25:1 Fuel Mix Chart – Rich Oil Mix Guide for 2-Stroke Engines
Some older or heavy-duty 2-stroke engines require a richer fuel-to-oil ratio, such as 25:1. This mix provides maximum lubrication and cooling—especially critical for vintage engines and those without modern materials or synthetic oils. This page offers an easy-to-follow 25:1 fuel mix chart, mixing tips, and answers to common questions.
What Does a 25:1 Fuel Mix Mean?
A 25:1 fuel-to-oil ratio means you need 25 parts of gasoline for every 1 part of 2-stroke engine oil. This is a significantly oil-rich blend, offering robust protection, especially for engines running hot or under heavy load.
For example:
- 1 gallon of gas (128 oz) requires 5.1 oz of oil
- 1 litre of gas requires 40 ml of oil
This ratio is often used in vintage chainsaws, outboard motors, and other early-generation 2-cycle equipment.
Why Use a 25:1 Ratio?
A 25:1 ratio is used when:
- Engines are older and built to looser mechanical tolerances
- Manufacturer recommendations call for a higher oil content
- You’re using mineral-based oil rather than modern synthetic options
- Engines are operating under extreme loads or high heat
Benefits include:
- Improved piston and cylinder lubrication
- Reduced engine wear in harsh environments
- Peace of mind when protecting irreplaceable vintage machines
To make it easier you can use the following old school reference chart.
Metric (Litres:Millilitres)
| Fuel (L) | Oil (mL) |
|---|---|
| 1 | 40 |
| 2 | 80 |
| 3 | 120 |
| 4 | 160 |
| 5 | 200 |
| 7.5 | 300 |
| 10 | 400 |
| 15 | 600 |
| 20 | 800 |
| 25 | 1000 |
US (Gallons:Fluid Ounces)
| Fuel (gal) | Oil (fl oz) |
|---|---|
| 1 | 5 |
| 1.5 | 8 |
| 2 | 10 |
| 2.5 | 13 |
| 3 | 15 |
| 4 | 20 |
| 5 | 26 |
| 7.5 | 38 |
| 10 | 51 |
| 15 | 77 |
How to Mix 25:1 Fuel Correctly
- Choose a clean, approved fuel container
- Measure the correct amount of oil using a marked bottle or measuring cup
- Add oil first, then gasoline
- Shake thoroughly to ensure the oil and gas are fully mixed
- Label the container with the ratio and date for future reference
Tip: Only mix what you need for the short term. Premix degrades over time, depending on the temperature where it is stored it might only last as long as a month.
Common Engines That Use 25:1 Mix
- Vintage chainsaws (pre-1980s)
- Classic outboard motors
- Older leaf blowers and string trimmers
- Vintage motorcycles or scooters
- Small industrial or agricultural engines
Important: Always check your owner’s manual or engine tag to confirm the required fuel ratio.
2 Stroke Mix App
If you’re looking for something to help with other ratios and remind you how old your fuel is, please take a look at our app for iPhone and Apple Watch on the front page?
FAQs – 25:1 Mix Explained
Q: Is 25:1 too much oil for modern engines?
A: Yes. Most modern 2-stroke engines are designed for leaner mixes like 50:1 or 40:1. Running such a rich mixture in these can cause excessive smoke, carbon buildup, and plug fouling.
Q: Can I use synthetic oil at 25:1?
A: Yes. Synthetic oils provide better lubrication and cleaner burns. Some users even run slightly leaner (e.g., 32:1) with high-grade synthetic oil, but only do this if you’re sure it’s safe for your engine.
Q: What if I accidentally mix too much oil?
A: While a bit of extra oil usually won’t harm your engine, it may cause fouled spark plugs or increased carbon deposits. If the mix is significantly off, it’s best to remix properly.
Q: Does 2 stroke fuel go bad?
A: Yes. Premixed fuel starts degrading after 30 days. Using a stabilizer can extend it up to 90 days, but fresh fuel always performs best.
Q: How do I dispose of old premix safely?
A: Never pour fuel down drains or on the ground. Take it to a local hazardous waste disposal or recycling center.