Alpine Club Forum

OK, some things to clear up here. The measuring the ration by counting the rotations of the pinion shaft to one full rotation of the wheel works just fine. Keep in mind however that if only one wheel is off the ground, the output to the wheel will be 2 times going through the diff. so seeing 2.2 or there abouts, with one wheel turning times 2 gives you the 4.22. If it had been less than two turns of the pinion you'd have the 3.89.

Tire size does NOT change the rear end ratio. Nothing changes the rear end ratio other than changing the internal Ring and Pinion gears. Tire size (Roiling Radius) in circumference, meaning a Taller skinny tire vs a short low profile tire DOES change the engine's revs per miles per hour... a Tall tire travels farther with each rotation, lowering the engine RPM for any given speed vs a short tire. So in essence changing tire height will effect RPM at any given real speed, like a radar gun. Indicated (Speedometer) speeds will vary with changes to tire height or actual rear end gearing changes, or Both.

The best way to to check this, and the most fun, is to get out on the freeway on some pleasant stretch of road, find the mile markers on the side of the road, reset the trip odometer to zero right as you pass a mile marker (noting the mile marker mile number), and drive 10 miles. 100 miles even better!. When you hit that 10th or 100th mile marker, note what your trip odometer indicates. If you traveled 100 miles and the trip odometer indicates anything more than 100 miles, 120 for example, then your speedometer is showing 20% over speed. 50mph real speed would indicate 60mph on the speedometer. If on the other hand your trip odometer only showed you had gone 80 miles, then your speedometer would be indicating 20% under real speed. This is where you get tickets! 60mph indicated would be 72mph real speed.

It's always good to know where you stand in regards to Speedometer accuracy. My 2013 Hyundai Elantra coupe which measures the wheel speed electronically, and should be dead accurate, isn't. it shows 12% high, verified by mile maker check, and side of the road Radar Gun signs we have hear in Washington state. It makes no sense to me why it is this far off with factory size tires. Personally, I think it's a clever plot to indicate high so the mileage warranty runs out quicker. I hit 50,000 miles 12% sooner than I should, and that was the end of the used car warranty. Very Clever...

Richard, this is why I like this site...answers to questions. At 77 and having had a stroke, I forget a lot of BASIC stuff and it makes me mad after 50 years building cars...but it is what it is. A bigger diameter does make for slower revs but has ZERO effect on ratio. Roughly 4 turns of the driveshaft makes one turn of the wheel/tire, no matter its diameter. THANKS for the detailed answer. Now if I could just remember where I put my wire strippers.

Still working on trunk, tank, and rear wires/lights. If I can I will post pix. Battery is now in right rear fender strapped into a base box, I have a raised floor with a mat cover to help stop movement, and I was able to keep the spare where Alpine put it. The yellow car with the hardtop is my spares car...except the top is going on this car. I have luggage that fits on the new floor and anchors with a small net. The small boxes to the right are spare tune up parts, (alternator, points, rotor, cap, coil, fuel pump, etc.). Tools and a fire extinguisher mount in the left fender, and they are anchored. A simple rod will hold up the trunk lid...Boot.



Edited 1 time(s). Last edit at 2021-06-09 02:34 PM by Charlie Cheap.

Enjoying watching the progress..

Cant wait to see pics of it on the road!!

THANKS ErnestoV for the vote of confidence. My hope is late next summer and driving to some gathering with the wife. AC keeping us cool, smooth ride with new suspension, great MPG with overdrive, and nice music I record for long trips. One can dream.

Charlie, you should NOT just drain and replace the diff oil. These cars are notorious for having ring gears that are loose. The Brits bolted them on, bending soft washers over the bolt heads to prevent them backing out. The fly in the ointment is the soft washers. When the Brits say soft, they mean dead soft. The washers are prone to thinning, the theory is normal rear end vibes cause micro movement between the bolt head and the washer. Anyway, they thin, causing the bolts to loosen. My first Alpine had about 70,000 miles on it, which was enough that the bolts had thinned, the bolts loosened and two of them were sheared off. The rest were removed by finger, no wrench required. So, any Alpine rear that you intend to put on the road should have the pumpkin removed and the ring gears inspected. Soft washers should be removed and lock tite used on the bolts.

Bill

THANKS Bill, for the heads-up on the rear axle ring gear. I'll order the gaskets and remove the pumpkin before installing. Doing body work now and stripping paint. With sensative eyes it takes longer than I expected, but I am getting there. It stays warm in Texas into October, and I hope to paint it at the end of this month...September.

I too thank you Bill. I am doing a rear end swap from Series 4 to 5, and was going to swap the pumpkins to retain the 3.89 gearing. Now I know to inspect closely the ring gear bolts. Shearing off ring gear bolts is NEVER a good thing... but it makes me wonder how that happened...unless they ALL broke. Even if loose, how could there be enough movement to shear them? if several had fallen out, leaving 2 or 3 left behind, that were loose, maybe then, and then the ring gear would spin freely on the carrier, leaving someone stranded on the road side. None the less, I will inspect thoroughly. Thanks or the info.

Hey Charlie..

Checking in to see how you are doing and on the progress on your car..