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  • July 03, 2020, 06:16:58 PM

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Author Topic: Tire size and speedometer  (Read 155 times)

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Online Longhunter

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Tire size and speedometer
« on: June 21, 2020, 11:22:18 AM »
Hello all,
             Looking through all the literature and posts here I see nothing stating Eagles ever came with 16 in rims. So with that here are a few questions. I have attached a pic of an Eagle I recently purchased and am referencing.

1. Did AMC ever offer 16s on Eagles?

2. Are the rims in the attached pic original ones that would have come on an Eagle?

3. Looking at the markings on the tires in the pic it appears they are 215 60 16s. Can someone tell me how I would go about determining how "different" my actually speed is versus what the speedometer may read with the 16s that are on it? Is my speed faster or slower than what is showing on the speedometer? Is there a calculator online or something?

4. Is there a way to "change" the speedometer cable or something else to be in alignment with the bigger tires and rims so it reads correctly? I thought there were different "cog" type things you could somehow change to accommodate different size tires. Maybe that is inside the transmission or transfer case?

5. Looking at the pic it appears there is ample clearance and the previous owner states he has never had a problem with the tires on the car. So is there a reason in anyone's opinion I should not keep these size tires and rims on the car?

Thank you.

Offline johnbendik

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Re: Tire size and speedometer
« Reply #1 on: June 21, 2020, 10:12:49 PM »
What you need is the formula to determine wheel/tire circumference when you know the three determining things: the wheel diameter (given in inches), the tire width (given in millimeters), and the aspect ratio.

It's a little complicated because you have to convert between the two systems of measurement.  Here's the overall formula:

If B = tire width in millimeters
   C = tire aspect ratio (for example, the '60' in  215 60s)
   D = wheel size in inches

Circumference = 3.1416 *  ( ( (B * (C * 0.01)  * 2) / 25.4) + D)

By comparing the circumference of your original wheel/tires and your proposed new ones, you'll know how much your speedometer and gearing will be off.  For example, I'm running 235/55ZR-17 tires on 17x7 wheels on my '84 wagon, which are within an inch (circumference-wise) of the  original 215/70-15s.  Here's a link to my post on the changeover :http://forums.amceaglesden.com/index.php?topic=42205.0

I have the formula above  set up in an Excel spreadsheet, if you can use that.  It makes it easy to try different combinations.  Let me know if you want it.

Good luck,
JB
1984 Eagle Sportwagon
258 - 6 cyl
5-speed !

Offline vangremlin

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Re: Tire size and speedometer
« Reply #2 on: June 22, 2020, 12:19:39 PM »
To answer some of your other questions,

1.  To my knowledge, Eagles were never sold new with 16" wheels.

2.  Those do not look like original rims.

3.  Here is a link to compare tire/wheel sizes and it will tell you the change in speed:  https://tiresize.com/comparison/

4.  After you go through the calculations to determine how much of a difference there is in the speed is with your current wheels/tires versus stock, there is a speedometer gear where the bottom of the speedometer cable attaches.  Changing that will get the speedometer closer to the actual speed.  You will probably find that the different tire size does not make much of a difference in what the speedometer reads and it may not be worth the effort to change the speedometer gear.

Good luck!
1981 Kammback 258 - "Pepe"
1980 Coupe 258 - "Ginger
1972 Gremlin X 304
1978 Gremlin 4 cyl 121 - sold
1964 TBird 390 - sold

Online Longhunter

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Re: Tire size and speedometer
« Reply #3 on: June 23, 2020, 10:11:07 AM »
Gents,
          Thanks for the info.

Johnbendik, that was a great accomplishment getting those tires in there. I am trying to keep mine as stock as possible so will probably change the 16s back to 15s once family gets it home.

Vangremlin, that tire calculator was right on time. I am keeping that for sure. The formula Johnbendik sent was making my head explode. He must be a rocket scientist!

Regardless, plan on probably going with some 225/65/15s in good weather for a wide grip and then change to 195/75/15s when it gets snowy/icy. Just have to play with the tire calculator to see what makes sense.

Thanks again.

Offline Canoe

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Re: Tire size and speedometer
« Reply #4 on: June 23, 2020, 11:13:24 PM »
Do NOT rely upon calculating circumference from the NOMINAL tire sizes on the sidewall. Those are just nominal sizes. You need to find the manufacturers' actually published circumference for that model in each nominal size. There can be some surprising differences compared to the ideal nominal size (they pick and label each size to the nominal size closest to the design's actual...).

But as vangremlin said, compare how much of a difference there will be between the original AMC Eagle stock size (which will be nominal) and the tires you're looking at.

Note that if you are using /70 or /75, you can balance with Airsoft pellets (not the biodegradable ones) loose in the tire as balancing beads for dynamic balancing. Can't run into issues by losing a weight (like getting home on pavement after losing a weight or weights on a trail). Always perfect at the speeds you need it.
I've used them with 225 and 235. I weigh out an amount for each tire and put them in four sandwich bags and the garage empties each bag into a tire as they mount them on the rim. They thought I was nuts, but
(arrrgh - I can't find the file where I calculated what weight my tires needed... had this posted on the EaglesNest) Won't work for /65 and lower, as those need to be balanced side to side too.

Online Longhunter

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Re: Tire size and speedometer
« Reply #5 on: June 24, 2020, 12:00:23 PM »
All,
     Thanks for the info on the Airsoft pellets never heard that one before. Will look into it when I get the new tires.

Just an update. As some of you know I am in Syria on a military deployment so family (20 year old college daughter and 21 year old college daughter boyfriend. Both good kids and both at The University of Tennessee on ROTC scholarships) are picking up my 1985 Eagle Wagon from CA and driving it to TX.

