Charlie

I am just now getting my 968 on the road after buying it 7 or 8 months ago. It is my understanding from the gear ratios at 3000 rpms I should be doing around 75??? Is this correct? At 3000 my speedometer shows 67mph? My tach could be off?

odurandina

if you're running a 25.65" tire you'll achieve 25.0mph/1000 rpm.

the vast majority of you run 25 inch tires, with 16" and 17" tire combos.

so you'll be quite a bit short of 75mph at 3000 rpm.

see post #160

https://rennlist.com/forums/968-foru...-968-a-11.html

Charlie

I'm running 255-40-17.

If it a simple ratio the I should be doing 73 at 3000 rpms.

odurandina

probably close to that.

but if you give me your make and model of tire,

i can calculate your speed at 3000rpm to within 0.00001%

Charlie

Pirelli PZero Asymetrico

Tom M'Guinn

There are many free speedometer apps for iPhones that use GPS to calculate speed. I'd get one of those and see if it's your speedometer. If the speedo is off, then (shameless plug warning), build one of these:

http://members.rennlist.com/tom86951...r%20Page1.html

odurandina

you're running a 25.3" tire w/ 822revs/mile

3000rpm = 74.03mph

like Tom says, your gps app will confirm this.

btw,

i'm making 75mph @ 2311rpm on my 285-35-18 Contisportcontact 3's running the 0.600 6th gear.

Charlie

Thanks guys. I wish I had I had a .600 6th gear as an overdrive, or a little lower final drive ratio

Tom M'Guinn

By the way, I've also noticed that some tachometers are way more accurate than others on these cars...