Tach readings for different rear ratios

[Willard M.]

Yesterday I was driving my 64 fuel coupe with 3.36 rear down a lovely Maine road at a reasonable speed. The tach said about 2200 and the engine wasn't real happy. By dropping it into third all was well with the world as that solid lifter motor just sang. This got me wondering about my 4.11 rear sitting in the garage and thinking it really belonged in the coupe. Are there any charts giving me engine speeds for different rear end ratios? For example, at 60 mph what would be my engine speed with a 3.36,3.55,3.70,4.11 etc?

Bill #2659

[Jim K.]

Re: Tach readings for different rear ratios

Hi Bill:
I actually wondered the same thing but with tire size thrown into the mix. I ended up doing a lot of math and I think I made an excel spread sheet... I may still have it somewhere if you want me to e-mail it to you...
Jim

[Duke W.]

Re: Tach readings for different rear ratios

The tach/speedo readings do not change as you swap axle ratios without installing the appropriate speedometer gearing, and without the correct speedometer gearing for the installed axle, the speedometer will be inaccurate.

Top gear revs @ 60 = axle ratio times tire revs/mile. The speedo is calibrated for 760 revs per mile (6.70-15 and 225/70R-15) and did not change for 7.75-15 and F70-15, which are both 775.

The 30-30 cam and FI have poor torque below 3000, so if you want maximum accleration at highway speed, downshifting to second is called for.

In a top gear pull from 60 with 3.36 axles in both a 300 HP and 375 FI, the 300 HP will initially pull away. The 300 HP engine makes more power up to about 3500.

Duke

[Harmon C.]

Re: Tach readings for different rear ratios

A good way to get the speed readings in todays world is a GPS. I belive a GPS is very close to being correct and you can compare it to the speed your speedometer is reading. In my C-5 it is a perfect match at any speed. Hand held GPS's can be found for $50.-$80.
Lyle

[Doug Flaten]

Re: Tach readings for different rear ratios

Assuming your tach is correct and that your speedo is reading correctly for your rear axle and tire combination, then you can easily calculate the engine rpms for different gear ratios by multiplying the curent engine rpm by the ratio of new axle ratio divided by the current axle ration.
If your example of 2200 rpm is at an indicated 60 mph, then a rear axle ratio of 3.55 would increase the speed to: 2200 x (3.55/3.36) = 2324 rpms
Similarly 3.70: 2422 rpm and 4.11 : 2,691 rpm
Doug

[Mark #28455]

real easy at about 80 MPH

In my 69 and 70 cars, at 80 MPH, the 3.08 runs about 3100 RPM, the 4.10 runs about 4100 RPM - you get the idea!
Mark

[Mark #28455]

real easy at about 80 MPH

In my 69 and 70 cars, at 80 MPH, the 3.08 runs about 3100 RPM, the 4.10 runs about 4100 RPM - you get the idea!
Mark

[Terry D.]

Re: Tach readings for different rear ratios

Bill

Go to www.keislerauto.com click on Chevy and then the speed analizer on the left. You plug in tire size and it will tell you the speed at any given rpm. It compares the speed with a stock 4 speed to their 5 speed but it will tell you what you want to know without doing all the math yourself.
Terry

[Terry D.]

Re: Tach readings for different rear ratios

Bill

Go to www.keislerauto.com click on Chevy and then the speed analizer on the left. You plug in tire size and it will tell you the speed at any given rpm. It compares the speed with a stock 4 speed to their 5 speed but it will tell you what you want to know without doing all the math yourself.
Terry