Gear Vendors Overdrive Unit, Chevrolet & GM 3 Sp Automatic Transmissions

Gear Vendors Save Gas or Diesel Fuel,  Extend Chevrolet or GMC TH350 and the Turbo 400 Mileage, 20% increase in fuel economy, Extend Engine Life, Start Saving Money Today!

Does your Chevrolet or GMC need more power to climb hills or tow with heavy loads, plus at the touch of a button taller gearing for cruising down the highway with the lower engine RPM's and improved fuel mileage, see application guide.  You are also going to actually make more horsepower and enjoy overdrive cruising better with a Gear Vendors overdrive.  For cruising you will have a dash switch that allows both manual and automatic operation.  Gear-Splitting is that feature which has made Gear Vendorss so popular.  Our Gear Vendors overdrive for Chevrolet GMC are available for automatic transmissions using:  TH350 and the Turbo 400. Our Chevrolet GMC overdrive transmission provides 500 to 600 less RPM's plus 20% increase in fuel economy. You need an auxiliary overdrive transmission capable of gear splitting behind your transmission. The toughest built, best performing auxiliary overdrive transmission you can buy is the Gear Vendorsover drive from

See How It Works, Guaranteed Amazing Performance for Your Chevrolet or GMC

How it works, gears are multipliers of torque and our Ratio Splitting capability allows you to shift to a gear with more mechanical advantage than the next stock gear in automatic transmissions and wide ratio manual transmissions. This also means engine rpm drops about half what it did without the Gear Vendorss and so rpm is sustained - higher torque and rpm on shift = more Horsepower.   When shifted into overdrive final gear reducing both rpm and torque to reduce the manufacture of HP when steady cruising to improve fuel mileage and decreases wear on your engine.  Two-year warranty rated at 30,000 pounds applications for Chevrolet and GMC, most beneficial when used in commuters, tow vehicles, 5th wheels and Motor homes. Our overdrive kits are very complete and are designed for your specific vehicle, detailed installation instructions are included in kit, and installation is also available. 20% increase in fuel economy with Gear Venor Overdrive See video presented by Gear Vendors at SEMA 2008 in Las Vegas.

Related to fuel economy, performance, and emissions — Gear Vendors overdrive units include state of the art electronics circuits that keep your vehicle in the optimum performing gear. Whether you are driving a older non-computer controlled vehicle or a late model with all the latest emissions computers our electronics provide the interface to ensure maximum performance and correct rpm without lugging or over reving in the wrong gear. The OE factory programming remains intact and our computer processor just makes it aware of the actual ratio 1,000 of times per second.  In older vacuum or throttle linkage transmissions the stock governor on the output shaft instantly slows down the moment the overdrive shifts and so automatically moves the shift point in any overdrive gear up by the correct 28.6%.  So both electronic and non-electronic automatic transmissions gain a group of features including a passing gear or climbing gear that is automatically available without your intervention and is at the split between 2nd and 3rd where you really want it.  Need more information on how to save gas or diesel fuel? 

Gear Vendors's Complete Kits Ensure a Smooth Installation for You

Our Gear Vendors overdrives are supplies with specific adaptation kits for each model of transmission model, the kits are  very completed.  By supplying specific extension housings to each model/brand transfer case Gear VendorsS is able to have a better product with increased engineering benefit to the final installation. Fact is, when you are in 4X4 there is very little desire to use GearSplitting and overdrive unless you are racing (and we do make the in-between kit for that purpose), it's 2wd where the benefits of overdrive and additional gears are most needed. All our kits bolt directly to the rear of the transmission, you can manually control the unit or we suggest you flip switch when leaving city limits sign and turn it off when you get to next city limit sign.

No additional crossmembers are required and you retain the factory ground clearance instead of moving the transfer case rearward which puts it at or nearer the middle of the wheelbase where ground clearance is most critical. Read customer testimonial.

Free Technical Support, Get Expert advice about your vehicle's performance with our Gear Vendorss Overdrive. Call toll free at 800-216-1632. More fuel milage 20%, 600 RPM reduction in Engine RPM means longer life and you save Money!

You get all the benefits of the GearSplitting performance and overdrive economy (see your appropriate transmission under GM/Dodge/Ford 2wd) and the best auxiliary transmission you can buy. Give our overdrive expert a call toll free 800-216-1632 he can answer your questions and help you pick the correct model for you application, also get free telephone support during installation when you purchase your parts    Free shipping within the continental United States only.

