1 - 12 of 12 Posts

Joined

·
2,192 Posts

But a higher gear ratio will also get you moving from a stop much faster. It's a trade off between quicker take off and better gas milage as highway speeds.

The gears are actually a torque multiplier. Generally the thought is to select a set of gears that will get your vehicle into it power band (the RPM level where the engine is producing it best power) as soon as possible through all the gears. There is some formula to this, off hand I don't have it or have it memorized, most people don't really use it.

In the past you could have a vehicle that had good acceleration off the line or you could have a vehicle that got acceptable fuel economy but not really both. With today's overdrives you can actually have the best of both worlds. So anytime you select a set of gears you should or it would be my advise to consider the ratios in the transmission. A 2.73, for example behind a 350, at 60 Mph with 26" dia tires your engine should be turning about 2100 RPM. Give or take a few RPM this is going to be a constant. If you change tires then that changes the dynamics of the rear end gear ratios.

Factory sets the gears in accordance with some formula that sets a ratio of a certain amount for each gear change. Hot rodders are mainly concerned with getting the engine into its power band as soon as possible and keeping it there as long as possible. For example: the 2.73:1 in my 76 is behind a 350 with a final drive ratio of 1:1. My the specs on my cam say that I should expect to start producing power between 1800 RPM through 6500 RPM. If you note that with the 2.73:1 rear end gears @ 60 I am only producing 2100 RPM which is just inside the power band but inside that power band at 60 not at take off.

Before deciding on a differential gear ratio, let's look at a few facts. The GM 700R4 four-speed's final drive ratio (forth gear) is 0.70:1 (20 percent) of that on a 400 or 350 Thd. Installing 4.10:1 gears would provide the same effective final drive ratio as a 2.87:1 ring-and-pinion. This is much better than the 3.11:1 and 3.08:1 final ratios without an overdrive. The stock GM gearing is suitable for street use given EPA constraints and Gas prices at the time of manufacturing.

If we calculate the rpm at freeway speeds with 4.10s, we have to consider speed, tire size, transmission final drive ratio, and ring-and-pinion ratio. For our calculations, we can use the following formula: mph x ring-and-pinion ratio x transmission - final drive ratio x 336/tire diameter = rpm. For example, 60 (mph) x 4.10:1 (differential gear ratio) x 0.70 (overdrive ratio) x 336 (constant)/26 (tire diameter) = 2,225.4 (rpm). Applying the same formula for 4.56:1 gears gives us 2,475 rpm at 60 mph.

Applying the same formula for a Thd 350 or 400 with 4.10:1 gears gives us 3,179 rpm at 60 mph. The 700R4 has a first gear ratio of 3.06:1 and about 40% reduction to a 1.63 Second and third is 1:1. With proper calibration the 700R4 can be a good overall transmission. It provides a good economy on the highway and good out of the hole times for a fun street machine without being particularly hard on the wallet.

A good site is : http://www.ringpinion.com/tech/techind.html this will give you additional information.

Joined

·
881 Posts

Tire Height and RPM Calculator

i would guess so if you wanted to bad enough but generally no, and most all transmission shops will probably say no also. In the Overdrive catagory you have a fare selection of OD trnsmissions that have different ratios. Well actually three that I know of. The 200R4, 700R4 and the very expensive 4L80e. Each offer a different gear ratio and each have their strengths and weaknesses. Well not the 4L80e I don't know if it having a weakness at all other than the outragous cost of installing and running one.guru54 said:i know he was talking rearend but i was just wondering if u can switch the transmissions gear.

I was reading where GM is putting out a newer version of the 4L60e (700R4 computer controlled) its called a 4L65e and they did that, to my understanding, for the new 405 Hp. vettes. It can handle more torque than the older 4l60e did, I don't know if they changed the ratios.

1 - 12 of 12 Posts

Join the discussion

El Camino Central Forum

A forum community dedicated to Chevy El Camino owners and enthusiasts. Come join the discussion about performance, modifications, troubleshooting, maintenance, and more!

Full Forum Listing
Explore Our Forums

Recommended Communities

Join now to ask and comment!