Darkness7

I currently have 4.10s, but after a crappy 1/4 mile time with 20" rims (weight/diameter) I've gone back to the drawing boards...

Currently I have a 275/35/20 rear tire (4.10 gears), but my mechanic said that by running the 20's I basically put the gears back close to 3.55 or 3.73... Is there logic in this? Sorry I have no experience with all these ratios, etc.

My cure to the problem (at least I hope) is that I have a pair of 17" stock wheels that I want to mount a set of drag radials to.

The two DR's I'm contemplating are MT's 275/40/17 or M&H's 275/50/17's, but I don't want to run into the same problem with diameters...

MT's (275/40/17)
4.3" sidewall
25.7" diameter
786.1 revolutions/mile

M&H's (275/50/17)
5.4" sidewall
27.9" diameter
723.9 revolutions/mile

Which one should I go with? Also, at the track I will retain the front 20's (255/35/20) because I have Brembo brakes and the stockers won't fit, and I have the car lowered with Steeda Ultralite's. I don't want to put the MT's (40 series) on and have the rear sit lower than the front .... Anyone have experience in this?

One more question.... I have a complete back up rear end and my mechanic was persuading me to put 4.30s in it since I run 20's on the street.... He says that with the 20's it would be like 4.10s, and at the track with the 17's it would be a beast on the 60ft times...

Hope this makes sense...

Stkjock

the 20s hurt due to weight and lack of side wall, that size is 27.6" its not that much taller then stock, so maybe your gearing comes down to a 4.04, you mechanic is way off IMO

the shorter tire will give you more gear, I run a 26.4 rear tire and a 27.25 front and its not an issue

the smaller rim/tire combo should help your times as you'll have more sidewall to work with and less rotational weight

stealth_GT

all that HP is being used to spin those big *** wheels... why not just get 17s for the track??

you don't have TPMS so you don't have to worry about messing it up!

(-:

grampa_stang

Mechanic is wrong about your 20's. With that size tire your running...you're just slightly taller than stock...so yeah...maybe the 4.10's are now 4.04's. Stkjock is on the $$

Darkness7

Great info. Thanks. So what side wall on the DR should I go with then? The 40's or 50's? The 50's will be taller thus I might lose some gear.... ?????

Absolutely, that's why I just picked up some stock 17's for the track!!!

Stkjock

well its a balancing act of sorts, a bigger side wall gives a better launch, but the taller tire will cut some gear. what size shot are you running? and have you been down the 1/4 to know what your traps are?

Darkness7

Yea, but it was ****ty times due to the 20's I had on there. You've actually responded to one of my other threads....

https://mustangforums.com/forum/stre...ml#post6165517

My times are posted there.

I don't plan on running anything over a 100hp shot as of now. I don't have the fuel support or plugs to go any higher. I don't want to run lean and blow something up like I've read on here...

157dB

Get a new mechanic.
One who has a tape measurer and a sense of proportion.
Its not the rim diameter, its the sidewall height with 20s.
There is a lot less flex which translates to quicker
loss of traction with any given amout of HP.
The 4.10s are OK. He just want that gravy $$ again that
he got when you had him install the 4.10s. He saw you
disappointed with the 4.10s and is helping you throw $$
at the problem (him).
http://www.miata.net/garage/tirecalcold.html
http://www.1010tires.com/TireSizeCal...?action=submit
Run the #s and see if he is not lying to you.....

Drop to a 17" rim from the 20s and get tires that have the same
outside diameter or Revs/Mile and the sidewall will be there without
power loss from taller tires....