Hello,
I’ve noticed that my car is not as peppy as I would have expected from dead stop and I was suspecting the rear gear ratio.
Background… 1969 XR7 Convertible, 351 M-Code, 4-Speed.
The car was born with a 3.25 posi, I know that’s no race car ratio, but I wouldn’t necessarily call that “highway gears” either. I suspect it’s been swapped out for whatever reasons prior to my ownership, here are my clues.
The less than expected performance from dead stop.
I did the old spin the tires and count the driveshaft rotation check and I calculated a ratio of ~2.70 (and not posi either). The 2.70 would explain the lesser performance from dead stop.
So my question is does it make sense that the rear-end ratio changing from 3.25 to 2.70 (~17%) ties into the speed-o inaccuracy drop by the roughly the same amount? I keep trying to figure out if the Speed-o would go up or down by the 17% with the ratio going down. Every time I rationalize that this seems correct, I question my logic that it would be the other way!
Any insight would be appreciated.
Thx,
Jay
Appreciate the responses, but calculating the rear-end ratio is not my question, I’m pretty confident I did that correctly.
I’m wondering if the change in gear ratio is causing my speedometer’s incorrect readings.
Does a 15% increase in actual speed from speedometer reading vs. a 15% increase in wheel spin vs. engine RPM?
Seems to me that it would, I’m just trying to get confirmation or explanation on the relationship between the two.
Thx,
Jay
If somebody swapped the gears and did not swap the speedometer driven gear to the proper one for the new gears yes it does. When I did my T-5 swap in my White 70 my speedometer was way off until I had the rear gears done. In that case it made it read closely. I swapped the gears in my 71 from 2.75 to 3.25 and it now is way off. I need to acquire the correct driven gear for it. I told the shop I had one. But when I pulled out my little box of gears it was gone. I must have sold or given it away.
It absolutely makes sense that changing from 3.25 to 2.70 ties into the speed-o inaccuracy and poor launch. If your car was born with 3.25 and the speedo is wrong, that tells me the gear wasn’t changed when it went to 2.75.
I swapped a 2.75 to 3.50, and it predictably showed the wrong speed…but my 351C went from disappointing to screaming. I went from a 16-tooth to a 19-tooth and the speedo is dead nuts on. Cost me $1000 - out the door, but ridiculous improvement. For your 3.25, you’re probably in the neighborhood of the yellow 18-tooth, If I remember right, there’s a 5 mph difference per tooth, but MDL can calculate it exactly for you. They’ve got awesome service.
Appreciate the confirmation.
I’m actually thinking of putting in the correct 3.25 posi 8" that the car was born with.
I see them on eBay for about $1000.00 shipped.
That way I can enjoy the “launch” and have an accurate speed-o!
Thx,
Jay
I’ve research that and I can’t find a consensus on the 8 vs 9" from factory issue.
428’s definitely had 9" rear ends, 351W is where the debate comes in. I’ve heard different people argue 8 vs 9 and claim they have an 8 and know for a fact it was built that way, I have no idea what it came with, but it’s 8 now and I’m not about to invest in a 9" upgrade.
You may be correct, but if you swap a 9" for an 8" wouldn’t you have to change the wheel axles as well along with other parts (meaning $$)?
All I can say is the prior owner of this car was very frugal with the upkeep of this car and if they blew out there 9" rear, I would imagine they would do the absolute cheapest way to fix the car. Swapping a 9 " to an 8" I would think would be more expensive than just fixing the exiting 9".
But you could be correct.
Jay
They all came with a 9”. Mustang guys get confused as most 69 Mustangs came with an 8” behind a 302 or six cylinder. You can pretty well count on it that the 9” got traded out and replaced by an 8” because the 9” was worth about double the 8”.
All 1969-1973 Cougars left the factory with a 9" rear. You could have a car where absolutely no options were ordered and it would still have had a 9" rear when it was produced. A lot of things can happen in 50 years so perhaps someone in the past swapped an 8" into your car, anything is possible. Are you absolutely sure that you have an 8" rear?
I could definitely see the prior owner selling his 9" if it would make him a profit!
I assumed he just broke the 8" during some point and put in a lower ratio for better gas mileage. Mid 70’s were tough for muscle cars!
Possibly the confusion is coming from Mustang owners as I was doing my research.
Thx,
Jay
Check your door tag info under axle. That’ll tell you what was in there from the factory. That and a Marti report. But you have it right with the speed numbers. You want to go 28 at say 1000 rpms, but you turning a 1000 rpms as before and putting that to the 2.70s where you would only need say 800 rpms for 28 MPH.
3000RPMs, 25" tires, 3.25 gears = 69 MPH
3000RPMs, 25" tires, 2.70 gears = 82MPH
2500RPMs, 25" tires, 2.70 gears = 69MPH
Probably burned up the clutches and just put in what was on the shelf to get it down the road.
Thanks for confirming what I suspected with the speed vs gear change.
The tag info is as follows…
Axle Code - “R” - 3.25 Traction-Loc Rear Axle. (nothing about 8 or 9").
Trans Code - “6” - 4-Speed Close Ratio Manual Transmission.
Engine Code - “M” - 351-4V V-8 Engine.
The Marti report also has the “Heavy Duty Suspension” listed as an option, I would have thought that would have gotten you a 9" if 9" rear ends weren’t stock on all as mentioned by Cathouse.
I’m going to say it was a 9" only because an 8" posi in 69 was a rare bird. This also had to see special ordered, IMO, either by a individual or the dealership. 351W 4bbl, 4 spd, 3.25 posi, heavy duty susp, wasn’t the usual setup. sounds like someone wanted a Eliminator type setup in sheep’s coatings. Your pic from last night, that’s a 9". I think someone changed out the gears for better gas mileage. As for the axles, the same 28 spline was used in both the 8 & 9 due to housing width being the same. My parts interchange bock shows many times it was done. Saves production costs.
The article you linked said to see if you can get a socket on the lower bolt, if you can directly get it on you have an 8", if it can’t quite get on because of interference from the housing it’s a 9". I was able to get the socket directly on the bolt, so 8" rear.
I always understood the 9" rear ends have a “Bulge” on one side of them and the 8" is more evenly rounded (like what my picture indicates).
I’m still going with an 8" rear-end.
This You tube video shows the differences as well, and if correct I currently have an 8".
