I have 165 x 15 tyres all round but would like to fit larger diameter tyres to the rear as My semi auto beetle has the 4.375 to 1 differential and I'm doing around 3600rpm at 100 KMH
I know many tyre sizes are now disappearing but hopefully there is one that will lower My revs at speed..
at a reasonable price..
My wheels are 6 inches wide chromed steel

cheers

LEE

3600rpm at 100km/h doesn't sound that bad. There's a reason that these cars have short gearing, it's because they have the aerodynamics of a brick and are somewhat underpowered.

In response to your question, Nankang do the CX668 in a 165/80R15, which is about as tall as you're likely to get without going to 4wd or truck tyres. You could also go to a 205/65R15, which is actually 2.5mm taller than the 165/80R15 and has a bit more meat to it.

I just swapped my 165/80r15s for 185/80r15s on the back of my bug and I'm very happy with the results. Both in performance and looks..........



Dia has increased from 645mm to 677mm.

New tyres are Hancooks.

As for the biggest dia I had 787.4mm on 6in rims on my old bug.

215/65/r15 about 26 inch cheap tooooo

You could also use the 205/70/15 tyre and get a bit of a cal-look stance happening

http://www.michelin.com.au/Home/Products-Services/pattern-detail/PassengerCar...

Michelin does tyres in this size in the cheaper XM2 range

http://www.csp-shop.com/cshop/technik/getriebeberechnung5.php 

Use the gear ratio calculator above so you can get a real idea of what your revs will be to see if it's worth it though

Cheers

Shaun

remember, greater rev's = greater cooling (ie: fan rpm speed). i wouldn't aim any less than 3000rpm, but i'm no expert. Would be my gut feeling if i was in your shoes.

And as the ad says for the 1500 series, "its top speed is its cruising speed", and the top speed would involve revving its nuts till it can't go any more lol.

ALL air cooled VW cars had the top speed as the cruising speed

as most other makes needed an overdrive to acheive this and most didn't have it..
although many English cars did offer this as an option in later years.. MG with overdrive etc..

in the 50's and 60's not many car makers had the cruising speed as the top speed... lol

When You see all those Baja beetles with huge rear wheels and tyres their engines are working much harder

the 1300 & 1500 semi auto beetles had 4.375 to 1 which is the 1200 manual beetle diff size. Later 1600 semi autos had 4.125 to 1 ratio - similar things happened with manual beetles with the 3.88 to 1 diff ratio instead of 4.125...

VW doesn't say anything about driving at 80 klms an hour everywhere apart from You get better economy..

I don't think it will make any difference, ..
if I go too big a tyre then I'll be in a lower gear revving the engine more going up hills...

LEE

A good tyre calculator site

http://www.tacomaworld.com/forum/tirecalc.php 

When I had my 1916 in Tweety trike with VW type 3 auto I went from 295/50/r15 to 275/60/r15. 8% larger diameter. Really showed up when towing our camper trailer in that it didnt like hills and got hotter sometimes overheating...and the camper weighed 300 kgms.

Went back to the 295's

Hi Lee

As you have a 4 vent engine lid maybe you should aim for a tyre that would give a final drive equal to a 1600 Superbug semi auto.

Steve

2 aspects to the solution to this problem that have not been addressed in these answers so far.

One is that the original rolling diameter of the stock tyre size needs to be respected and preserved with regards to braking balance and suspension.
The other is that the solution to balancing engine rpm vs top speed, if the maximum tyre diameter is observed, is found with modifying the diff ratio.

If you are concerned about your top speed revs, and, if you think your engine and semi auto trans can handle it, get a taller diff Lee.