Not a preferred gear ratio, but approximately 3.5% longer.

Hello!

I now have legally increased the size of my tires on my A4 Avant AFN by approximately 3.5%. Instead of 205/60R15, I am now using 205/65R15, and I have also had this size officially registered with the TÜV (German technical inspection authority). Although the tires have a slightly larger circumference, my speedometer is still 5% (at 120km/h, and even more at lower speeds) inaccurate, according to measurements taken by ADAC (German automobile club). The speed displayed by the climate control system is probably quite accurate, as it is currently showing a slightly lower speed.
Furthermore, I have inflated them to 3 bar at the front and 2.5 bar at the rear, which should further increase their circumference. The tachometer reading was 2 bar.

I'm curious to see if and how it affects consumption. If I can't find any increased consumption, then it would actually be a 3% saving!

Greetings
Udo

Udo wrote:

I have now legally increased the tire size by approximately 3.5% with different tires on my A4 Avant AFN. Instead of 205/60R15, I am now using 205/65R15, and I also had this size registered during the TÜV (German technical inspection).

Hi

I'm quite surprised about the TÜV registration, especially if it happened "out of the blue."
For my 195/60 14-inch tires instead of 185/60 14-inch, I obtained a certificate from the internet, which clearly stated that no speedometer adjustment is required.
And that's exactly the point that the TÜV inspector questioned quite precisely (besides the roadworthiness, of course, since I'm not driving the aluminum wheels according to the inspection report).

Or was about your case, was there any other document, e.g. Manufacturer's approval?

Quote:

Furthermore, I have inflated them to 3 bar at the front and 2.5 bar at the rear, which should further enhance the overall size. The tachometer reading was, however, 2 bar.

You can forget about that wish. Roughly speaking, it only works the other way around: Reducing the pressure below the manufacturer's recommendation reduces the rolling circumference, but above that, the tire no longer "grows" significantly (before it bursts ).

Quote:

I'm curious to see if and how it affects consumption. If I can't find any increased consumption, then it would actually be a 3% saving!

Probably, you will experience a higher fuel consumption with the same driving style, as indicated by the speedometer and odometer:
1. due to the constant increase of 3% in real speed
2. due to the 3% larger actual distance covered.


Greetings
Udo

Hello Ulf,

I called the TÜV and asked what I need to do to get an alternative tire size registered. Answer (approximately): Put on the tires and come by to check for clearance, and since the tires have a larger circumference, I should also bring a tachometer reading.
Had the tires fitted (same wheel as 205/60, AEZ 7JX15), went to the ADAC testing center for the speedometer check (10 EUR), and then to the TÜV.

When registering, the TÜV inspector initially paused, because the ABE (approval document) for the wheels did not include the 205/65 tires, but then still charged 39 EUR.

The inspector then positioned the car with the front left wheel on wooden blocks and the rear right wheel on wooden blocks, checked the steering, made a turn, checked again, and then performed a test drive (driving in a circle until the brakes squealed + full braking) and that was it.

Regarding consumption, I will report whether I am now (apparently) consuming 3.5% more or whether the lower RPMs compensate for it...

Greetings
Udo