Gear ratio, mph calculator?

bajabugman · 12-05-2009, 11:15 AM

I was wanting to know if anyone knew of a good gear ratio/mph calculator online. I am looking for a calculator where you punch in your gear ratios, final drive, tire size, and engine rpm and it calculates how many mph you will be traveling. Any help would be greatly appriciated, thanks in advance.

Wyvern · Wyvern's Garage

This one is cool as it has many options

Gear Ratio Calculator - Porsche 993 Turbo

skinnerd · skinnerd's Garage

Super cool one Brian.

I did a simple one in Excel....but not as elegant.
Here's a screen capture.
Could send anyone the Excel spreadsheet if they wanted.

Wilhelm · 12-05-2009, 06:16 PM

Good info. However I dont know what gear ratios and ring/pinion I have. ( I am working on getting that info)

Quick question for the Early 911 guys: I have a 71 911 S with the 901 transmission, 16 inch wheels, 205/55/R16 front and 225/50/R16 rear.

At 4000 RPM in 5th gear I get 75 MPH. Would this indicate "short gears" or a 7:31 ring/pinion ?

In other words, is this RPM/ Speed in 5th gear "normal" ? Something in my setup seems "short" !

Thanks !

Matt Monson · 12-06-2009, 11:14 AM

'71 S is most likely an 911/01 gearbox with A,Ga,O,V,ZA. Almost all 901 based bearboxes run a 7:31 R/P. Heres some more information for you:

901 transmission - The Porsche 914 Wiki

skinnerd · skinnerd's Garage

Wilhelm:

I plugged a 7:31 and the ZA 5th gear along with your 225/50R16 tires into the calculators.

Both gave me about 88mph at 4000rpm.

So it does look like you've got some shorter gears in your gearbox.
Almost to the tune of your 5th gear is like what a normal 4th gear should be pulling.
The calculator shows 72mph at 4000rpm for a typical 4th gear that should be in your car.

Your top speed is down about 20 mph from what is should be at 7000rpm (130 vs 150mph).
I'd guess your car accelerates pretty well though at lower speeds even for a non-torquey 71S.

Wilhelm · 12-06-2009, 05:39 PM

Thanks Matt and Skinnerd;

My 71 S has a 2.7 RS spec MFI motor so yeah...the acceleration is fantastic ! And the gearing does seem short. One of these days when 901 box needs a "freshening up" I will find out exactly what I have in there. Previous owner wasnt sure.

Highest speed I have had it up to was a 125 MPH indicated. Didnt check the RPM...I was busy ! Felt like it would have another 15 MPH in it though. I would guess top would be around 140 MPH.

Thanks for the info guys !

mack6820 · 12-06-2009, 06:14 PM

i found out mine was off with a GPS. I hope the GPS was accurate. I was actually showing to be off by 7mph.

Matt Monson · 12-07-2009, 11:45 AM

My calculations suggest that you may have a "W" 5th gear in there. But without cracking it open you can't know for sure. It's probably a lot of fun when you are on it hard but not so much fun heading down the highway.

I've got an "X" top gear in my car and even that is a little too short for more than 1/2 an hour on the open road.

Hcarraro · 12-07-2009, 01:24 PM

If you have an excel spreadsheet, I have an excellent model I would be happy to share. I can't figure out how to add this file so you will need to email me at hcarraro@hughes.net and I will send you a copy of the spreadsheet model.

To use the model, one just plugs in the tire size and the model calculates speed at 1000 RPM intervals beginning at 2000 and ending at 8000 RPMs. You will need to know your gear ratios (letter ratio or number) to determine speeds at a particular RPM and in a particular gear.

What is sad is that I don't know who did the model so I can not give credit. However, I am almost positive it was found on Pelican.

Take care,

Henry