Changing gears - pros Vs cons on size.

chromebumpers

chromebumpers

solid fixture here in the forum
Staff member
I know that my car originally had a 2.56 gear ratio and then someone changed to a whole other rear which now has a 3.73 ratio. What effect does this gearing have on the motor or trans? On a non-racing application, a car that had no other changes done. FYI, I don't know if this matters 14" rims then vs now at 215/75/15 today. Does the road speed read faster or slower on this car's speedometer?
 
Speedometer will read faster and odometer will rack up mileage faster. You won't want to drive on the highway. You will stop frequently at gas stations as you'll use a lot more fuel. I have a 2.41 rear and a 3.73 rear for my Firebird. I use the 2.41 for road race tracks, land speed races and trips. I use the 3.73 for the strip and usually leave it in a while afterward for some quick acceleration around town but it gets old pretty quick with the engine revving so high (short tires) and I put the 2.41 back in.
 
you can compensate for the rather large change in rear gear ratios (2.41:1 vs 3.73:1) roughly a 60% change , in the way the engine rpms to ground covered per, tire rotation, with a manual transmission and changes in driving styles but a change that drastic with an automatic transmission is sure to work FAR better with one of those ratios that the other.
you'll definitely find you'll spend a gear deal more time between gear shift points with the 2.41:1 rear gears , and rarely see the car reach the upper range gearing during street driving.
btw buy new differential gears, not used ones, those are a Risky buy,
if these gears were run with poorly set up tooth contact pattern,
you will inherit noisy gears, no matter what you do.
have an experienced shop or mechanic set up the rear gears,
if they are not correctly installed they will be ruined or at least noisy
in fact your differential can differ just enough, dimensionally,
to cause new issues to come up with previously run gears from a similar differential,
that worked ok in that differential
READ THE LINKS AND SUB LINKS
http://garage.grumpysperformance.co...late-gear-ratios-and-when-to-shift-calcs.555/
gmtransgeart.png

do a few calculations, think about the rpm band
YOUR engine makes its best power in,
and YOUR tire diameter
and YOUR cars weight
and YOUR rear gearing,
and YOUR trans GEARING
as a general rule if your looking for brisk street performance,
(especially with an over drive transmission)
youll want the transmission first gear . multiplied times the rear gear ratio to fall in the 10:1-10.6:1 range
and youll want the engine to cruse at 70 mph in the lower end of the engines effective torque curve, If you take the time and effort to do all the math and accurately match the rear differential gear ratio, transmission gearing and tire diameter youll eventually find a clear consistent advantage to use of a transmission with an over drive top gear ratio of near .70-.75 in most cars and a rear gear ratio in the 3.73:1-4.11:1 range for performance cars.

http://garage.grumpysperformance.co...late-gear-ratios-and-when-to-shift-calcs.555/

http://garage.grumpysperformance.co...r-gears-at-the-transmission.12179/#post-59006

http://www.wallaceracing.com/gear-speed.php

http://vexer.com/automotive-tools/speed-rpm-calculator

https://www.summitracing.com/expertadviceandnews/calcsandtools/gear-ratio-calculator

http://spicerparts.com/calculators/transmission-ratio-rpm-calculator

http://www.differentials.com/technical-help/differential-gear-ratio-calculator

http://vexer.com/automotive-tools/1-4-mile-ET-HP-MPH-calculator
 
Last edited:
Leave the 3.73 Rear Diff gears in the 1968 Catalina Richard.
Least of your concerns.
 
87vette81big said:
Leave the 3.73 Rear Diff gears in the 1968 Catalina Richard.
Least of your concerns.
Click to expand...

I'm just doing what I do with every car, making sure it's mechanically right before I move to the next project.

I'm feeling better lately, most of my pain is gone so I'm getting more done. The weather will cool down soon making it more enjoyable to work on things myself. The Charity Truck will be easier than first planed, in fact I have to slow down to let donations get started otherwise I won't be able to do another charity project - and I'm already started thinking of that now. This is why I'm pushing to get the ragtop done asap, I hate having projects unfinished. Sitting too long and they become a problem and worthless to me.
 
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