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Old 09-22-2003, 19:02   #1 (permalink)
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Question Tyre size selection

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?
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Old 09-22-2003, 19:17   #2 (permalink)
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Re: Tyre size selection

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.
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Old 09-22-2003, 23:44   #3 (permalink)
 
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Re: Tyre size selection

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.
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Old 09-23-2003, 03:00   #4 (permalink)
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Re: Tyre size selection

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
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Old 09-23-2003, 16:51   #5 (permalink)
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cool

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)
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Old 09-23-2003, 21:00   #6 (permalink)
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Re: Tyre size selection

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.
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Old 09-23-2003, 23:53   #7 (permalink)
 
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Re: Tyre size selection

My tranny is starting to go at 230000 miles. Still the original.
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Old 09-24-2003, 22:28   #8 (permalink)
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Re: Tyre size selection

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.
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Old 09-24-2003, 22:34   #9 (permalink)
 
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Re: Tyre size selection

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