E85 / flex-fuel conversion: UK vs US vs Germany
Running ethanol changes the emissions behaviour of an engine, which is why an uncertified conversion kit is treated as tampering rather than tuning.
All three markets reach the same verdict, conditional. What differs is the rule each one uses to get there, which is the third column.
| Market | Verdict | The rule that decides it |
|---|---|---|
United Kingdom DVSA / GOV.UK |
Conditional | Must still meet its exhaust-emissions requirement The catalytic converter and lambda sensors must keep working. A quality flex-fuel kit that keeps the car compliant is the safe route. |
United States EPA, NHTSA and state codes |
Conditional | EPA-recognised certified conversion system A factory flex-fuel vehicle needs nothing. Converting a non-flex car needs a certified system, plus a CARB EO in CARB states. An uncertified kit is treated as tampering. |
Germany StVZO / KBA |
Conditional | No retrofit approval scheme exists The conversion changes emissions behaviour so the type approval lapses without inspection, and the only route is an individual approval with evidence. Kits without German test documentation are effectively unregisterable. |
Germany is the strictest by structure: unlike LPG, there is no retrofit approval scheme for ethanol at all, so individual inspection is the only route.
Why the answers differ
Can you convert a car to run E85?
Ethanol changes the emissions behaviour of an engine, which is why an uncertified conversion is treated as tampering rather than tuning in all three markets. The US has the most developed framework, recognising certified conversion systems federally and requiring an exemption order on top in CARB states, so a compliant path genuinely exists. The UK requires only that the car still meets its emissions requirement with the catalytic converter and lambda sensors working. Germany is the strictest by structure rather than by attitude: unlike LPG, there is no retrofit approval scheme for ethanol at all, so the only route is individual inspection with evidence, and kits without German test documentation are effectively unregisterable no matter how well they work.
Verdict at a glance
The same three markets, grouped by where they land.
Conditional
ConditionalUnited Kingdom, United States and Germany. Permitted, but only once the condition in the table above is met.
Sources
Primary legislation and regulator guidance behind each market's verdict.
- UKModifying your vehicle's emissions (GOV.UK)
- UKTell DVLA about changes to your vehicle (GOV.UK)
- USClean alternative fuel vehicle and engine conversions (US EPA)
- USAftermarket, performance and add-on parts (California Air Resources Board)
- Germany§ 19 StVZO - Erteilung und Wirksamkeit der Betriebserlaubnis (gesetze-im-internet.de)
- Germany§ 21 StVZO - Betriebserlaubnis für Einzelfahrzeuge (gesetze-im-internet.de)
Other cross-market comparisons
This is a comparison of national rules, not legal advice for your vehicle. Rules change, and the US row in particular summarises federal law where one exists and the common pattern across states where it does not. Confirm the position in your own state or region before you buy or fit anything. Sources last reviewed July 2026.