For classic GM TH350 transmissions, converting to an electric speedometer is a common upgrade for reliable, modern readings. This guide highlights top options tailored to different needs—whether you’re restoring a vintage ride, outfitting a daily driver, or dialing in precise speed readings for performance setups. Each product is chosen to suit specific buyer types, from purists to performance enthusiasts. The following brief table compares the key picks at a glance.
| Product | Best For | Why It Stands Out |
|---|---|---|
| B&M 20299 | Classic restoration with robust protection | Billet aluminum, leak prevention, easy maintenance |
| DAYTONA Adapter | Motorcycles/ATVs converting to electric speedometers | Simple mechanical-to-electric conversion, recalibration required |
| Antrys TH350 Kit | TH350 restoration with seals and secure mounting | Comprehensive seals and bracket for reliability |
| GOLKNHA TH350 Gears | Gear-driven speedometer upgrades | Multiple tooth options, included housing |
| GOLKNHA TH350 21-Tooth | Higher customization for gear ratios | Extra gear and housing kit |
B&M Racing 20299: Transmission Speedometer Port Plug
The B&M 20299 is a billet aluminum transmission speedometer port plug with an engraved B&M logo. It helps protect the transmission from contaminants during use or storage and provides a replaceable O-ring seal for simple maintenance and leak prevention. This option is best for owners seeking a clean, durable plug that preserves the original hydraulic system while maintaining a tight seal. Caution: Not a full gear replacement kit—it is a port plug meant for protection and sealing rather than gear engagement improvements.
DAYTONA Motorcycle Speedometer Conversion Adapter
The Daytona 93390 adapter converts a mechanical speedometer output to an electrical signal suitable for electronic speedometers on motorcycles or ATVs. It’s best for riders who want to convert existing mechanical gauges to electric readings with calibration per the installation guide. This is a flexible solution when the vehicle uses a mechanical drive but a digital or electric display is preferred. Limitation: Recalibration is required to match the new sensor output to the chosen electric gauge.
Antrys TH350 Speedo Housing Kit
The Antrys kit addresses a complete TH350 speedometer setup (1969 & up) with a steel housing, extra oil seal, O-ring, retainer bracket, and bolts. It ensures a secure drive assembly, prevents leaks, and maintains accurate gear engagement. Best for restorations or builds where durability and leak prevention are priorities. Choose this if you want a robust mounting and a secure seal to protect fluid levels; avoid if you only need a simple plug without housing components.
GOLKNHA TH350 22-Tooth Gear with Housing Seals
This GOLKNHA package provides a 22-tooth speedometer driven gear with seals and a housing kit, designed for GM TH350 transmissions. It’s best for owners needing a gear-driven solution with seals to prevent leaks and ensure reliable operation. It’s a good match if you’re aiming for precise speed readings across a range of tire sizes. Limitation: Gear tooth count must match your transmission and tire ratio for accuracy.
GOLKNHA TH350 19-Tooth Gear and Housing Kit
The 19-tooth gear and housing kit from GOLKNHA includes a 19-tooth driven gear with seals and a housing kit with retainer bracket. It is best for setups needing a lighter gear ratio or specific speedometer calibration. Best for users who need a compact, integrated kit that includes seals and hardware. Caution: 19-tooth configuration may not suit all rear-end gear ratios—verify compatibility with your tire size and gearing.
Etersec 345215 Speedometer Block Off Plug
The Etersec 345215 block-off plug is designed for GM TH350 and TH400 transmissions (1968-1981). It seals the speedometer port with a precision O-ring for leak prevention while providing a clean delete option when a speedometer is not used. Best for builds where the speedometer is removed or temporarily disconnected. Limitation: Not a speedometer conversion kit; it blocks the port rather than converting it to an electric readout.
Speedway Motors TH350 Master Gear Kit
The Speedway Motors TH350 Master Gear Kit includes drive and driven gears, a housing bullet, clips, bolts, and O-rings. It is designed to deliver accurate speed readings across various tire sizes and rear-end gear ratios. Best for enthusiasts who want a complete, all-in-one kit for flexible speedometer setup. Caution: Requires careful matching of gear counts to your tire and rear-end setup to achieve accurate readings.
Buying Guide: Key Considerations for TH350 Speedometer Upgrades
- Purpose and vehicle type: Are you restoring a classic, or tuning a modernized street car? Choose components that align with your goals, such as protective port plugs for preservation or full housing kits for reliability.
- Gear ratio and tooth count: Ensure the driven gear tooth count matches your tire size and rear-end gearing to achieve accurate speed readings. When in doubt, consult your drivetrain reference or a professional installer.
- Seals and leak prevention: Prioritize kits that include seals, O-rings, and brackets to prevent leaks and maintain proper fluid levels.
- Compatibility: Confirm that the kit supports your specific TH350 model year and that the hole size (7/8″) and mounting arrangement match your transmission.
- Installation scope: Decide between a simple port plug, a complete housing kit, or a full gear set with housing to determine labor time and complexity.
- Calibration needs: Some mechanical-to-electric conversions require recalibration of the speedometer. Check the installation instructions for calibration steps and reference points.
Frequently Asked Questions
- What is the TH350 speedometer port plug used for?
Answer: A port plug seals the speedometer port to prevent leaks and protect the transmission when the speedometer is not in use or when a protective seal is desired. - Do I need to recalibrate my speedometer after installing an electric adapter?
Answer: Yes, many electric speedometer installations require recalibration according to the gauge’s instructions to ensure accurate readings. - Can I replace only the speedometer gear on a TH350?
Answer: Yes, driven gear replacements are common for matching tire size and gearing, but you may also need a housing kit for a secure seal. - What should I look for in a TH350 gear kit?
Answer: Look for correct tooth counts, included seals and brackets, and hardware such as O-rings and retainer bolts for leak prevention and reliable engagement. - Is a complete housing kit better than a simple plug?
Answer: A complete housing kit offers a more secure mount and leak prevention, making it preferable for reliability on restored or heavily driven vehicles. - How do I know which tooth count to choose?
Answer: Tooth count should align with your specific rear-end gear ratio and tire diameter; verify with your vehicle’s drivetrain specifications or a professional.