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I've got this gear calculator (link is to an MS Excel file) specified to my car.
I've still got a 2 liter, but I've installed a 6mt (it's a v7 but the ratios are the same as the US STI). The only thing funky is that I retained my rear differential but used a ring and pinion off of a 1994 Subaru Legacy Turbo car which, I thought was 1.1:1 but now I'm confused.
When I am travelling at 80mph in 6th, my rpms are 3750. The chart shows that I should be travelling at 94mph.
Is it that the stock WRX has a 3.90 output from the center diff and a 3.545 rear diff which would require the stock r&p to be 1.1:1. Or is it that the r&p that I bought was a 1:1 because the stock wrx rear is 3.545 and the STI is 3.90?
Either way, I thought that I knew what all my ratios were, but I think I was mistaken.
You have a JDM gearbox. 1-4 are the same, but you have much shorter 5th and 6th gears . Also, the 1.1:1 refers to the center diff on some drivetrains. You definitely need a 3.90 rear diff, and that's definitely what you got. If you had a 3.54 diff, you'd have blown up your center diff already.
Originally posted by BAN SUVS You have a JDM gearbox. 1-4 are the same, but you have much shorter 5th and 6th gears . Also, the 1.1:1 refers to the center diff on some drivetrains. You definitely need a 3.90 rear diff, and that's definitely what you got. If you had a 3.54 diff, you'd have blown up your center diff already.
1-6 are all shorter vs the 5pd lol
I think it's his tire size also. so, are you saying you should be going faster or slower than that figure?
Yeah, that's a pretty tall tire. Switch to 215/45, 225/45, or even 215/40 (what I would use in S03s on a wagon). You'll have slightly shorter overall gearing which will give a slight reduction in ETs. However, while that affects the relation of actual speed to what the speedo says, it doesn't change what the speedo should say at a given RPM in a given gear
I was saying that his 1-4 are correct in his calculations because JDM vs. USDM are the same in the 6MT, but 5th and 6th are taller in non-JDM 6MTs. That's why his calcs were off in 5th and 6th gear. Also, I know the JDM 6th gear is shorter than US 5MT 5th gear, but I'm not sure about the US 6MT.
Okay, I did some more checking into this, and I think I have your answer. First, I used the gear ratios found in the EcuTek DeltaDash Road Dyno as gospel for what your ratios are. I don't doubt that they are accurate, as this is information that has been made public anyway. According to that, your 6th gear ratio is 0.842. By plugging that in under 5th gear in the spreadsheet you posted, it came up with 83.67 mph after I corrected your tire size to 215/45r17- which is still not right. The spreadsheet also gave the RPM as 3585.5 when I plugged in 80mph. Again, close but not quite. At first, I thought "oh, that's just speedo error- all cars have about that much error built in." However, your indicated speed (80mph) should be higher than your actual speed. But the spreadsheet says 83... hmm. So, then I looked at some more things. According to the Road Dyno, 6th gear gives you 22.3 mph per 1000 RPM. 80/22.3= 3587.44 RPM, so the numbers are close enough. That's what it SHOULD be. So, more head scratching. Finally, I took a look at the spread sheet formulas. Ah ha! You are calculating backwards from tire size and RPM through the diffs to get RPM. So, I decided to try it working from the engine forward. 3750/0.842(6th gear ratio)=~4453.7. The answer to that divided by 3.90(diff ratio) is 1141.2 approximately. That is the RPM of your wheels. Now, according to TireRack.com, your Yoko tires in 215/45R17 do either 843 or 845 rotations per mile depending on if the tires you have are AVS Sport(845) or AVS ES100 (843). This means that at 60 mph, the tires do 845 roatations per minute (845 because I'm only doing the math once. It will be close enough either way. ). At 80 mph, they do 4/3 that many rotations. (4/3=80/60) Thus, your theoretical tire RPM at 80MPH is... 1126.7. So, we have a difference in opinion of only 15 rpm now between what the tire is actually doing according to the speedo, and what it should be doing according to the theoretical math assuming a tire diameter of 24.6" exactly. That's an error of just 1.3%. But... it's still in the wrong direction. According to that you're really going 81 mph when the speedo says 80. Hmm... Okay, let's check the assumptions I've made. Doing the math on 215/45R17 tires indicates their diameter is 24.6". However, a 24.6" diameter wheel does 819 revs per mile, not 845. I guess Tire Rack has bad info in that particular tire, or Yokohama doesn't care if its tires are taller than their stated size says they should be. So, let's do it again, this time with 819 revs per mile. 4/3*819= 1092 wheel rpm. So, you're still going faster than your speedo says you should. So, the only explanation I have left is a simple one. Either your speedo is off low, which is very bad if you like to speed, because you're going even faster than you think you are, or your tach is off. I imagine it is possible for your tach to say 3750 when you're really getting ~3590 like the spreadsheet says you should be. I don't suppose any of the local police would be kind enough to help you check your speedo readings against actual speed using radar would they? That would possibly enable you to rule out tach error if the radar says 83 when the speedo says 80, like the spreadsheet says.
Okay, so I read through my post again and realized something- if the 215/45R17s do 845 revs per mile, then 80 mph would be 3700 rpm almost exactly. I can easily buy that 50 rpm is getting lost between tach error, and your estimation of where 3750.00 rpm is on the gauge and where 80.00 mph is on the speedo. So after all that, if the 845 revs per mile figure is right, at least with brand new tires, your speedo is okay. And with worn, shorter tires, the difference gets multiplied and you end up going a bit faster than your indicated speed. Keep this in mind on your next set of tires.
Edit: one last thing before I drink this off my mind. If you plug in 215/40R17s, you get 80.79 mph in 6th gear@3750 rpm. Food for thought.
I don't know whether to thank you for your hard work or to bypass the formality so we can put on our lab coats and get to work. If I ever become a race car driver, will you be my chief engineer?
Seriously, you are one smart dude.
I don't know about getting a cop to help me out (although Mach5 in the HIIC is a Military Policeman. Maybe he could help out.)
What I can and will do is hook up the DD and then the UTEC and see how things correlate.
The tires are AVS AV1-45i. Probably no longer manufactured. The are very worn, though, so that does lend itself to that extra 50 tire rpms.
So I guess what I'm having problems with is the correlation between tachometer and wheel rpms. The spreadsheet is still saying my crank should be turning ~3500rpms, the tach is still telling me another story. Pardon my cranial density.