gear ratio question

TClovis · Apr 6, 2006

I've asked a question similar to this one but I'm still a little unclear.

With a M21 trans and 5.12 gear ratio with 15's, what am I looking at?

I know gas mileage will be ridiculous but will I screw something up for around town driving? I want to hold off a little on changing gears due to expenses but I want to know if I can realistically drive the car with that set up.

zdld17 · Apr 6, 2006

5.12:1?? you are looking at start line ratio of 11.2 and lots of gas. Didnt think anyone ran those gear on the street except for pulling lots of load.

You could use just 3rd and 4th and reverse.

TClovis · Apr 6, 2006

I'm new at this: What does 11.2 mean? What about speed/rmp's?

JimM · Apr 6, 2006

11.12 is thew overal ratio in fiirst gear, 2.2:1 x 5.12:1

if I figure on a 255-60-15 tire, you're at 4138 rpm @ 65 mph. I wouldn't take it on the highway much.

TClovis · Apr 6, 2006

So, from the sound of that it may not be too bad for around town driving. Do you agree Jim?

zdld17 · Apr 6, 2006

You will be the first one away from the traffic light, hopefully you are the first one there. This application is totally performance minded. But you can play with it.

kencar69 · Apr 6, 2006

I would not run that combination on the street! 4100 + rpm's at 65 mph will be very hard on your engine, and I think it would shorten its life considerably. I was running 3.73 in my rear with a turbo 350 doing about 3100 rpm's at 65 and while it was doable, it always felt like the car was waiting to shift into 4 th gear.

pdq67 · Apr 6, 2006

The way to do this is to install tall, skinny rear tires.

Way back there, I ran Firestone Town& Country, 9.50/14, mud&snow tires that were right at 29" to 29.5" tall on my stock, 14" x 6" painted steel rims vs stock, D-70/14's at 25.3" to 25.5" tall.

Speedometer at 3050 to 3070 rpm went from 70 mph to about 82 mph!!

I had to raise my rearend up about 4" to 4.5" using coil-over, overload helper springs to keep the tops of the wheelwells from sitting on top of them. Worked fine but did look funny!!

Old-school all the way!!

Topend went from about 130 to 145!!

pdq67

TClovis · Apr 6, 2006

Wow, now I'm nervous about this.

If I went ahead and dropped down to factory gears, which are 3.73 what would the RPM/speed look like with that combo? I don't know how to do this and it is a $900.00 job in Clovis, Cal to change out gears, so I was trying to just get by for a little while.

Larger Dave · Apr 6, 2006

Actually factory gears were more like 3.07:1. I have run 5.13 and 5.38's on the street. Street being defined as driving 14 miles to the What'a'Burger to pick up a race then home again. With 5.12:1 gears you can start out in fourth gear. But everyone around you will figure you don't know how to shift out of first.

TClovis · Apr 6, 2006

My BU 12 bolt shows a 3.73 gear ratio but it was changed out to a larger carrier in the 70's and 5.12's.

Thanks for the help.

JimM · Apr 6, 2006

3.73's would lower your rpm @ 65 to what, 2900 or so?

It would be much more practical, but still pretty buzzy on the highway.

I don't think anyone's saying you "can't" drive it around like that, but you'll be avoiding running at 55 plus any more than a few miles at a time.

pdq67 · Apr 6, 2006

Let's put this, this way....

Our engines were made to run 3 to 3,100 rpm at 70 mph all day long for the rest of their lives!

Trouble is now, that is like MAYBE 17 mpg!!!

I got 15 with my hopped up 406, (M-20, 3.31's and 26.5" tall L-60's), driving about 60 mph on the highway back in '86 when gas decided to go up a bunch again! Driving about 80 miles a day, daily commute when i was in town!! It was like $6.00/day in gas back then.

That's why I bought a new '87 Chevy Sprint ER/Suzuki Swift, 3-banger 5-speed hatch, b/c it got 50+ mpg running 70 to 75 mph!!! AND the darn sucker did it for 200,000+ miles up until it wouldn't pull itself due to a 100 percent shot clutch at 227,000+ miles AND the engine and tranny were FINE!! Body, beat all to heck tho or I woulda put a new clutch in it back then..

Tell me about GM's quality will you!!!

pdq67

TClovis · Apr 7, 2006

Well, I've decided to go back to 3.73's after this discussion. I'd like to cut the corner because of expense but I also want to enjoy the ride (town & freeway)

Thanks.

zdld17 · Apr 7, 2006

3:73 will be just fine on a 4 speed car,, I did this for 35 years at about 3300 rpm at 70 until recently when I went Tremac, down to 2200 range. As someone mentioned,, these motors were built with 3000 all day rpm in mind but the gas price per gallon has got us thinking differantly now. Goferit.

69RS-Yenko · Apr 9, 2006

If you still want to have the out of the hole grunt as well as good drive manners. I'd look into a 3.90 which is a good all round power gear, not to steep, not to mid range ok top end. Ran that ratio for years and pulled a ton of cars with 4.10s that is with a M-21 which has a strong 2nd gear which works perfect with a 3.90. It's a hard to find gear but if you look you'll find one, mostly used in road race cars.

gear ratio question

TClovis

zdld17

TClovis

JimM

TClovis

zdld17

kencar69

pdq67

TClovis

Larger Dave

TClovis

JimM

pdq67

TClovis

zdld17

69RS-Yenko

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