Features Getting My Old Speedo to Match a Modern Drivetrain Swap—What’s the Trick?

I’ve run into a bit of a head-scratcher after swapping in a more modern transmission behind my flathead. The idea was to get better cruising ability on the highway, and in that respect, I’m happy—no more feeling like the engine’s screaming at 60 mph. But now I’m noticing a few unexpected things that I’m hoping you guys can help with. First off, the overall feel behind the wheel just isn’t what I was hoping for. The shift points don’t feel as natural, and throttle response at low speeds seems off. It’s like the car’s balance changed in ways I didn’t predict. I’ve been wondering if this means I should rethink my rear gear ratio, or maybe even look into different clutch options. I figured the trans swap would be mostly plug-and-play, but it’s opened up a can of worms I didn’t see coming.

One of the most noticeable annoyances has been the speedometer. At its core, the speedometer is the gauge that shows the driver the vehicle’s actual speed at any given moment. It’s something that becomes easy to overlook until it stops giving accurate information. I realized this when I noticed mine was reading as much as 20 mph too fast in some situations. While modern setups might rely on a live speed monitoring application that sends accurate digital data straight to a screen or device, my setup depends on that mechanical connection between the trans and the gauge. I kept my original speedo because I love the traditional look of the dash, so swapping it out isn’t something I want to do. I came across a tool that lets you calculate the right speedo gear based on tire size, trans type, and rear gear ratio, and that inspired me because it gave me a way to approach the problem logically instead of guessing. But I’m still curious what others have done to sort this out.

Another thing I’m noticing is that while the car feels great cruising at higher speeds, driving around town is less enjoyable now. First gear feels taller than it used to, and I find myself slipping the clutch more when pulling away from stops. I’m also wondering if I should have paid more attention to how this swap would affect carb tuning, since the engine’s working in a different RPM range most of the time now. I’m not sure if I’m just overthinking it or if other guys have run into these same kinds of issues after doing similar swaps.

I’d like to know if others have found solutions that helped them get that tight, connected feel back after doing a drivetrain swap like this. Did you end up changing more parts than you planned to get the car to feel right? And if you kept your stock speedometer like I did, how did you deal with getting the speed reading to match reality without gutting your dash? I appreciate any ideas or experiences you’re willing to share.

 

Your new transmission must not have as low a gear in first as the original transmission did.

 

You didn't mention what transmission. But using the T5 as an example it comes with 4 or 5 common gear sets that all have different ratios. So potentially you knowingly, or not, now have a higher low ratio than you like. Changing rear axle may not fix it as if you go lower you will also affect the amount of OD ratio. Fixing the speedo is fairly straight forward after you know the cable rpm per mph your speedo is looking for.

 

I'll chime in with this question regarding my model A Coupe with an original Model A speedometer.. I've been wanting to order a custom-made speedometer cable to hook a Borg Warner 4 speed (looks like a t5 but only four gears with top gear 1:1) to my Model A speedometer. I'm running an 8" Ford with a 3.55 ring and pinion. My question is, is the RPM of a speedometer cable to the indicated speed relatively similar for an old speedometer like a stock Model A compared to a later model speedometer? I hope my question makes sense.

Lynn

 

@Gallus , welcome to the HAMB from Illinois .
Speedometer questions can often be answered by @e1956v .
Let's see if he can help @lake_harley and @Gallus .

 

Speedhut, maybe others, has a Speed Box. GPS driven with a cable output to your original speedometer. Dead accurate. I like mine.

I did just a diff gear swap from 4.10 to 3.36 in my 50 Buick. Yep, town driving became more 2nd gear. Slip the clutch a little in 1st. Dropped 650 RPM in 3rd.

Ben

Ben

 

Thanks pprather. I talked with the gentleman with the speedometer service in Colorado some time ago but forgot how to reach him. The link with his user name will let me get in touch with him again.

Thank you.

Lynn

 

Cable revolutions per mile will be the same on the speedometer. Making up a cable is not a problem and I can help with calibration afterwards with a driven gear change or a speedometer ratio adapter. Just give me a call or email from the website.

 

I'll have to measure again the length that I need and will get in touch. Thanks!

Lynn

 

A vote for Rusty. The Best.

 

Rusty @e1956v made the cable to mate the stock speedometer in my '54 Ranch Wagon to an AOD trans. Excellent work, fast turn around, fair price, and reasonable shipping. Rusty is the real deal. You won't be disappointed.IMO

 

Rusty knows

 

Funny, my wife won't let me wear my old "speedo" with my modern (current) body

 

Hello,

I have been reading all about the speedometer services and samples that have been posted by this HAMB vendor.

His knowledge and service is top notch. Reading the posts gives me the old feeling of our neighborhood shop like his that had this guy that was so knowledgeable in all speedometer services. When our Impala had 3:55 gears for a long time down from 4:56, the speedometer was off. He changed it in 20 minutes.

So, any time we needed service, it was a short jaunt to the old shop to get quick service. He knew exactly what to do when we explained what was the problem.

Jnaki

The articles I have read about @e1956v and his work, it just seems like the old service we were used to in our hot rod cruising days.
https://www.jalopyjournal.com/forum/threads/great-hamb-vendor-speedometer-service.1333164/

 

Like @Moriarity said , Details !
The
Accurate speed can be solved
Speed readings .

Cruise and Performance speeds
Issues .
Transmission issues and gearIng ,
No matter if
O-D 4 speed manual
1-3 gears
Or
5 speed manual
1-4 gears
The Gear combinations are wrong for your
Wants or Application & Rear Gear & Tire height .
Typically between each gear change
No more then 1,000 ish RPM drop
@ WOT ,,,


I think this way , gears be for O-D gear.

Just like walking up a flight of stairs
Same Pace , Rpm , each step higher you
Go with out extra effort ,
Then the Over Drive would like
Skipping 2 - 3 stair's

May be I am wrong .

 

A 1000 rpm speedometer cable will read 60 miles per hour, that is the standard calibration number for nearly all speedometers.
Its the rear axle gear ratio and tire diameter that determine what speedometer gear set is needed to get 1000 rpm cable speed at 60 mph. Year, make, model, transmission type, or rear axle type makes no difference.

 

Smart gal

 

Yeah, I'm not quite as svelte as I once was.

 

Ummmm..... it seems that the speedo is working, just not accurate, correct? No one mentioned (yet) that the most likely solution, is to change the speedo gear in the transmission.