New tires
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Senior User
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New tires
I have a 65 Ford F100, I've changed the third member out and the ratio is now a 300 instead of the original 354 rearend. This has helped some what on allowing the engine to run at a lower rpm but, I would like to reduce the rpms (at 65 mph) just a little bit more. The engine is a 352 cubic inch. I currently have 235/75R15 tires on the truck. They are needing replaced (now) and was wondering if anyone out there has a suggestion for a little bigger (taller) tire not a wider but just a taller tire. Thanks Kevin
Cargo Master
Joined: Jul 2006
Posts: 3,014
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From: Concord, CA
Ah, I will show you one of my favorite links.
The first page goes into the basics of how to decipher tire sizes in the different nomenclature over the years. Helps in finding equivalents. At the top right is another page titled "wheel dimensions & matching wheels and tyre". Click on that and go 2/3s down to tire and wheel size calculator. You can plug in your current tire on the left to get rolling circumference and then a new size on the right and then calculate the difference between the two. Say you go to a 235/80-15 and you will see a taller tire that rotates 3.2% less and shows you traveling at 72.24 mph when your speedometer says 70 mph which equals less rpm.
Car Bibles : The Wheel and Tyre Bible Page 1 of 4
Very helpful for me in choosing new tires for my Cougar as I went from a 14" wheel to a 15" wheel and didn't want much change in my speedometer or odometer readings.
The first page goes into the basics of how to decipher tire sizes in the different nomenclature over the years. Helps in finding equivalents. At the top right is another page titled "wheel dimensions & matching wheels and tyre". Click on that and go 2/3s down to tire and wheel size calculator. You can plug in your current tire on the left to get rolling circumference and then a new size on the right and then calculate the difference between the two. Say you go to a 235/80-15 and you will see a taller tire that rotates 3.2% less and shows you traveling at 72.24 mph when your speedometer says 70 mph which equals less rpm.
Car Bibles : The Wheel and Tyre Bible Page 1 of 4
Very helpful for me in choosing new tires for my Cougar as I went from a 14" wheel to a 15" wheel and didn't want much change in my speedometer or odometer readings.
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John I have not yet changed the speedo gear just going by the sound of my engine and my own ear. I've had a police put a gun on me so I know what my speed is. I'm trying to get a little taller tire so if I need to run 65 mph or 70 mph down the road I can for an exetened time, say up to 4 or 5 hours. I would prefer the engine run at a bit lesser rpms. I know when I get the right size tire on the truck then I'll need to change the speedo gear. Thanks John for all your knowlegde and help.
Cargo Master
Joined: Jul 2006
Posts: 3,014
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From: Concord, CA
There in no 85 aspect tire that I know of. You'll have to settle for an 80. If you want to see all that is made then go to the Tire Rack web site and search by size. They have everything made.
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I've been to the car bible tyre site, very helpful. I put in 235/85R15 and it came up (conversion chart) 30.7 X 9.3 / 15. So I went to a tire site enter these numbers as close as possible and they 30 X 9.5 X15 came cup with several tires. I guess these tires will work for my truck. Any suggestions would be appreciated. Thanks Kevin
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My 65 is a 2 wheel drive will these tire look funny on the truck since they are going to be taller and has anyone out there had this size tire 30 x 9.5 x 15 on there 2 wheel drive. Thanks Kevin
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From: Spokane, WA
I've seen 2wd's with up to a 31X1050R-15 on the rear. The 30X950-15 will fit no problem. It is going to be a 6ply (load range C) tire versus the 4 ply P235/75R-15 so they will be a little pricier.
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Joined: Oct 2009
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From: Dallas, Texas
What I mean is it seems you found what you were looking for and now you are not sure if you want it. I for one, don't think these trucks need larger than the 2.35/75x15 tire. That is already larger than would have come on the truck (Yes, I know you could have ordered it another way, but around 7.75-15 would have likely been on it in the lot.)
I would reconsider going to a numerically lower rear end ratio instead of taller tires if you have to have fewer rpms at 65mph. If you have done this once then it should not be too much of a challenge to change down to a 2.70 or so?
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Thanks Greg; I never did think about another 3rd menber. I'd guess that a 270 rear end would most likley come out of an old 61 thru 66 six cylinder. I was just vey fortunate the first time, one of my friends told me that a local person had the 300 third member. I called and sure enough he had it. He said that he thought this was the only one in this area. I gues what I/m asking is where would I start liiking for a 270. Thanks in advance Kevin
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From: Dallas, Texas
I can't find any listing for lower than the 3.00:1 in the '66 sales info. and that was for a Ranchero.
Looks like 3.25 was the lowest truck ratio available.
You may be looking at OD options, since I still would avoid the tire adjustment.
Looks like 3.25 was the lowest truck ratio available.
You may be looking at OD options, since I still would avoid the tire adjustment.
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From: Spokane, WA
Use a third member out of a Galaxie, 2.75 available. Most of the six cylinder trucks had lower (higher numerically) gears to make up for the lack of power. My 6 cyl 62 has 3.89's. Just make sure its the same spline count.