Thread: RPM's to high?

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I have a 700R4 with 3.89 rear gears and 30 inch rear tires. At 70 mph, my tach reading is just under 2200 rpm. I wanted to be able to cruise at 70-75 on the Interstate and get decent mileage, so that is why I chose this combination. I would say 2500 rpm at 65 mph is a pretty good combination.

Originally Posted by 34_40

I finally got the coupe out and went onto the highway to see how it would do. All went well, nice smooth ride, no shakes or rattles / noises. Even the exhaust was real smooth! I was worried that it would drone and cause me major annoyances.

My only "concern" was the engine rpm at 65 was a touch over 2500 rpm. Do you guys consider this to high? the current combo uses an AOD with 3.90 gears. Tires I think were 38" tall...

Anyway, what do you think?

Mike,
Something's wrong with your numbers. That AOD has a 0.67 to 1 OD ratio, right? If you're running 3.90 rear gears in OD a 28" tall tire would have you at 2,038rpm at 65mph. To get it up to 2500 in OD would be something in the range of 22"diameter on the tire. Were you maybe not in OD? At a 1.0 ratio in third a 33" tall tire would have you cranking 2581rpm. Sure your tach is right? Using a "fudge factor" to get the units right, RPM = (MPH x Gear Ratio x 336 x Final Drive Ratio)/Tire Diameter in inches (65mph x 3.9 x 336 x 0.67)/28" = 2038rpm. If you're in third, (65 x 3.9 x 336 x 1) = 3042rpm. Something's not right here.

Do you maybe have an AODE? That has a 0.7 OD ratio, which would put your numbers a little higher, but still not matching up with your 2500rpm at 65mph intro.

Originally Posted by rspears

Mike,
Something's wrong with your numbers. That AOD has a 0.67 to 1 OD ratio, right? If you're running 3.90 rear gears in OD a 28" tall tire would have you at 2,038rpm at 65mph. To get it up to 2500 in OD would be something in the range of 22"diameter on the tire. Were you maybe not in OD? At a 1.0 ratio in third a 33" tall tire would have you cranking 2581rpm. Sure your tach is right? Using a "fudge factor" to get the units right, RPM = (MPH x Gear Ratio x 336 x Final Drive Ratio)/Tire Diameter in inches (65mph x 3.9 x 336 x 0.67)/28" = 2038rpm. If you're in third, (65 x 3.9 x 336 x 1) = 3042rpm. Something's not right here.

Do you maybe have an AODE? That has a 0.7 OD ratio, which would put your numbers a little higher, but still not matching up with your 2500rpm at 65mph intro.

Thanks Roger! Here's what I know to be fact.
A = rear is 3.90:1 ratio
B = Tranny is a "91" AOD and I can count the shifts, so I know it's in "4th" (od).
C = Tires ??? I don't know the diameter. I'm in a hotel in central mass but I'll be home friday evening so I'll measure that over the weekend.
D = Gauges... HMMMmmm.. I hadn't considered they may be "mis-calibrated".. esp. the tachometer, I have no reason to doubt it..

Originally Posted by 34_40

Thanks Roger! Here's what I know to be fact.
A = rear is 3.90:1 ratio
B = Tranny is a "91" AOD and I can count the shifts, so I know it's in "4th" (od).
C = Tires ??? I don't know the diameter. I'm in a hotel in central mass but I'll be home friday evening so I'll measure that over the weekend.
D = Gauges... HMMMmmm.. I hadn't considered they may be "mis-calibrated".. esp. the tachometer, I have no reason to doubt it..

OK, the AODE applications list is the 1992–1994 Lincoln Town Car, 1991 and up Ford F-Series Truck, 1993–1994 Ford Crown Victoria, 1993–1994 Mercury Grand Marquis, and 1994–1995 Ford Mustang which means unless your tranny came from a truck it's an AOD with the 0.67 final drive ratio. Do you recall what size your rear tires are, like 255R15, or 225R15 or whatever?

Originally Posted by rspears

OK, Do you recall what size your rear tires are, like 255R15, or 225R15 or whatever?