So yesterday they picked her up. Brake master cylinder had what appeared to be brown yogurt/pudding in it. Oil needs changing for sure and transmission fluid was dark and needs a change. Other than that the only oddity I cannot figure out is the fuel gauge. It works and shows an accurate reading when the car is turned off. But when running does not show accurate amount of fuel. Any ideas?

Yesterday after picking it up they drove it from Lucerne Valley CA to Las Vegas NV with no problems. Today going to get an oil change and clean and bleed brake system. Anyone live in Las Vegas want to take this on for payment? After getting the fluids changed they will head out from Vegas to the Grand Canyon and spend the night. The next day from Grand Canyon to Albuquerque, NM for the night. Next day on to Fort Hood, TX.

Questions:
1. What brake fluid should I use? Is it DOT3 or DOT4? Is there really a difference in the two?
2. Is there a reason to use synthetic oil or is regular 10W 30 good to go?
3. I read some posts here concerning hoe Fram oil filters suck. Is this still accurate?

Offline Canoe

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Re: Tire size and speedometer
« Reply #6 on: June 25, 2020, 12:05:22 AM »
...
So yesterday they picked her up. Brake master cylinder had what appeared to be brown yogurt/pudding in it. Oil needs changing for sure and transmission fluid was dark and needs a change. Other than that the only oddity I cannot figure out is the fuel gauge. It works and shows an accurate reading when the car is turned off. But when running does not show accurate amount of fuel. Any ideas?
...
Questions:
1. What brake fluid should I use? Is it DOT3 or DOT4? Is there really a difference in the two?
2. Is there a reason to use synthetic oil or is regular 10W 30 good to go?
3. I read some posts here concerning hoe Fram oil filters suck. Is this still accurate?
  • I've only seen DOT3 specified. I don't know if DOT4 is acceptable. I use a DOT 3 that is rated for 550F. Still have some sealed bottles. Don't think it's available any more.
    Quote
    https://en.wikipedia.org/wiki/Brake_fluid
    - a vehicle that uses DOT 3 may also use DOT 4 or 5.1 (a temperature upgrade) if the elastomers in the system accept the borate compounds that raise the boiling point
    - cannot be mixed with DOT 5.0, which is silicone based
    Watchout out for a proper cleaning & bleeding of the master cylinder. The rest of that job is easly done with a cheap suction device to pull fluid from a freshly stocked master through the lines from the caliper/cylinder in the correct line order. Don't let the master run low while doing that. As their lives depend on brakes, the issue is getting someone who can actually do that correctly/safely instead of just thinking they can.
  • Use synthetic oil. More stays up on the bearings, etc., and makes for less wear & tear on start. Less break-down in heat.
    As it's been a while since a change, I'd be recommending:
    A. A flush (ad it around the corner from where you're getting the oil changed, run for the prescribed time, then a gentle drive around the corner to the shop) and the oil & filter changed before they head out, and change the filter and top up the oil a couple of hundred miles down the road; or,
    B. Change the oil & filter before they head out, and change the oil & filter again a couple hundred miles down the road.
  • Just stay clear of Fram oil filters; too many corners cut over the years. 'The' filter to get used to be WIX, aka NAPA Gold, aka Carquest Blue, with the NAPA Gold the most expensive source and the Carquest Blue the least expensive source. I get the filter with the slightly physically longer body/can that is more commonly seen on jeeps, just to have more filter area, just because. Don't recall the part # from Carquest Blue. I can check my spares tomorrow. That info may have changed, since I bought a supply and have been changing oil & filter with those. Likely a lot more recent info on that on Jeep forums.
    The few times I've had to use an oil-change place, they've been happy to use the filter I supply.

Trans fluid. DO NOT LET THEM POWER-FLUSH IT. Stirs up all sorts of crap that will trash the trany within a year (at least that's what has happened to me and everyone I know who ever had that done).
A trany filter change is a good idea, but then you've got a gasket to replace too. Make that easy by getting the updated Chrysler gasket that is a reusable gasket out of a hard plastic with rubber beads, good for multiple changes. Super easy to use; fits perfect, no fuss, no muss; whoever is changing the filter will appreciate that when they're putting the pan back on. I got the part number from EaglesNest or EaglesDen. Should be here somewhere.
Dexron II is no longer made or under licence. III and IV are said to be backwards compatible for transmissions, but that's unclear for use in transfer cases (most say yes, some say no).

Offline johnbendik

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Re: Tire size and speedometer
« Reply #7 on: June 25, 2020, 10:48:59 AM »
This has nothing to do with your Eagle, but thanks for serving in Syria.

And based on the adventure you described, your daughter (and her boyfriend) has inherited your toughness and bad-assery.  So congratulations on that.

JB
1984 Eagle Sportwagon
258 - 6 cyl
5-speed !

Offline Canoe

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Re: Tire size and speedometer
« Reply #8 on: June 25, 2020, 01:58:45 PM »
... 'The' filter to get used to be WIX, aka NAPA Gold, aka Carquest Blue, with the NAPA Gold the most expensive source and the Carquest Blue the least expensive source. I get the filter with the slightly physically longer body/can that is more commonly seen on jeeps, just to have more filter area, just because. Don't recall the part # from Carquest Blue. I can check my spares tomorrow...
I have one spare left. Carquest Blue #85522
http://weblink.carquest.com/acl/?mfgName=CFI&partNumber=85522
This doesn't look like the longer one, but it also matches the one currently on my 258. I think my memory is playing with me.

 

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