215/75/15 as opposed to 215/70/15

ford
f100

#1

I inherited a 1965 F100 that my dad ordered new.136,200 original miles.I should have known better but got in a hurry today to replace the tires and had the 215/75/15’s put on when the old ones were 215/70/15.I told them I wanted the 215/75 thinking that was on it.I was not questioned.

So.Other than a slightly taller tire,will my speedometer be way off? I was so intent on getting a whitewall tire that I told the store to put the 215/75’s on the truck out of haste and not looking again at what was on there.

The truck is mainly used to haul my dirt bikes and very light loads.Nothing else.Perhaps i helped it a little since it has the 3 on the tree transmission and like a 411 rear? It does get wound tight at 60-65 mph which I try to not run.

The tires are Firestones and are 50,000 milers.

Thanks


#2

Doesn’t appear to be a big deal. Compared to the old tire (not speedometer reading-more on that later), you will be off a bit. The tire calculator I use says at 60 mph for the old tire, you will run at about 62 mph, 3.6% or so higher.

The clinker is that the old tire wasn’t OEM that the truck speedometer was calculated for. If you have the old owner’s manual, you might check and see what was spec’d at the time and then use an on-line tire calculator to figure the difference for you. My memory fails, but the old tires were probably a higher profile than the 70-profile tires that were being used,and closer to the 75 profile tire you put on. You may very well be closer to the original speedometer calibration. If you have a measured mile speedometer check, I would run and check the real readings, but I think overall you are OK.


#3

I’d be surprised if the taller 75 series tire wasn’t actually closer to your truck’s ORIGINAL tires than the 70 series tires that were on it. Alas the way tires are sized has changed since your truck was new. The tire shop that installed them should be able to tell you.


#4

Not a bad answer for a KU fan… I need to learn to type faster on my phone.


#5

THANKS for the replies. I have been bumming all day thinking that I messed up.I realize that the tires are not the original size but had assumed that my stickler dad had already figured out the old to new conversion.He was like that and this truck was his pride and joy having ordered it new and all.
anyway,thank you kindly for the replies and for making me feel tons better for my oversight that may well have been for the better.


#6

We’re one of those “houses divided”. We have KU, KSU, Baker U, and PSU representation so far in the extended family, and that just counts the bachelor degrees.


#7

Bias ply 7.50X15s were the likely original tires. 7.50 inches is approximately 190 mm. The aspect ratio was likely about 80 but there was no such specification at the time. There has been a great deal of improvement in tire performance since 1965. If the speedometer is near correct the current radials are much safer than 4 ply nylon OE tires.


#8

Just borrow someone’s GPS which should give you actual speed on the screen. Take it for a ride and compare the speedo reading with the GPS actual reading. If there is an error once you know what it is you can drive with confidence.

Most car speedo’s are off a bit on the high side anyway. My Honda Civic is spot on, but my other cars are off 3 and 4 mph on the high side. I don’t tell my wife because she likes to go a bit fast so the error keeps her from getting speeding tickets.

I’m certain there is plenty of clearance with these tires in the wheel wells so they will be just fine. More likely they are closer to what came on the truck. 70 series tires were “wide oval” tires of the period and certainly wouldn’t have been the OEM size on a pickup. 70 series tires came as an option on my '67 Mustang and were “sport” tires of the era on things like GTO’s. My first set on the Mustang were bias ply (not radial) and only lasted 10,000 miles. I couldn’t afford new tires every 10K miles so I found some Dunlop radials in 70 series that were pricey but lasted 50K miles. All this is to say your dad’s OEM tires weren’t so hot and probably were shot in about 20K miles.


#9

I’ve always wondered just how accurate those GPS speedometers are… but I guess it stands to reason that they’d be pretty darn close. My thought on the tires themselves goes with what another guy said… 275/75 should be closer to being right, if still not a little low… And I’m also questioning the accuracy of a speedo that happens to be close to half a century old, in the first place.

I’d definitely check it against one of those roadside radar displays a few times or one of those GPS speedos or SOMETHING.

Funny how I get on here and see jayhawkers and all of that. I’m in Hutch, myself!! Small world!!