Does wheel size alter gear ratios?

[Orrsum]

i am wanting to upgrade my Navara wheels from 16 inch to 17 inch and go from a 255/70/16 tyre to a 265/70/17.

Somebody told me that if i increase too much i will not be able to move off the mark when towing :-/ due to the fact that it will alter the gear ratio.
can anybody explain this to me or offer some advice.

[PADDLES]

yes, the increased rolling diameter will alter your final drive ratio. ie. revolutions of the motor compared to revolutions of the wheel. i doubt it will affect you to the stage of not being able to move, but it will place extra strain on your clutch and drivetrain when taking off loaded. you will have lower rev's for the same km/h speed effectively.

[FNQCairns]

Yes it will alter the ratio, your speedo will be out a little also, I fully suspect that you will be able to tow Ok it's not a huge amount your contemplating.

Try a google on "tyre size calculator" for more details, don't forget the yanks write tyre as tire I think.

cheers fnq

[Orrsum]

thanks Guy's. i have noticed that the speedo is out by about 6klm, according to my GPS ( it says that i am going faster than i really am) hopefully this will bring it closer to the actual speed. i think the final difference is about 40 mm.

[Orrsum]

I doubt it will affect you to the stage of not being able to move, but it will place extra strain on your clutch and drivetrain when taking off loaded. you will have lower rev's for the same km/h speed effectively.

not good news about the extra strain on the clutch, its very light as it is.

[Crocodile]

Hello Orrsum,
It is all to do with the outside diameter of the tyres.
Try the tyre websites for specifications, eg. Toyo.
225/70/16 OD of 0.726m.
265/70/17 OD of 0.804.
That is about a 10 percent increase in diameter, which equates to a 10 percent increase in distance travelled per revolution of the wheel.
Would it make the car harder to get rolling?.
Yes it would, by 10 percent.
Remember that this is 10 percent in all gears, you may have trouble holding fifth on hills.
Acceleration will be reduced in all gears.
Will 10 percent be noticable?
Unless you get a scientific reply from someone who has made the same change, you don't know.
You may be able to do a quick swap with someone who has taller tyres to try it out.
You speedo will under-read by 10 percent, 100 indicated will 110 actual, watch out for radars.
Be aware that there will be issues of legality to do with non-standard tyre diameter, your insurance company can decline a payout if ther is a claim.

[Orrsum]

the navara 'Outlaw' came out with 17inch wheels and the ST-X came out with 16inch, thats why i didnt think it would make too much difference.

[GBC]

I had 10% bigger tyres on my last hilux.
The effect was noticeable in that it was more sluggish, harder to hold top gear, and most importantly - the brakes hated it and did not work anywhere near as well as they should have. They were also illegal in my case (33").
There are upsides for a higher volume tyre offroad which is why I did it.
A tyre shop will be able to tell you the diameter of the outlaw tyres.

[Orrsum]

the original is 765OD and the new ones will be 804OD so its not even 10% my calc is 4.85% increase or am i working it out incorrectly?

[chisel]

Note that technically, legally in qld you can only increase the overall diameter of your tyres by a maxiimum of 15mm.

[Wahoo]

bigger tires threw my speedo out by 11K's, clutch hated it big time

[BenDover]

If they arent the same brand tyre you had before. Best thing to do is measure the difference physically. Tyres arent tyres anymore in the 16 range and I HATE THEM. eg.. A maxxis 275 70 16 isnt the same size as a bfg 275 70 16. It can vary from manufacturer by upto 45mm. The jargen is 16's are a better fitment and handle better on the road. All true. But they cost alot more aswell!!!

You will lose power and eco in height and width. But your getter much more millage then the odo shows unless you get it recalibrated. Change your diff gears to suite.

[sempre]

Had a similar problem with my car , its a nissan gu , put a set of 315 - 70's on it and had to change the diff ratio's to 4:6's , goes like the clappers now but speedo is about 10% out .
Sam.

[Orrsum]

Note that technically, legally in qld you can only increase the overall diameter of your tyres by a maxiimum of 15mm.

isnt that all going to change in July? i heard to a max of 50mm.

[Orrsum]

had to change the diff ratio's to 4:6's


Change your diff gears to suite.

how do you alter the gear ratio?

[kokomo]

Change the Diff Ratio and all order will be restored...

Did it on my Brothers Surf HIlux..

Easy and cheap..

Go see a mechanic

[Noelm]

as a general rule, you do NOT change the diameter of the wheel and retain the same "profile" tyre, you get a tyre that has a lower profile and the 'rolling" diameter remains almost the same, so what I am saying is,,, if you had (say) 15" wheels and had 75 profile as in your tyre size as 15 X 265 X 75 when you go to (say) 17" wheels, you would then fit 17 X 265 X 65, that will ,maintain the overall diameter, these dimensions are "just for instance" and you will need to go to a good tyre service and ask for proper advice, or, you can alter the speedo drive, as this is what is done on cars that are available with various wheel sizes and diff gear ratios from new, or you can just do it and put up with it, but remember speedo readings could bring you undone at speed cameras and stuff!

[ak73]

or just fit a speedo corrector kit from Jaycar for $50

[snatch]

Get a gps and test your speedos current accuracy. I found that when I fitted Bridgestone duelers to my D40 that to get a tyre that fitted I had to go slightly oversized. It actually corrected my speedo as it was slightly over indicating with the standard factory rubbish.

Going to an oversized tyre increased my 2000rpm speedo reading from 97 to 100kph and when tested against a gps it was accurate. In other words the circumference of the tyre eincreased by roughly 3 %. Hope this helps.

[Orrsum]

Get a gps and test your speedos current accuracy. I found that when I fitted Bridgestone duelers to my D40 that to get a tyre that fitted I had to go slightly oversized. It actually corrected my speedo as it was slightly over indicating with the standard factory rubbish.

Going to an oversized tyre increased my 2000rpm speedo reading from 97 to 100kph and when tested against a gps it was accurate. In other words the circumference of the tyre eincreased by roughly 3 %. Hope this helps.

thanks for that, its what i was getting at, my speedo overstates my current speed by approx 6km/hr. i can get bigger tyres for the current rims its just that i have come accross some 17inch ones at a really good price.

thanks again.

[White Pointer]

i am wanting to upgrade my Navara wheels from 16 inch to 17 inch and go from a 255/70/16 tyre to a 265/70/17.

Somebody told me that if i increase too much i will not be able to move off the mark when towing :-/ due to the fact that it will alter the gear ratio.
can anybody explain this to me or offer some advice.

G'day,

Make sure it is legal according to the owner's handbook or the tyre placard. If not, you will need a modification plate to recompliance your vehicle. This is critical for insurance.

The tyre seller should be able to tell you the rolling diameter of your OE W&T and the ones he wants to sell you. They have to be exactly the same! If you are buying a set of 5 it is not so critical, except make sure it fits. But if you are keeping one of the originals as a spare, and the diameter is different, you will blow up your diff if you put it on.

The low profile tyres will be softer and may run power pressure. They will stake easier and may not have the load index rating to maintain your carrying capacity, including towing loads.

White Pointer