen to one). These are just examples. There are hundreds of different combinations to choose from when deciding on the final drive gear ratio. This ratio is calculated by dividing the number of teeth on the ring gear by the number of teeth on the pinion gear.

A 700R4 transmission has the following gear ratios which must be multiplied by the gear ratio in the rear differential to arrive at a final drive ratio.
First gear: 3.06:1
Second gear: 1.63:1
Third gear: 1.00:1
Fourth gear (overdrive) 0.70:1

Now, if we used, for instance, a 3.00:1 rear gear, then multiplying 3.00 times 0.70 would reveal a final drive ratio of 2.10:1 in fourth gear. Depending on the torque produced by the motor, a final drive ratio this tall might lug the motor to the point where you would have to use more throttle to pull the vehicle and might therefore negate the advantage of lower revs which were meant to increase fuel mileage. On the other end of the spectrum, first gear would be about right. 3.00 times 3.06 would give you a 9.18:1 final drive ratio for starting off from a stop.

If we used a 4.00:1 rear gear, then multiplying 4.00 times 0.70 would reveal a final drive ratio of 2.80:1 with a first gear ratio of 12.24:1 which would work very well for starting off with a heavy load.

Of course, tire size works into the equation as well. A taller tire like a 30" tire will give an EFFECTIVELY higher final drive ratio. A shorter tire like a 26" tire would give you a shorter EFFECTIVE final drive ratio.

As an aside, the 700R4 is less than ideal in a drag race car because of the very wide spread between first and second gear. The revs drop drastically and may drop the motor out of its best operating range.

OK, tutorial is over. Let the discussion begin

Tech - I was running right with you until you added the tires into the quandary.

So if I went to a 700R with my current rear end gears I wound end up with a final rear drive of 1.91:1 in fourth which wouldn't yield enough power to get out of my own way let alone somebody else.

So now, With my 28" tires, I going to have to look to changing out my rear gears from the existing 2.73 to at least a 3.55 to maintain a balance of power and efficiency. Guess I'm going to have to wander on down to the Corvette shop and see if they have the rear gears I need. I got a feeling this ain't going to cheap or painless.

Bob, I seem to remember a thread on this subject from years ago and as well as I can recall, it seems the consensus was in the 3.50 to 3.70 range as a reasonable balance, so you're choice of 3.55 would seem to be right in the ballpark.

The 3.55:1 ratio is very popular in hot rodding and for your application seems about right also. But I think this is a Ford ratio. In 10 bolt rears chevy has 3.42:1 and 3.73:1. I would lean more towards the 3.73:1 with your over-drive.

Kitz