RPM's too high at 70 mph for 2.75 gears

toastintx · Apr 25, 2016

ok need a little help 1973 Mach 1 with FMX 3 speed auto and standard 2.75 rear gears. my problem i think the rpms are way too high. at 60 mph running 2600 and at 70 mph a +3000. this is too high right? what can cause this. the cars seems to be running ok but wow. i was thinking about going with 3.50 gears but that would be crazy rpms. whats my problem. oh and running 26.6 inch tires

73pony · Apr 25, 2016

Is your tach right. With 2.75 gears and a 1:1 ratio (3rd gear in FMX) you should be at 2,433 rpm.

Don65Stang · Apr 25, 2016

Is it stuck in 2nd gear?

toastintx · Apr 25, 2016

maybe my tach needs to be calibrated. and yes i hear the tranny shift to 3rd. can the tach be calibrated?

73pony · Apr 25, 2016

Could be the prob. 2nd gear on FMX is 1.47 I think.

toastintx · Apr 25, 2016

bought the car back in nov and all seems to be running great, but this is a little worrisome

73pony · Apr 25, 2016

Do you have another tach or aftermarket one you can run some wires too (from coil, ground, power) temporarily and test drive and compare the two?

toastintx · Apr 25, 2016

I guess I will try and find one to hook up so I can check. Maybe that's why I get crap gas mileage. I know that these cars aren't great on mpg's. But....

barnett468 · Apr 25, 2016

.

Speedo is also likely off some.

barnett468 · Apr 25, 2016

.

Did you physically verify the rear gears by spinning the tire?

Might not have 2.75 anymore.

.

73pony · Apr 25, 2016

.

Did you physically verify the rear gears by spinning the tire?

Might not have 2.75 anymore.

.

Good point. Also to verify speed do you have a smart phone? If so download a free GPS speedometer app.

toastintx · Apr 25, 2016

Yes Barnett468 just did that tonight, open gear one wheel off ground two complete rev on tire 2 3/4 turns of the yoke

73pony · Apr 25, 2016

Then there is one of three thing going on.

1 speedo is way off

2 tach is way off

3 Trans is stuck in 2nd

toastintx · Apr 25, 2016

Just downloaded a phone GPS app so I can check mph. Process of elimination now. Thanks guys foe your advise, you guys rock!

73pony · Apr 25, 2016

Let us know what you figure out.

toastintx · Apr 25, 2016

Will do Jason

Bill73Ragtop · Apr 26, 2016

That kind of rpm is more in line with a 3:25 rear end gearing than 2:75. If it was stuck in 2nd, that would be pretty obvious and rpms would be way up there at 60 mph.

73pony · Apr 26, 2016

I am curious what you do find out with the speedometer. If that is what is off here is how it calculates out....

2.75 rear

1.00 final ratio

3000 RPM

26.6" Tire Height

Puts your speed at 86 mph

toastintx · Apr 26, 2016

Yeah that's what I thought I'm guessing Speedo is good cause I wasn't going that fast to hit 3k lol

RPM's too high at 70 mph for 2.75 gears

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