Speedo Ratio Box – 

Der Fritz SRB-

For Vintage VDO Speedometers

Used in Aircooled Volkswagens and Porsches

Introducing the Fritz SRB – Speedometer Ratio Box – Vintage VDO
Finally, a proper bolt on solution for DIY recalibrating vintage VDO speedometers used in classic Aircooled Volkswagens and Porsches with OEM style M18x1.5 threaded input

The Fritz SRB is a fully mechanical, inline ratio gearbox that allows you to adjust your speedometer reading in minutes — no electronics, no permanent modifications, no sending your gauge out for service. I designed a step spread of about 2.5% with a goal reading within +/- 1mph o f GPS at 60mph.

Simply choose your desired ratio by selecting pulley sizes, install the box inline using factory M18x1.5 cable ends, and dial in your speedometer exactly where you want it. Underdrive (slammed cars) or swap pulleys side to side for overdrive (big tire guys).
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Why It Exists
If you’ve lowered (or raised) your car, changed tire diameter, swapped transmissions, or installed a different gauge, you already know the problem: Your speedometer reads wrong.

Vintage VDO speedometers are purely mechanical. They rely on a spinning magnet and calibrated spring tension. Small changes in tire size, stance, cable drag, or internal wear can throw them off — and often not in a perfectly linear way.

Sometimes it reads 5–10 mph fast.
Sometimes it’s accurate at 30 mph… and completely off at 65.
Sometimes it’s just inconsistent due to age and cable wear.

Until now, your options were:
• Live with it.
• Send the gauge out for recalibration.
• Somehow adapt a noisy domestic ratio box with mismatched threads.
• Keep pretending that stock 36hp can actually bury the needle, while getting passed by a Prius.
The Fritz SRB was designed specifically for our cars.
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How It’s Different
Unlike traditional steel-gear ratio adapters, the Fritz SRB uses a precision toothed pulley and belt drive system to control output shaft speed.
Think supercharger drive — but for your speedometer.

This design offers several advantages:
• Ratio steps in 2.5% increments up to 12.5% using GT1.5 Fine Toothed pulleys and 6mm belts
• Ratio steps in 2.5% increments 15%-30% using GT2 toothed pulleys and 6mm belts
• Quiet operation (no steel gear noise inside the cabin)
• Smooth, backlash-free engagement
• Easily swappable pulleys with multiple ratio options using a small selection of pulley sizes
• Pure mechanical goodness, no modern electronic nonsense.
• No Special skills needed or requirements to ship to anyone to reconfigure, adjust, or rebuild.
Change ratios with basic hand tools. Reset the tensioner. Done.
It’s built for enthusiasts who actually work on their own cars or some Porsche guys who pretend to

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Two Versions Available
V1 – Direct Mount
Threads directly onto the back of the speedometer gauge.
V2 – Remote Mount
Installs inline using dual cables, similar to the factory 1970s EGR speedometer setup (Supers and Bay Windows).
Uses commonly available OEM-length cables designed for EGR-equipped VWs.
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Fitment
Designed for mechanically driven, cable-operated VDO speedometers with M18x1.5 threaded connections, including:
• VW Type 1 Beetle & Ghia
• VW Type 2 Bus
• VW Type 3
• VW Thing
• Many Aircooled Porsche models
• Any 6V and 12V systems (mechanical drive only – no electronics

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Built for Real-World Cars

I designed the SRB after dealing with inaccurate speedometers on my own lowered cars. I wanted something adjustable, adaptable, and DIY-friendly — something you could tune in your garage, or reconfigure when you change tires, stance, or even swap the car entirely and worked on Stock 6V systems. I started designing my own DIY box, and it soon escalated into what you see here. I figured, Im not the only VW who wants one, so I made a batch.

This isn’t a generic adapter made for domestic applications. It’s purpose-built for vintage VDO Gauges specifically.
Vintage VDO speedometers are simple, mechanical things — which I like — but they don’t automatically adapt when we start modifying the car the way VW people always do. After failing to tweak the tension spring, busting 60 year old plastic time after time and refusing to pay for constant recalibration by specialty shops that 1000 miles away every time I changed a setup, I decided to build something adjustable that I could tune in my own garage. The SRB exists because aircooled Volkswagens are never left alone, and I wanted a clean, mechanical way to recalibrate the speedometer without permanent mods, electronics, or reinventing the whole system every time I changed the stance or tire size. I based it the pulley tooth count and belt design making 2.5% changes for simplicity sake up to 12.5%.  and a different pulley family for 15%-30% in larger steps for those in need.