Gear ratio craziness

Hi David- I don't remember which thread it was in, but I posted it. I forgot to mention I copied this from the Samba (transaxle expert Bruce Tweddle was the one responsible for the info). Here it is again-

 

How to determine r&p ratio with trans in the car-

jack up the car so both wheels are off the ground- E brake off, shifter in 2nd gear. Turn one tire so the valve stem is at the top. Turn the engine over by hand, counting engine revolutions needed to get 1 complete revolution of the tire.

9 engine revs- 4.375

8 1/2 revs- 4.125

8 revs- 3.88

 

I believe Larry Jowdy had a comment of it not working with an open differential unless some drag was put on the other tire? Anyway, I hope this helps, and let me know if it works. Al

 

Thanks for that, Al. I'll try some drag ( lower one to the ground or put a wedge under the other wheel ) ..would it matter which rear wheel ?

 

The Bride just opened up a large bottle of Merlot and put another log on the fire. I may not get at wheel spinning till the morning after....

A big bottle of merlot and another log on the fire? Enjoy the night. Let me know how the wheel thing works tomorrow...Al

David-

 

Playing the odds, I'd be 95% certain you've got a .89 4th-- almost everybody does.

 

If my math is right, and it's a 4.12 R/P and a .89 4th, it'll be 3.67125 turns of the engine for one turn of wheel in the final drive. If it's a 3.88/.89 it'd be 3.4532/1. If you had a 4.12/.82 it'd be 3.3784/1 

Thanks for that guys...you're all right. I dropped the left wheel onto the ground and got 3 1/2 + turns so Stan's bet at the 3.67125 turns for a .89 4th seems to be what I have.

 

Now...about those gear/rpm/tire size calculators...I wonder if they consider that the bottom of a tire is somewhat flat when sitting on the road in a fully loaded car ?

 

Not trying to split hairs here, but trying to be as accurate as possible. My tires are skinny P155/80/R15's and measure only 14.5" diameter from the center of the hubcap nipple down to the garage floor therefore giving a diameter of only 29". So the math goes...one wheel turn will be 3.14 x 29 divided by 12 + 6.01 ft. At 3,000 rpm we multiply 3000 x 6.01 then divide by the final ratio of 3.67 and we get a whopping 4919 feet. Divide that by 5280 and we now have 55.89 mph at 3,000 rpm with my rig and that seems very consistent with my experience.

 

That translates to 3,220 rpm at 60 mph and 3,756 at 70 mph which also is consistent. With a 3.88 r&p rpms only go down about 6% from 4.125 and would give 2,820 ...3,026 and 3,530 at those speeds but the 3.44 r&p would bring those rpms down to 2,500...2,685 and 3,132 at those speeds. That's where I want to go. The stock EJ22 torque curve seems to indicate this to be a reasonable range to be in.

David-

 

That's a lot of math-- I do it the lazy-man way, and just use a calculator.

 

Regardless, that torque curve will pull a stock-geared Type 1 with a 3.44 easily. It'll be nearly perfect.

 

If I might suggest spending a bit more and getting it built to "Pro-Street" specs, it'd be a good idea. I know you have the low-budget ethos going on (and believe me-- I respect that), but sometimes an extra $500 is money well spent.

 

BTW: thanks for the information on the Bully Clutch. I believe I'm going to go in that direction. I've had a copperhead disc for several years that I can't get to stop chattering unless it's smokin' hot. I'm going to try the spring-centered deal and hope it holds up.

Glad to hear it worked out, David. And I've also wondered about the diameter of the tire that the car sees as well; rpms always seem to be a little off of what's calculated and I'm sure that's why. Btw, the diameter of a 155/80-15 is 25"- a 29 or 30" tire is a very tall tire.

It makes sense to me to get a new tranny into the car while the engine's getting changed and I wanted to get the gearing sorted out while the topic was still active. Yep...I'll spring for some extra hardware in it too. That would be money well spent. Thanks for the input, Lads...

If you can, see if your tire manufacturer lists rotations/mile for your tire. Actual tire circumference for odometer purposes will be less than stated size, but if you can't find that info, 3% lower is a good estimate.