My Auto V6 Fiero runs around 3750 RPM's @ 75MPH and that seems a little high to me. At 80MPH it gets close to 4000RPM.
Stock 3spd AUTO tranny.

Is this about right for a stock AUTO 3spd V6 Fiero?

The gas mileage on the car seems to be a little low even though it is a 3.4 pushrod V6.

I got around 20MPG on the last tank, so I am trying to see if it is the high RPM on the daily commute or possibly something that needs to be fixed that is causing me to fall short of mid-20 MPG's.

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86 GT - Red, Auto, 3.4 V6
88 5.0 Mustang

"Don't even consider keeping up with the Joneses!! They're broke!" - Dave Ramsey

quote Originally posted by LookingForAFiero: Is this about right for a stock AUTO 3spd V6 Fiero?

Yup.

Mine runs 3300 at 75mph. You could be a tad high assuming stock tire size and tach reading correctly. Is your TCC locking up? That would cause a higher than normal rpm.

im running about 3400 at 75. 2.8l auto i average 30 mpg on highway.... and really bad intown.

You've got to remember, that these cars were built in a time when the speed limit was 55 mph, so GM would have probably geared the car to get the best mileage at 55 mph not 70-75 mph

i get 20 if i drive like a maniac
and 16 if i drive like my grandma
isnt that wierd.
not a typo
20 like a maniac
16 like a grandma
ps yeah the revs are normal
all autos were like that.
just the engineering

[This message has been edited by BlackGT Codde (edited 10-20-2008).]

quote Originally posted by LookingForAFiero: My Auto V6 Fiero runs around 3750 RPM's @ 75MPH and that seems a little high to me. At 80MPH it gets close to 4000RPM. Stock 3spd AUTO tranny. Is this about right for a stock AUTO 3spd V6 Fiero?

Your RPMs may be a bit high. My `86 GT with automatic and 3.4L V6, runs stock diameter, 215/60R15 rear tires, and is turning at just under 3400 RPM while cruising at 75 MPH, just about what Hudini and LowNPro reported with their Fieros:

quote Originally posted by Hudini: Mine runs 3300 at 75mph.

quote Originally posted by LowNPro: im running about 3400 at 75.

Are your rear tires 215/60R15s, or a different size, LookingForAFiero?

It all depends on the FDR. If you have the 3.33:1 your RPMs at 75 MPH are correct. If you are running the 3.06FDR then they are high by a few hundred RPM's. Check the FDR from the codes under the hood on the build sticker.

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87GT - 3800SC Series III engine, 3.4" Pulley, N* TB, LS1 MAF, Flotech Exhaust Autolite 104's Custom CAI 4T65eHD w. custom axles, HP Tuners VCM Suite.
87GT - 3.4L Turbocharged engine, modified TH125H
" I'M ON THE LOOSE WITHOUT THE JUICE "

If one also takes into consideration the rear tires' outside diameter, then the point posted by PFF member, Dennis LaGrua, makes more sense as RPM is a function of both the outside tire diameter of a car's driving wheels (the rears with a Fiero) and the car's FDR, or final drive ratio:

quote Originally posted by Dennis LaGrua: It all depends on the FDR. If you have the 3.33:1 your RPMs at 75 MPH are correct. If you are running the 3.06FDR then they are high by a few hundred RPM's. Check the FDR from the codes under the hood on the build sticker.

On my stock automatic-equipped, TH-125 `86 Fiero GT, there are, unfortunately, 39 three-character codes in the under-the-hood location mentioned above.


Does anyone know specifically what 3-character code designates what final drive ratio of the Fiero that one has?

quote Originally posted by BlackGT Codde: i get 20 if i drive like a maniac and 16 if i drive like my grandma isnt that wierd. not a typo 20 like a maniac 16 like a grandma ps yeah the revs are normal all autos were like that. just the engineering

not all Autos are like that , the 87 L4 TH125 has the "best" gearing 1:2.81 if i remember correct 88V6 has the worst 1:3.33
= my 87 L4 is at 3000RPM at 75mph ( running 16" 225 tires on the back )

i made this list here a while ago : http://www.format-service.de/trannyspeeds.htm

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1984 Fiero Sport
1987 Fiero
1984 Fiero SE
1999 Chrysler Grand Voyager 3.3 LX LPG

*** nuclear winter cures global warming ***

[This message has been edited by Fieromaniac (edited 11-03-2008).]

quote Originally posted by project34: Does anyone know specifically what 3-character code designates what final drive ratio of the Fiero that one has?

See this link from the Michigan Fiero Club, this link, or this link. Most (all?) GM build codes for passenger car axle ratio begin with 'F' or 'G.'

Calculating the expected engine RPM for any speed is easy. The formula is:

RPM = (tire revs per mile) x (transmission gear ratio) x (rear axle ratio) x (speed in mph) / 60

The top gear transmission gear ratio for most non-overdrive transmissions is 1:1. Revs per mile is based on a tire's loaded 'rolling radius,' rather than its 'overall diameter.' (Note that the actual in-service revs per mile value will increase slightly as the tread wears.) www.tirerack.com is a good resource for finding the exact 'revs per mile' spec for any specific tire. For example the spec for a BFG Traction T/A in 215/60R15 is 825 revs per mile, and the axle ratio in my Formula (TH125 automatic) is 3.33:1 (build code GX3). Thus the expected RPM at 75 mph (in 3rd gear with the torque converter locked) is:

RPM = 825 x 1.0 x 3.33 x 75 / 60 = 3434

quote Originally posted by LookingForAFiero: My Auto V6 Fiero runs around 3750 RPM's @ 75MPH and that seems a little high to me.

Given the same tire size and rear axle ratio as my Formula, 3750 RPM would be about 9% high, but it's still probably within the range of tachometer error, speedometer error, and tread wear combined. Then again, it also suggests that you might want to check to be sure that your torque converter clutch (TCC) is engaging properly.

[This message has been edited by Marvin McInnis (edited 11-04-2008).]