Tire size change-affect speedo?

[lakeside]

I want to put a set of 65R-16 235 tires as opposed to the called for 70R-16 on the F150 Rear for winter, will this affect the speedometer reading. (Got a real deal on the pair), Thanks Dan

 

[old]

Your tires should be marked something like this. 235-65R-16. The 235 is the diameter the 65 it the width and the 16 is the rim size. In theory you can go narrower or wider and not change diameter so again in theory going from a 65 to a 70 will not effect your speed-o

 

[Jon Hagen]

check this site
tire sidewall numbers

 

[Greg_Ky]

Simple answer is yes. You will be traveling slower than your speedometer will indicate because you will be putting on a smaller diameter tire. The side wall on the 65 series tire will be 5% shorter than the 70 series. Total diameter of the tire will be 10% shorter.

 

[RodinNS]

Yes... a 65 series is a lower profile than a 70 series tire, so you will now be driving slower than your speedo suggests.
There is probably an electronic correction built in on that truck that can be programmed for the tire size... if you wanted to bother. Without checking tho, I'd say the correction is probably under 5%.
I swapped a set of 235/75R15's onto my Ranger that should have 215/75R15's and best I can tell, that's a 10% correction... and that's a lot bigger jump that you're taking.
All I know is that when I get in traffic doing 100K posted, I'm doing 90... and I'd say that's fairly accurate given I've been in traffic behind the local constabulary... so not likely there was anyone speeding.

Rod

 

[rustyfarmall]

Can't really be calculated without knowing the diameter of the 70R-16.

the 235 is the diameter of the tire in millimeters.

The 65 means the sidewall is 65% of the diameter which is 235 mm.

So a 235/65R-16 tire will not stand up as tall as a 235/70R-16 tire.

 

[Greg_Ky]

235 refers to the width of the tire not the diameter.

 

[Greg_Ky]

This should help clarify.
Poke Here

 

[Anonymous-0]

If its later than mid year '98 the Ford garage can calibrate the speedometer in the computer with their laptop. Before that it takes a gear change on the sensor. A good Ford parts department has a book on the gears and many gears available.

Gerald J.

 

[RodinNS]

235 is the width in mm...
soa 70 series has a sidewall height of 164.5 or 6.47"

The 65 series will have a height of 152.75 mm or 6.01".
That would make the rolling radius of the 70 right at 22.47" and the 65 series at 22.01".

Circumference would follow at 141.18" on the 70 and 138.29.

If I did my figures right, that means the 65 series tire has exactly 97.95% the circumference of the 70 series...

Rod

 

[Uncle Ernie]

Here is my favorite Tire comparator. I use it all the time when we want to put wider tires on a rig without throwing the spedo way out of wack.
Tire Size Comparator

 

[Mark - IN.]

65 is smaller diameter than 70, so will be going slower than speedometer says.

Years back I read an article on 4x4 pickups and lots of folks desires to put huge mudders and lift kits on them. The jest of the article was that going to such huge difference made a difference in stopping, because of course the diameter difference through the computers off for ABS. ABS per the article calculates stopping distance by speed based off of the transmission, so for instance the trans might say are doing 50 MPH and calculate maybe 100' to come to a stop, but with the larger tire diameter might really be doing, oh say 55 or 60 MPH and really need, oh say 105' or maybe 110' to stop, and the next thing one knows is driving up on top of the back of someone's car at a red light because the calibrations are all whacked out. For testing purposes, the magazine used a then new 88' Chevy 4x4 to make their point, also pointing out that when changing tire diameters, really oughta take to the dealer and have the chip recalibrated to the new tire diameter. One thing's for sure in your case, by going to a smaller tire diameter, if your speedometer says that you're doing 55 and the speed limit is 55, you won't get a ticket for speeding because you'll be going slower than the speedo says. Will affect your fuel economy negatively too, but I don't know that it will by much.

Good luck.

Mark

 

[Farmer Alf]

If your main concern is your speed, use your gps or borrow one. Mine has a built in speedometer so I assume they all do. Drive at a few typical speeds and make a note of your speedo error.

 

[rustyfarmall]

Yes, 235 is the width, not the diameter. Don't know what I was thinking of, but there is no editing feature on these forums so a mistake is a mistake for eternity.

 

[bc]

Whenever I buy new tires, I always find a level place on the interstate and do about a 5 mile test with a stopwatch with the cruise set at 60 or 70 mph. I use the airplane markers or else the mile markers if nothing else. My watch and even my cell phone has a stopwatch with lap timer on it. Then run the calculations. If I'm going to drive down the road at 69 in a 65 or 74 in a 70, I want to be darn sure the speedometer is clocked out right. I would assume that with worn tires, I would be running slower than what the speedometer says. It is easy to do a stopwatch test anytime.

60 seconds x 60 minutes = 3600 seconds in an hour. 3600 divided by the mph you set on your cruise equals the seconds it should take to drive a mile. 3600 / 60 = 60 seconds. 3600 / 70 mph = 51.4285 seconds. 3600 /(divided) by the seconds on your stopwatch = the mph you are driving.