new tires 4 swb... top speed lower?

[air-cool-me]

For ref. SWB 1967 with

2.7 1977
'E' Cams
9.5:1 factory Porsche pistons
MSD ignition
PMO carbs
revs to 7000
speedo was rebuilt by north Hollywood speedo but wobbles a bit so i don't know how accurate it is.


the 901 was re-geared by the previous owner apparently..

at 3000 it reads 55mph

at 6000 in 5th it reads 110ish mph with old hard\crappy 195/60/15's (haven't had a chance to "test" this in a while) 24.2 inches diameter

I want to run toyo RA-1 on my deep 6's and a tire size suggested was 205/50/15 turns thats the only size that will fit.
http://www.toyo.com/docs/tires/product_spec.asp?lpid=18968&name=Proxes%20RA-1%99&category=sport

the new Diameter ends up being 23.1 inches.



Right now the top speed is perfect. At the very end of the straight at Pacific Raceways i am just a hair past 7000.

How will the shorter tires affect my top speed?

thanks for any help you can give.. not sure if there is a quick way to find this out before i start crunching numbers.

[304065]

Nick,

Obviously the shorter tires will lower your top end. In 1967 the normal was geared AFMSX but if it's been re-geared you really need to know what's in the box before you can make any decisions. Its' easy enough to check by pulling the gear stack, search here.

An AFMSX would give you 114 at 6K with a 24.2 diameter and if you reduce size to 23.5 you'd be down to 110 mph.

[Early_S_Man]

Nick,

I have no doubts that you could go with 195/65-15 on the front and 215/60-15 on the rear with your deep 6s and no clearance problems ... the 215/60-15 would be 25.2" tall and improve your top speed back to the same as with OEM 165-15s or 185/70VR-15s of the day.

As a last resort you could go with some 195/65VR-15s all around and they would still be 25.0" tall.

http://www.tirerack.com/tires/Spec.jsp?make=Continental&model=ContiTouringContact+CV95&partnum=965VR5CV95&vehicleSearch=false&fromCompare1=yes&place=28

http://www.tirerack.com/tires/Spec.jsp?make=Pirelli&model=P6000&partnum=965WR56000N1&vehicleSearch=false&fromCompare1=yes&place=12


http://www.tirerack.com/tires/Spec.jsp?make=Bridgestone&model=Potenza+RE960AS+Pole+Position&vehicleSearch=false&fromCompare1=yes&partnum=965HR5RE960PP&tirePageLocQty=%26partnum%3D965HR5RE960PP

http://www.tirerack.com/tires/Spec.jsp?make=Bridgestone&model=Potenza+G+009&partnum=965HR5G009&vehicleSearch=false&fromCompare1=yes&place=15

http://www.tirerack.com/tires/Spec.jsp?make=Pirelli&model=P6+Four+Seasons&partnum=965HR5P6V2&vehicleSearch=false&fromCompare1=yes&place=20

http://www.tirerack.com/tires/Spec.jsp?make=Yokohama&model=AVID+H4S&partnum=965HR5H4S&vehicleSearch=false&fromCompare1=yes&place=22

http://www.tirerack.com/tires/Spec.jsp?make=Michelin&model=Energy+MXV4+Plus&partnum=965HR5MXV4EPL&vehicleSearch=false&fromCompare1=yes&place=33

http://www.tirerack.com/tires/Spec.jsp?make=Michelin&model=Energy+MXV4+S8&vehicleSearch=false&fromCompare1=yes&partnum=965HR5MXV4ES8&tirePageLocQty=%26partnum%3D965HR5MXV4ES8

[304065]

Good suggestions for all of us Warren!

I was looking at the factory spec book where they give different rolling radius figures for the tire at rest and at speed! Those pre-radial tires must have had significant centrifugal growth for the factory to mention it!

[air-cool-me]

thanks for all the help guys.

I'm not ready to pull the gear set tonight but it sounds like i still have the same 5th gear.

"I have no doubts that you could go with 195/65-15 on the front and 215/60-15 on the rear with your deep 6s and no clearance problems"

I understand i could find the right sizes in another brand but that isn't really what im looking for.

I want an "R" type tire for track duty thats why i am limited to a 205/50/15 if i want to use a toyo RA-1.

if it really goes from 114 to 110 then i should be fine..

thanks for the help guys.