Tyre size selection - Ford Forums

JBonDSS · 09-22-2003, 20:02

If I can change tyre size from 275/75/15 to 285/75/15 or 31/10.5/15 the speedo reading is 1.9 % too slow
(as per the miata tire size calculater http://www.miata.net/garage/tirecalc.html)

will that lower the revs on the hwy or increase them?

Will the motor have to work harder or less getting of the line?

351w · 09-22-2003, 20:17

yes your motor will work harder if you put taller tires on it, and your brakes will have to work harder as well but your rpm's will show a lower number on the tach than it did with smaller tires.
but to go from a 275-75-15 to a 31-10.5-15 is not a difference that should cause any problems.

89F150 · 09-23-2003, 00:44

Yeah i dont think you'd have any problem with changing to those tyres and i dont think you'd see much difference.
My speedo is out about 6 ks at 100kph.

ducky · 09-23-2003, 04:00

Re your tyres be careful. I had a ford explorer and the tyres fitted by ford were 235/75/15, after about 5 years of use I changed them to 31/10.5/15 and had travelled about 20,000 klms and the gear box gave way, I was advised that it was possibly due to the oversize tyres. Fuel ecomony dropped and much slower on the take off. After having a replacement gearbox fitted similar syptoms were starting to show up again. Sold the Explorer and purchased a F150. A friend of mine who works for a Ford Dealership put larger tyres on his Explorer and also lost ecomomy. He then went back to the original size

JBonDSS · 09-23-2003, 17:51

mmmm interesting
yeah i found the tyres are 235/75/75 so the comparison to the 275/75/15 is 8.2% slow reading from the speedo

I'd rather not break a Auto trans nor loose that fuel economy at this stage (still on petrol)

351w · 09-23-2003, 22:00

i run 275-70-15 on my truck and it works really nice but i also have mods done to increase my HP.
actually i find that the 275's are wider and a little heavier than the 10.5's that i had before.
i prefer the 10.5's and i am looing around right now at buyin a set before snow starts falling.
also i have to say this to DUCKY i would not recommend myself putting that size tire on a 4 cyl or 6cyl bronco or explorer but this is an F series post and the parts used on the F series trucks will hold up alot better than an explorer would with this type change.
my truck is an 86 with over 150,000 miles on it and i never lost a tranny or rear but then again i take care of them and change fluid in them.
but oversize tires will add wear for sure.

89F150 · 09-24-2003, 00:53

My tranny is starting to go at 230000 miles. Still the original.

351w · 09-24-2003, 23:28

time for a rebuild anyway 89f150.

jbon if you want a slightly bigger tire try a 30-9.50-15.
they arent as big as a 31-10.5 but still look good and give you about 1 inch to 1 1/2 inches more ground clearance than the stock tire size.

89F150 · 09-24-2003, 23:34

Yeah i was planning on a rebuild about 6 months ago but ended up buying another car.