RPM to Speed Calculator

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RPM to Speed Calculator

The racecar doesn’t have a speedometer, or at least not an accurate one. So when people inevitably ask “How fast?” I have to reply “about 5500 RPM” or some such number.

I am rewarded with a blank stare that says

I don’t SEE a second head, but there must be an explanation for what just came out of your mouth.

While that conversation will always be awkward, the below information may be able to help a little.

SAAB V4 Gearbox Exploded SAAB Gearbox Parts Diagram

Background

The SAAB three cylinder two-stroke and four cylinder four-stroke V4 cars all used nearly the same gearbox. Early cars had a three speed while later cars had a four speed. Early housings were cast iron, then cast aluminum, then finally cast aluminum with strengthening ribs on the outside. Bell housings changed from the two-stroke to the V4 but the gearboxes all bolt up. All the (four speed) gear ratios were the same. Ring and pinion ratios, however, changed a fair bit over the years.

NOTE: All of the above does not include the SAAB Sport and Rally competition parts.

What this means is that if you feel like changing gearboxes (or ring and pinion sets), you can tailor the ratio to your needs.

SAAB Ring and Pinion Sets

Below is a table of all the ring and pinion sets that have been available for the SAAB two-stroke/V4 gearbox.

 

Model Pinion gear teeth Ring gear teeth Final drive ratio
SS&R 6 35 5.83:1
?? 7 40 5.71:1
96 (3 spd)
95
7 38 5.43:1
Grp 1 (SS&R)
GT 750
’65 Sport
7 36 5.14:1
’66 MC 850 8 39 4.88:1
Sonett V4 9 42 4.67:1
?? 9 38 4.22:1
“Nurburg-Ring” 9 36 4.00:1

If you aren’t sure what final drive you have in your old Saab gearbox… Simply pull the fill plug for the transmission out and shine a light in there. Spin the ring gear around and look on the edge of it until you see a series of numbers. For example, on the ring gear in Dad’s Quantum Formula S it had the number 481 scribed into it. Then, stamped into it was the sequence (with spaces): 800373 42-9 9 67.

The scribed number 481 is the ring and pinion matching number (they are always a matching set). The stamped number 800373 is the part number of the diff set. 42-9 is 42 teeth on the ring gear and 9 teeth on the pinion gear. 9 67 is September of 1967.

Calculate

Then you need to be able to convert arr-pee-emm (RPM) into road speed using the final drive ratio you’ve chosen.

I’ve failed miserably at getting the javascript and calculator form to work within this WordPress post. None of the plugins or workarounds I’ve tried work. So I created a basic HTML page and will send you there to do the calculations. I’ve placed it on my FormulaSAAB site as I use it there as well. Just plug in the appropriate numbers and click the calculate button.

Calculator

SAAB Sport and Rally

In addition to the stock gears, there were three SAAB Sport and Rally gear sets fabricated. Special 1, Special 2, and Special 3. These gear sets provided close ratio options for those competing with their SAABs.

Special 1 Gear Set

Close ratio sets “Special 1” and “Special 2” are the most commonly found, but are still extremely rare and difficult to come by. The “Special 3” set definitely existed because I’ve seen pictures on the internet (that means it must be true), but I’ve never seen one available for sale so don’t think of it as an actual option.

 

  • Special 1 provided longer 1st and 2nd gear’s, used the same 3rd gear, and a shorter 4th gear
  • Special 2 provided even longer 1st and 2nd gears, a longer 3rd gear, and the same short 4th gear as Special 1.
  • Special 3 split the difference between the Special 1 and Special 2 1st gears, used the same long 2nd and 3rd gears as Special 2, and the shortest 4th gear of all.

 

@1000 rpm STD CR1 CR2 CR3
1ST 3.9 mph 4.3 mph 5.2 mph 4.9 mph
2ND 6.5 mph 7.3 mph 8.5 mph 8.5 mph
3RD 10.5 mph 10.5 mph 11.4 mph 11.4 mph
4TH 16.2 mph 14.8 mph 14.8 mph 14.4 mph

My Dad and I put together a spreadsheet that calculates the speed in various gears, in each of the 4-speed gear sets, using 4 of the most common ring and sets plus the super-short SS&R 35/6 (5.83:1) that we have installed in the Sonett currently. It can be modified to use any tire diameter you choose by editing the number in cell B29. Click the spreadsheet icon to download the spreadsheet (located on FormulaSAAB.com).

spreadsheet

I have provided all the tire diameters we currently use, plus the stock size, for your convenience.

  • Stock: 23.5″
  • Dunlop L series 450-15: 24.0″ (fitted to the Quantum 1)
  • Dunlop L series 500-15: 24.8″ (fitted to the Formula S and sometimes to the Sonett V4)
  • Hoosier Speedster 185-65-15: 24.2″ (sometimes fitted to the Sonett V4)

 

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