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Engine Specific Info and Questions => IDI Engine => Topic started by: ftm1776 on August 08, 2014, 06:38:37 pm
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I'm not sure if I am figuring this correctly.........I get about 2700 rpm....seems a little high????????
1991 MK2 Jetta diesel with 5 speed transmission numbers 020 on driver's side of tranny and AWY 04120 on bottom.
Ratios from Bentley: 5th gear, 0.894; final drive 3.941
At 50 mph I figure 765 tire rpm with 11 inch radius as measured (13 inch rims)
What is the engine rpm???
I'm trying to get a 2000 rpm engine speed to check the oil pressure spec.
Can someone check my figures???
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This is our checker. Run your numbers through it and see what you get. I am almost sure this is in the stickies by now isn't it. I always have it handy as a favorite or bookmark. Suggest you do the same. Lots of info here. Note tranny code and each gear has its numbers accordingly.
http://scirocco.org/gears/
Enjoy we all have it.
If you are running 155/50/13 tires and rims as stock then 3085 is your RPM. At 2700 you must be running something else.
195/60/13 ???
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Thanks, ORCoaster !!!
Fun tool to play with.
I get about 2600 rpm at 50 mph using the calculator......
My tires say 165R13 82S???? So I used 82 as the sidewall ratio....
However, assuming that the tire compresses somewhat, I tried 75 and came pretty close to my figures.
Thanks, again.
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How old are your tires? ???
A measuring stick and some basic math will get you the correct ratio quickly.
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My tires are just a few years old. I did the basic conversion from mph to tire rpm and worked through the gear ratios to engine speed.
It came out pretty close to the "scirocco" calculator above. I am using smaller rims. Original was 14 inch, these are 13s. Smaller narrow tires seem to be better in the snow. 13 inch tires are a little bit cheaper.
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My 84 Rabbit has 175/70/13.
My 92 Jetta has 185/60/14.
The sidewall ratio has to be on the tire somewhere, you probably have 165/70/13.
Plugging in 165/70/13 gives you 2667 RPM at 50 MPH in 5th.
175/70/13 gets you 2602 RPM @ 50, not much difference.
185/60/14 gets you 2591 RPM.
http://en.wikipedia.org/wiki/Tire_code
The 82S is the load index and speed index, not related to tire size.
82 = 1,047 lb maximum per tire
S = 112 mph maximum speed
*Went out and verified tire size on the MK2, 165/60 didn't sound right last night. They are in fact 185/60/14.
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You guys do realize that the size is also a multiplier of the side wall right? example if you had a 200/50r14 your tire would be 200mm wide the sidewall from rim to tread would be 200x.5 or 50% of 200 or = 100mm...the 100mm is only the height from the rim on one side, so you double that and add the rim to get tire height...100+100=200mm then 200mm = 7.87" + 14" for the rim =21.87" tall tire. The first number is the width, the second is the ratio of how much of that width is sidewall. So just because you go to a bigger rim means nothing. That is why "within a few tenths of an in* you can run 155/80r13, 175/70r13, or 185/60r14 and it is *basically* the same tire height and your speedo is not off. Rolling resistance from a wider tire will be there but the rpms will be about identical. I always got the best mileage with 155/80r13 but they are so skinny they don't have much resistance on the road. Most people that say their mileage is off do not factor in if they have a non stock tire. 887.3 revolutions per mile on a 185/60r14 vs 925.8 on the 165/60r14 is about 4.3 percent. Your speed and mileage will be off accordingly. Run a non stock tire height and you have to change the gear in the trans, problem with that is VW didn't make a bunch to swap out.
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Here play with this one for tire sizing. I like the old diameter wheel over the new one thing. Gives you all the dimensions you want to ever see.
http://www.miata.net/garage/tirecalc.html
DAS
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This is the data that I find on my tires: 165/R13 R202 at one location and then 165R13 82S at another location. Other than that, there is no sidewall ratio. Several other multi-numericals near the rim edge, but nothing that looks like a ratio number.
ADDENDUM: I get why the measured distance from center wheel to ground is not the relevant dimension. We need to know how many inches the tire rolls with each revolution.....hence the circumference of the tire is what we need to know.