No.. CRS.. When I get home friday I'll try to get to the shop and get some data for us.. THANKS!

OK, I understand CRS but it's SUNDAY! You've been back since Friday night, and still haven't remembered to put a tape next to those rear tires so we can sort out this business of your rpm's.

Originally Posted by rspears

OK, I understand CRS but it's SUNDAY! You've been back since Friday night, and still haven't remembered to put a tape next to those rear tires so we can sort out this business of your rpm's.

I'll have to hang my head in shame... I WAS at the shop and walked by the car a dozen times but we had our excavator go down and it had to be ready for monday morning.. 2 days soaked in hydraulic oil and grease..

I can't wait to go back to work 'cause I need the rest!

I will try and run back tomorrow before I leave for central MA., then we can hammer it out! Thanks for your patience!

Originally Posted by 34_40

I'll have to hang my head in shame... I WAS at the shop and walked by the car a dozen times but we had our excavator go down and it had to be ready for monday morning.. 2 days soaked in hydraulic oil and grease..

I can't wait to go back to work 'cause I need the rest!

I will try and run back tomorrow before I leave for central MA., then we can hammer it out! Thanks for your patience!

Jus' pullin' your chain, Mike! Ya' gotta take care of the important stuff, or things come to a halt while you're traveling. It'll get figured out one of these days. Travel safe next week!!
Just remembered that my wife is flying out tomorrow for Connecticut, hooking up with a couple of early childhood girls from her neighborhood, and they're going up to a B&B in Maine (can't recall the town) for a few nights to catch up after too many years. Looks like the weather should be great for them!!

Originally Posted by rspears

Jus' pullin' your chain, Mike! Ya' gotta take care of the important stuff, or things come to a halt while you're traveling. It'll get figured out one of these days. Travel safe next week!!
Just remembered that my wife is flying out tomorrow for Connecticut, hooking up with a couple of early childhood girls from her neighborhood, and they're going up to a B&B in Maine (can't recall the town) for a few nights to catch up after too many years. Looks like the weather should be great for them!!

Thanks Rog...

Your wife from CT originally?

Originally Posted by 34_40

Your wife from CT originally?

Nope, three little girls from Leawood, Kansas (suburb of KC). One ended up in Connecticut, one in California, and we're still in the KC Metro. The Cali gal didn't want to get together in California; the Connecticut gal didn't like the idea of coming back to Kansas; so KS and CA are flying to CT. Crazy logic, but two of three were going to be traveling, and my gal loves to travel. Oh, and they're headed to Rockland, on the coast.

Originally Posted by rspears

Nope, Crazy logic, but two of three were going to be traveling, and my gal loves to travel. Oh, and they're headed to Rockland, on the coast.

That's cool! I hope she enjoys her trip and time with friends!

Roger, currently 25.5" height... my new target will be 27.5" tall...

OK, with a 25.5" tall tire, 3.90 gears in 4th with an AOD (0.67 OD Ratio) you're cranking 2238rpm at 65mph, give or take a few rpm for tire inflation. When you shift to 27.5" tall tires (which is around a 225 75R 15), you'll drop to 2075rpm, again at 65mph. If you're showing over 2500 it sounds like you've got a bit of tach offset/error.

Originally Posted by rspears

OK, with a 25.5" tall tire, 3.90 gears in 4th with an AOD (0.67 OD Ratio) you're cranking 2238rpm at 65mph, give or take a few rpm for tire inflation. When you shift to 27.5" tall tires (which is around a 225 75R 15), you'll drop to 2075rpm, again at 65mph. If you're showing over 2500 it sounds like you've got a bit of tach offset/error.

OK, Thanks Bud! I had an evening client meeting with my boss and on the way back home (just got in a few minutes ago) I was thinking I can verify my speedometer by comparing it with my gps! One of them (or both) may be off just a bit... Maybe next weekend I can do that cross check with the GPS.

Yeah, you're right that it could be the speedometer that's running slow vs the tach being high. Duh! I didn't even consider that side, and checking it with the GPS is a great idea. I do that all the time in the Jeep, checking the "real" speed.