Overall/average diameter of a 285/75/16 tire?

[Gary - K7GLD]

More specifically a Toyo M-55 10 ply E rated tire. I current have Michelin 265's installed, and with a bit over 40K miles, they measure out at 30 3/4 inches from the pavement to top of the tire.



My concern relates to issues in having room to install the spare in the proper location - looks to be less than 2 inches overall remaining space between the frame rails - might be a deal breaker if the new spare won't fit!



Anyone here have a 285 spare properly fitting under their bed?

 

[klenger]

According to the gear speed calculator on my website, the diameter is 32. 83".

 

[Gary - K7GLD]

According to the gear speed calculator on my website, the diameter is 32. 83".

HMMmmm - we might have problems - I'll have to try the spare first to see if it will fit, then go from there. I know a fair number of guys run the 285's, but never heard any spare mounting issues.



We'll see...

 

[24V-DSL]

http://www.toyo.com/tires/tire_specsheet.cfm?id=11



I have them, but didn't buy a spare.

 

[VicR]

Gary - just an added caution if you have an aftermarket rear-end cover (ie mag hytec) as those infringe on the spare tire space on a 2wd too.



-Vic

 

[SNiebaum]

just a fwiw, 285/75/16 is a metric/english tire size. 285 is 285mm of tread width, 75 is the sidewall height as in 75% of 285mm is the sidewall height. So a 285/75/15 tire is . 75x285mm = 213. 75mm = 8. 42in(213. 75mm/25. 4 mm/in) x 2(2 sidewalls per diameter) = 16. 84 in + 16 in(rim height) = 32. 84 in. The true dimension varies per manufacturer.

 

[klenger]

= 32. 84 in.

+/- 0. 01"



All that math is built in to the worksheet I used.

 

[MCummings]

Here is one thing I figured out today too.



The size of your rim has to be factored in as well. a 245/75R16 is not the same as a 245/75R17



A 305/55/R17 and a 305/55R22. 5 aren't going to be the same either! LOL



Just a heads up.





Merrick



EDIT: I glimpsed over teh formula's but now I see the rm height is factored in,, my bad.

 

[mcoleman]

Gary the Literature on my 285s says 32. 9.

 

[RonA]

Gary.

I run 285's on my first gen. Different brand. The spare is a tight fit. It is also pretty heavy. I have no problem , but some may have a hard time putting it up there without a floor jack or an extra set of hands.

Ron

 

[Gary - K7GLD]

OK, first, a big thanks to all who offered help - and after the dust settled on the tire shop floor, here's what we ended up with in real life.



Toyo M55 - 285/75/16's on stock rims measure out at 32. 75 inches at 60 PSI inflation pressure. They fit the spare tire space like THIS:



-



PRETTY dern tight!



The fit to the frame rail is just as tight on the other side, and REAL close fit of the tire tread to the Rip Rook 4 inch exhaust. After this pic was shot, I made up a sandwich of a smashed Coke can wrapped inside multiple layers of aluminum foil to shield the tire from the exhaust pipe, and will keep a close eye on it for heat issues.



I've updated the speedometer with the Smarty, and now with a 4% or so added "overdrive" will see how various driving and towing issues sort out - I'm especially interested in use of 5th gear mountain towing at 55 MPH in these narrower, winding Oregon grades - should comfortably cruise at 55, with power and RPM to spare when needed!



Cheers!

 

[klenger]

So, I guess I was off by 0. 08". I'll try to do better next time.

 

[MCummings]

So, I guess I was off by 0. 08". I'll try to do better next time.

[crazy guy from police academy voice]Thanks, 'cause you were really startin' to **** me off![/crazy guy from police academy voice]

 

[Gary - K7GLD]

So, I guess I was off by 0. 08". I'll try to do better next time.

I would imagine there would be minor size deviations depending upon wheel width and inflation pressure - my stock wheels undoubtedly are not optimum for this size tire - and that probably affects final measured size. .

 

[jrobinson2]

I've shoved a few different brands and styles of partialy worn 285s under mine, I couldn't get a nearly worn out 295 to fit



Jared

 

[PPettit]

Gary,

I would be interested in your driving impressions with the larger tires installed. I went from the 265/75-16 Toyo M55's to the 285/75-16 M55's on my '01 with the automatic. The truck really feels down on power now. Going over the passes empty it downshifts on grades that did not cause a downshift with a load in it before.



Phil

 

[Gary - K7GLD]

Gary,

I would be interested in your driving impressions with the larger tires installed. I went from the 265/75-16 Toyo M55's to the 285/75-16 M55's on my '01 with the automatic. The truck really feels down on power now. Going over the passes empty it downshifts on grades that did not cause a downshift with a load in it before.



Phil

HMMmmm - that seems sorta weird - there IS about a 4% difference in overall "gearing" going to the 285's from 265's, but that difference in diameter sure would seem too trivial to cause what you are experiencing - unless you happened to be at the ragged edge of downshifting with the 265's.



We'll see - our 5er is relatively light - a 24 ft Kit that weighs a maximum of about 7000 lbs wet and loaded - but previous mods with that load provided us with abundant power, and the ability to crest Sierra grades at 100 MPH in 6 gear if we wanted to! :-laf :-laf



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Biggest difference here in Oregon, is the narrower curvy roads, where it's more difficult to maintain decent road speed, so rather than lug the engine, or constantly down/upshift, I figured to go to the larger size tires and just plan to make 5th gear my "mountain" gear...