Classic Audi Technical Mech/Tech Suspension & Brakes » Alternative tyre size

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Old 17-07-17, 09:41 AM   #1
SwallownAmazon
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Default Alternative tyre size

Just a thought:
Has anyone fitted 205/60 x 15 tyres in place of 205/50 x 15 to an Audi 90 20v to reduce revs at a given speed? I know the speedo will be out but surely can be recalibrated. I am not interested in ultimate performance/roadholding, just driving around in a comfortable highish performance car that has given me a lot of pleasure over the years. My tyre dealer friend says that 205/60's were fitted as standard to Audi 90's "back in the day"; is he right or just going on number plate recognition software that may or may not be correct?

Rocket ship should be back on the road this week after the cam belt change and a few other niggly little things have been put right by the bloke who has looked after it since we acquired it in 1994. Sadly, I don't have the facilities to do major jobs on it now, especially as I can't get up off the floor without rolling over onto my front - old age!

Last edited by SwallownAmazon; 17-07-17 at 10:11 AM. Reason: RE-POSTED IN CORRECT SECTION.
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Old 17-07-17, 10:39 AM   #2
John.
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205/60's belong on the coupe. I'd stay with the 50 profile.
..........also if you had a prang and insurer spotted wrong size tyres, that'd be all they'd need to wriggle out!
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Old 17-07-17, 04:14 PM   #3
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How are you going to recalibrate your speedo?

I personally don't understand those people who want to decrease revs "just because", without good cause. Gear ratios have been chosen by engineers and they are as they are for some purpose. B3 quattros have shorter fifth gear to compensate for additional losses in transmission by having engine turning at rpm range where it generates more torque - so apparently in all B3 quattro saloons it's 3k @ 105 kmph in fifth gear - because A90q used a pair of identical boxes with these ratios, and I drove B3 A80q with 1.8 mono, and found there the same thing as well. It's also interesting to note that CQ uses essentially the same gearbox as B3 A90q, but with slightly shorter fifth gear, to compensate for bigger wheels, so it would still end up with 3k @ 105 kmph.

Anyway, back to technical solutions for your wish. First, wheel arches will barely accept that size - if it's fwd, it may rub in rear arches due to way how rear suspension works; I ran 195/65/15R on my A90q and it was fine, but then again - on quattro wheel moves up and down, not in circular motion. Also rim geometry becomes more critical and height and stiffness of the suspension more important. As for speedo, since it's size of B4/coupe, it also requires coupe or B4 speedo clock, otherwise there will be 5% less distance counted and speed indicated, per 100 km.



This is how did my A90q looked like on a 195/65/R15. As you see, the gaps are rather tight...
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Old 17-07-17, 04:51 PM   #4
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Way too big, stick with the standard 205/50 x 15 tyres, don't know about FWD drive ratio but my 90 20v quattro is geared around 22 mph per 1000revs giving a theoretical top speed of 140 at the red line and all the power is above 4000 so why do you want to drop the revs ?

Only the coupe's had the 60 seris tyres, saloons in B3 shape 20v format had 50 seris tyres, thats why the 20v coupe had same gearing as an S2 on same final drive ratio with a top speed in theory of 155 at 7000 revs. (coupe 205/60 x 15 tyre, S2 205/55 x 16 tyre, same rolling radius)

60 v 50 aspect ratio on a 205 15 inch tyre is a 7% increase in diameter way too much !
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Old 18-07-17, 11:27 AM   #5
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Quote:
Originally Posted by quattrodave View Post
why do you want to drop the revs ?

To reduce fuel consumption when cruising. Not interested in ultimate performance at my age !

60 v 50 aspect ratio on a 205 15 inch tyre is a 7% increase in diameter way too much !
That's why I asked the question. My friend in the tyre business used his employer's software to find the correct tyre size for my car very recently and he came up with 205/60 section on 15" rims as the recommended option. His software is clearly wrong and I insisted on 205/50 Hankooks which are apparently quite rare now (he had to get them in from his Portsmouth depot) hence my question about availability a month or so ago.
Thanks to all for the advice.
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Old 18-07-17, 03:54 PM   #6
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For fuel consumption reduction you have to look at different gearbox, not different tyres. Unfortunately you have 20v engine, it's both thirsty and made for high revs.
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Old 28-08-17, 09:15 AM   #7
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Quote:
Originally Posted by SwallownAmazon View Post
That's why I asked the question. My friend in the tyre business used his employer's software to find the correct tyre size for my car very recently and he came up with 205/60 section on 15" rims as the recommended option. His software is clearly wrong and I insisted on 205/50 Hankooks which are apparently quite rare now (he had to get them in from his Portsmouth depot) hence my question about availability a month or so ago.
Thanks to all for the advice.
different car, but im running 195/50/15 on my type 85 cq . 205/50/15's are just too expensive now imo.

original wheels were 14 inch, and since changing to 15's my speed no longer over reads and is now actually spot on at 30 mph according to the sat nav and roadside speed readers .
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Old 28-08-17, 09:33 AM   #8
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Quote:
Originally Posted by John. View Post
205/60's belong on the coupe. I'd stay with the 50 profile.
..........also if you had a prang and insurer spotted wrong size tyres, that'd be all they'd need to wriggle out!
If the 'computer' in the tyre shop is saying 205/60's are, then i doubt very much that the car insurer has any clue to the original fitment either.

These are old 'classic' cars , and information on the original factory specs will not be obvious.

Personally, i doubt they are going to care about a change in profile , not unless you were running a major stretched tyre or something.

Or you changed up to 17's or something. Then you might want to mention it.


Personally, my insurance company told me that on a classic car like mine they expect people to have changed wheels from a later audi, and they were not really bothered.

im running 15 inch imitation split rim speedlines from a later au di 80/90/coupe
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Old 28-08-17, 09:59 AM   #9
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Quote:
Originally Posted by Gaz View Post
im running 15 inch imitation split rim speedlines from a later au di 80/90/coupe


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Old 28-08-17, 01:28 PM   #10
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I am not going to chip in my 2 Penney's worth to this post about tire sizes and aspect ratios, but I have an interesting website in my book marks, have a look.
https://tiresize.com/tyre-size-calculator/
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