Tire change = Drive Ratio change?

[PoorManQ45]

It would seem to me that you could effectivly make your Q45 have the same total drive ratio of 4.08 gears by going with smaller rims/tires than OEM.

Is this true? If so, what would the overall height of the tire have to be to mimick a 4.08 rear-end. Also, why spend ~$1000 on new rear end gears when you could just buy a set of tires?

Also, changing to larger than OEM tires would cause the Speedo to read slower than you were actually going, correct? If that is true, wouldn't that cause the Governor to be raised? And, wouldn't larger than OEM tires allow you to get slightly better gas mileage because the Engine RPMs would be lower than OEM at the same speed.

[97Q45t]

It is possible, but not a wise choice.To stimulate a 4.08 gear change from a 90-93 Q with a final drive ratio of 3.538:1 and 215/65/15 tires (26.2" diameter), you will need to replace the tires with a set which has the overall diameter of 22.7". One size would fit that description is 195/50/15. My friend, that is the size of a civic tire. You would have to replace the the rims, and possibly tires every single time your car go over a moderate bump or pothole. Then it would cost more than $1000 for the gear swap.

[louiegz]

One size would fit that description is 195/50/15.

Not only that, that size tire would have a load rating of 82. Far short of the 95 load rating that you need for a 2 ton Q. It would not be safe.

[Rex]

As mentioned, it would require some very small tires which would look bad in the wheel wells, IMHO. But is a decent idea for some drag slicks assuming of course you can find some with the correct load rating.

[AZhitman]

All correct.

Lastly, larger tires reduction of overall RPM's and subsequent improvement of overall mileage would be offset by the additional horsepoer required to set larger tires in motion (unsprung weight/mass).

So while your hypotheses are correct in theory, they have little if any real-world benefit.

[PoorManQ45]

Ok, lets assume that a set of tires to simulate the 4.08 gears could be had with the proper load rating. Also, lets assume looks do not matter. Also, to circumvent the problem of a bent rim or poped tire(from tiny side-walls), couldn't you just get smaller rims, say 14~15in.

Would the 14 or 15in rims clear the Q45 brakes?

Would there be any other problems with going with smaller tires?

[elwesso]

I dont think 14s would clear the brakes... 15s dont leave mcuh room...

[AZhitman]

Sure. You could put "theoretical" tires on "theoretical" rims with plenty of "theoretical" caliper clearance and theoretically do all those things.

The load rating of the tires, weight capacity of the wheels, ridiculous appearance and brake clearance issues don't go away just because you want them to.

There's several Lincoln and Cadillacs in South Phoenix running around on 13" Daytons with zero offset and 185/50/13 whitewalls - Might want to log onto lowrider.com and ask them.

[BigBodyQ45]

There's several Lincoln and Cadillacs in South Phoenix running around on 13" Daytons with zero offset and 185/50/13 whitewalls - Might want to log onto lowrider.com and ask them.

I think I have seen an Infiniti too.....

[JT_MONEY_2004]

I JUST PISS MY PANTS !!!

HAHAHHAHHAHAHAHAHH

[vicQ45]

HAHAHAHA thats teh funniest thig ive EVER seen!

[AZhitman]

LOL - Good one, BBQ!

[elwesso]

I see those guys on caddy forums all the time

They get these itty bitty wheels, the big fleetwoods need about 99 load rating... "yeh man i gaht da 195-60-14, 88 load ratin'.. thass enough, ainnt it?"

Death on wheels

[DR.Q]

Some people really need to be slapped. I bet that thing handles awesome with ZERO contact patch!

[Jeff Williams]

The main difference in changing tire sizes, is that by going to a smaller tire size, would not net any more faster ET's because the car is now going slower, than before with stock tires.

The problem wiht bigger tiers, is weight, and mostly wind resistance at higher speeds. Taller tires usually give you worse gas mileage. I have experimented on my Nissan truck. I started out with 225/70/15 and went to 225/60/15, and tried 235/75/15 tires. The best mileage was the smallest tires, even though I was running higher RPM's.

A lot of drag racers, use the higher rearends, to offset the taller slicks they run on the back of the car.

The ral problem comes in, when the slicks "Stand" up. You have seen the top fuel cars, where the tires grow asround 25%

[Q45tech]

"I bet that thing handles awesome with ZERO contact patch!"

Tire size has nothing to do with patch size, the patch just rearranges its shape to support the weight via psi. Skinny width loooooooooooong shape.

The number of revoluution per minute gets increase so the tire has much much less time to cool when not in contact with road...............so these smaller diameter tires fry even in mild weather.

[DR.Q]

So a dump truck tire and a skateboard wheel(assuming it was inflatable and the same width) have the same contact patch with the same amount of weight on them?

[AZhitman]

Not a fair assessment.

They don't have similar overall surface area.

The skateboard-sized tire would flatten (desperately attempting to approximate a sufficient contact patch) until it ran out of surface area.

[PoorManQ45]

ROFL!!!

I think you may have taken it to an extreme.

[DR.Q]

It wasn't intended to be fair, I just took it to extremes to show how it does differ with diameter.

[AZhitman]

Great (albeit long) discussion here on this topic, illustrates both schools of thought.

http://forum.miata.net/cgi-bin...206;p=

[maxnix]

It wasn't intended to be fair, I just took it to extremes to show how it does differ with diameter.

It doesn't really. Think of the load carrying capacity that includes the weight of the load carrying device (tire wheel combination).

[AZhitman]

This matter can be easily resolved with an ink roller, a jack, a Q45, a tire gauge, a piece of butcher paper and 2 different width tires mounted on wheels.

[AZhitman]

HINT: I've seen the results (high school shop class) when the instructor ridiculed the enormous Gatorbacks on my '68 GS.

[DR.Q]

http://www.ffcobra.com/FAQ/handling102.html

[DR.Q]

http://www.kormanfastbmw.com/ttiredia.htm