Gear Vendors Overdrive Unit, Chevrolet & GM 3 Speed Automatic Transmissions

Gear Vendors GM Overdrive Kit Chevrolet Tubo 350, 20% More Mileage
  • You need quality.
  • You'll want reliability.
  • You want performance 22% Overdrive.
  • You want better mileage 20% more fuel economy!

YOU WANT A Gear Vendors


Bolt on Perfromance and Economy! 
  • Save Fuel
  • Extend Engine Life
  • Save Money
  • 30,000 lbs Capability


Select your vehicle from the following table to see detailed information on improving your vehicles performance and saving money today! For detailed information on General Motors 3 speed automatic transmission with our overdrive unit select from one of the following:

GM 3 Speed Automatic Truck/Motorhome   GM TH350 3 Speed Automatic Final Drive Ratio
GM 3 Speed Automatic Performance Car/Street Rod   GM TH375/TH400/TH475 3 Speed Automatic Final Drive Ratio
    GM 3 Speed Automatic Application Chart & Pricing

GM 3 Speed Automatic Truck/Motorhome

With the Gear Vendors dash switch in “auto” you will have overdrive just as if the vehicle came from the factory with our system. The dash switch can either be mounted in our under dash bezel or the custom in dash bezel like the Fleetwood Limited's that came factory with our overdrive. This overdrive will shift on and off without your attention and can be used while towing up to combined weights of 30,000 lbs. An interactive floor switch is positioned for the left foot (just like a headlight dimmer) to give manual control at any moment so that you can turn the overdrive on/off for performance and gear splitting.

Performance while accelerating or climbing grades is greatly improved by the availability of up to 6 forward ratios. With approximately 1/2 the drop in engine rpm on each gear change and the ability to downshift at 1/2 gear intervals (much sooner than with the stock only transmission) performance gains of over 40+horsepower and 60lb ft. of torque are typically minimum.

Most importantly you are getting the horsepower and torque without modifications in any way to the engine. Any other way of increasing power (chips, induction, exhaust, fuel) all put stress’s beyond the original equipment. With the Gear Vendorss OVERDRIVE you get this power because gears are multipliers of torque, plus the engine can be kept in a narrower (better performance) rpm range when power is needed. For this reason, if you do have performance modifications our overdrive also helps them work to their maximum potential. Not only do you get performance without engine modification, but the overdrive will pay for itself in fuel mileage alone. Add to this the wear savings you get from slowing the engine down 22% so that the same rpm's that currently take you only 500 miles will now take you 640 miles.

Most vehicles will experience mileage increase in the 20 percent region if driven at the same top speeds as they are without the Gear Vendors. Increased mileage and reduced engine wear is why all vehicles today come with factory overdrive.

The Gear Vendorss OVERDRIVE will do more for this vehicle then any Underdrive or Overdrive on the market. You will get all the features normally associated with an underdrive except a gear below first, plus all the economy ofoverdrive. Because Gear Vendors is the only product you can shift under throttleyou will have far greater performance and economy. The Gear Vendors comes with thebest money back guarantee . There is not room here to discuss every situation that a rises where additional gears and torque multiplication help. Look at the final drive ratioand then call and get one, you will absolutely be glad you did.

GM 3 Speed Automatic Performance Car/Street Rod

At about $2600 dollars (depending on models) the Gear Vendorssis not the cheapest way for you to get overdrive (a rebuilt wreaking yard 700r4 is probably cheaper) but the Gear Vendors is by far the best way. Horsepower handling is the first issue.  Our overdrive will handle 1200+ horsepower reliably. This is why you have seen Gear Vendors in 8 second 1/4 mile cars in publications like Hot Rod and National Dragster.   This is also why all GM Callaway Corvettes with automatics came with a TH400 and Gear Vendorss instead or the 700R4 which is the base Corvette transmission. If you’re running more then 1200 horsepower (and we have customers that are) then talk to us as there are some hydraulic changes we do for vehicles up to 2000hp.

Horsepower aside, there are several other reasons you will prefer a Gear Vendors to any factory overdrive automatic. You will have a dash switch that allows both manual ad automatic operation. With the switch in “auto” the vehicle will act just like it came from the factory with overdrive. The overdrive will kick in at 47 mph or whatever the EPROM is set for in our electronics and turn back off at slower speeds without attention of the driver. The ratio is .78 to 1 (see the final drive ratio charts). This equates to 28% faster vehicle speed at the same rpm. Or, take your existing rear gear multiplied by .78 (i.e. 4.10 X.78=3.20). In this example a vehicle with a 4.10 behaves just like it had 3.20s when the overdrive is engaged. This is just the right amount of overdrive as any taller would require a lock up converter or create excessive converter slippage. With the .78 to 1 ratio you’re good with any torque converter you chose for high performance street/strip.

