Tyre size/gear ratio/speed

[GARRICK.CLARK]

Hi guys.
I'm running a 1911 type 4 in my 1500 67 bug.
Gearbox is a standard 1500.
Tyre size is 205 60 15 on the rear.
Would I benefit from going to 205 70 15.
These tyres are 2 inch taller.
What would my 70mph versus RPM's be with the 60's fitted.
Plus with the 70 series.
I'm looking for it to be a good street car ,not so much for strip use.
What would be a good gear ratio tyre combo.
Thanks for any input G.

[Pablo2]

You'll get more help if you provide more thorough information.

"Gearbox is standard 1500" Does this come with 4.12 (8:33) R&P ? Most in the U.S. wouldn't know what "standard 1500" is.
Confirm that you have stock .89 4th gear.
Provide the tire manufacturer's listed circumference or diameter of BOTH tires rather than just a generic size and approximate difference.

[GARRICK.CLARK]

rojer that.
I'll be back soon with info soon.

[GARRICK.CLARK]

Right the tyres I have now are 205 60 15 =24.7 inches in diameter.
the tyres I want to try are 205 70 15=26.3 inches in diameter.
Its got a 4.125.1 final and an 0.89 4th

[Marc]

Should be on the high side of 2800 RPM @ 60 MPH, I don't foresee any problems with that engine.

[GARRICK.CLARK]

Cheers Marc.is the 2800 at 60 using the 70 series tyres.

G

[Marc]

Yes. With the 60-series it works out to just under 3000 RPM @ 60 MPH, just right for a stock/stockish Type I.

[GARRICK.CLARK]

Hi guys been driving this bug round now for a while. I have a 1911 type 4 in it. It drives ok just seems so under geared. Motorway driving is slow and have to keep to the inside lane. 70mph is over 3000 revs and just sounds like I'm thrashing it. What would you recommend I do over the winter to improve this. I have driven bugs with GT bug boxes installed and they are quiet good with all 4 gear lasting longer before gear changing. Is there any better gear combos than a GT BOX for fast street driving. Quick off the line with lots of gear changes isn't what I'm after.

Engine has about 100hp.
Tyres are 205 70 15.
Thanks guys for any input.

[Marc]

Here are simplified formulae for calculating RPM vs road speed (the "336" is a factor that allows the use of the tire diameter rather than its circumference):

MPH = (RPM x Tire height) ÷ (Final drive ratio x 336)

RPM = (MPH x Final drive ratio x 336) ÷ Tire height


205/70-15 => 26.299" diameter
Your final drive ratio in 4th: 3.67:1 ....therefore your RPM at 70 MPH works out to 3282.


I just got back from a 370mi roadtrip in my Bug, most of which was at 70 MPH and up, and my little 1600 Type I did just fine with slightly shorter gearing - I really fail to see the problem here.

235/45-17 => 25.326" diameter
My final drive ratio in 4th: 3.60:1 (3343 RPM @ 70 MPH)


Another way to look at this is to consider the gearing of a Bus that came with an 1800...185-14 tires are also about 25.3" tall, and the final drive ratio in 4th is 4.32:1.
At 70 MPH, the engine's turning over 4000 RPM. Your 3282 would be good for only ~57 MPH in one of those, and they can do that or better all day long pushing a big box through the air. With the little 66mm crankshaft stroke, I just don't see the need for gearing any taller than what you have now.

There are no late-model swingaxles in the USA (we got nothing but IRS since 1969) so I'm relying on info published on the 'net from here on...so you should verify it for yourself before diving in.
An "AS" box from a `73-up 1600 Beetle wouldn't help the gearing situation much since the .93 4th nearly offsets the taller R&P; it'd only drop your RPM at 70 MPH by about 60...although it would be stronger and a worthwhile improvement for you. If you can find a trans from a `71-`74 Karmann-Ghia (codeletter "AO") however, it should have the 3.875:1 R&P and a .88 4th - that would drop your RPM at 70 MPH by ~230...and that's as tall as you should consider IMO.

One can also convert any two-sideplate IRS box to swingaxle, but generally speaking anything with the 3.875:1 R&P has a single-sidecover case like my "AT" - the c1971/72 Karmann-Ghias are the notable exception; the IRS version is codeletter "AN".

[Pablo2]

... I really fail to see the problem here.
... I just don't see the need for gearing any taller than what you have now ...

+1
Air-cooled engines overheat when geared too tall. I've learned this the hard way.
Not sure what you're comparing to, if you believe your gearing is too short or too "sporty".