Another great feature of the Gear Vendors is that you will have a full 6 forward ratios when you switch to manual on the dash switch pull into low, accelerate, then hit our button (on the shifter or the floor), a green light appears indicating overdrive and your in first over which is right between 1st and 2nd . Then shift to 2nd and turn us off. Then rev back up and hit the button for 2nd-over. Then 3rd and 3rd-over. See more information on Gear Splitting in the Drag Racing section. The performance and sound of the motor is tremendous as it goes through 6 full throttle close ratio shifts. You are in each gear 1/2 the time you are now so the car is not only faster but it also feels and sounds much faster.  It is the gear under your seat that counts.

Gearing is what you are buying when you purchase a Gear Vendorss Under/Overdrive. If you take a few moments to study the gear chart for your transmission and rear end ratio combination you will se why our overdrive is so popular.

Final Drive TH350 3-Speed Automatic

Axle Ratio

 Rear End Ratio  






  Transmission Ratio

Final Drive Ratio


1st 2.52






Over 1.97






2nd 1.52






Over 1.19






3rd 1






Over 0.78






Final Drive TH375/400/475 3-SPEED AUTOMATIC

Axle Ratio

  Rear End Ratio 4.88 4.56 4.1 3.73 3.42 3.08
  Transmission Ratio

Final Drive Ratio

1st 2.48 12.1 11.31 10.17 9.25 8.48 7.64
Over 1.93 9.44 8.82 7.93 7.22 6.6 5.96
2nd 1.48 7.22 6.75 6.07 5.52 5.06 4.56
Over 1.15 5.63 5.27 4.73 4.31 3.95 3.56
3rd 1 44.88 4.56 4.1 3.73 3.42 3.08
Over 0.78 3.81 3.56 3.2 2.91 3.67 2.4

Please Click on Part Number to buy it online 

Chevrolet/GMC Application Guide 2 Wheel Drive 3-Speed Automatic

Part Number




3DFS350A Automatic 3-Speed T-350 9" Tailhousing. Replaces 9" Tailhousing.  See kit in above photo, TH350 Medium Length found in long wheelbase.
3DFS351A Automatic 3-Speed T-350 6" Tailhousing. Replaces 6" tailhousing. TH350 Short most common on the 350 series.
3DFS353A Automatic 3-Speed T-350 Shorty. Cut 2" off of output shaft. 70 1/2 to 81 Camaro/Firebird and Short wheel base street rods. 
3DFS376A  Automatic 3-Speed TH 375 9" Tailhousing. Replaces 9 1/2" tailhousing.  60's-70's cars with 27 or 32 spline output shaft and mechanical speedometer. Provide output shaft spline count when ordering. 
3DFS400A Automatic 3-Speed T-400 Mech Speedo. Replaces 4" tailhousing.  See TH400 Kit Big block Chevy and GMC.
3DFS401A Automatic 3-Speed T-400 Elec Speedo. Replaces 4" tailhousing. 88-91 3/4 and 1 ton with electronic speedometer.

Chevrolet & GM 3 Sp Automatic Transmissions 

We have a 1965 Chevelle Malibu SS Pro Street. It has a 355 CI Blown engine with a Crane Roller Cam, 488 spooled rear end, four link suspension, B&M TH 400 Automatic.  We took interest in Gear Vendorss when trying to figure out how tolower RPM in overdrive at highway speed. First consideration was to install a 700R4 with overdrive but didn't want to loose the beefy TH400. With the spooled 4:88 rear end and the 31x18.5-15 Mickey Thompson tires the Chevelle hooks up nicely and feels nothing but nasty. The only problem, at highway speed the RPM was not desirable. After receiving helpful information from Gear Vendorss it wasn't hard to make the decision on which way to go.
chevelle_malibu_ss_pro_street.jpg chevelle1965_engine.jpg

With the Gear Vendors under/overdrive, it has given a reduction of engineRPM at highway speed that we can live with. Considering the rear end gears the car has, it is a great improvement, plus the additional bonus of changing the 3 speed to a 6 speed. It makes the Pro Street even more exciting to drive.

Thanks for a great product.

Gary & Barbara 

We have included a frequently ask questions section that may answer some of your questions, don't see answers you need, just give us a call we will be happy to review your application with you. The following links provide specific application information, performance data that you can expect and the price data for your specific vehicle.